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EXPLANATORY NOTES

HYDROGEOLOGICAL AND HYDROCHEMICAL MAPSOF NEGELE NB 37-11

Getachew Zewdie (Chief Compiler)

Jiri Sima (Editor)

The Main Project Partners

The Czech Development Agency (CzDA)cooperates with the Ministry of Foreign Affairs on the establishment of an institutional framework of Czech development cooperation and actively participates in the creation and financing of development cooperation programs between the Czech Republic and partner countries.

www.czda.cz

The Geological Survey of Ethiopia (GSE)which is accountable to the Ministry of Mines and Energy, collects and assesses geology, geological engineering and hydrogeology data for publication. The project beneficiary.www.geology.gov.et (www.mome.gov.et)

AQUATEST a.s. a Czech consulting and engineering company in water management and environmental protection. The main contractor.www.aquatest.cz

The Czech Geological Service collects data and information on geology and processes it for political, economical and environmental management. The main subcontractor.www.geology.cz

Copyright © 2011 AQUATEST a.s., Geologicka 4, 152 00 Prague 5, Czech RepublicFirst editionISBN 978-80-260-0334-2

aquatest

Acknowledgment

Field work and primary compilation of the map and explanatory notes was done by a team from the Geological Survey of Ethiopia (GSE) consisting of staff from the Groundwater Resources Assessment Department; the Czech experts from AQUATEST a.s. and the Czech Geological Survey in the framework of the Czech Official Development Assistance Program. The team is greatly indebted to the Guji and Liben zone administration of Oromia and Somali regional states and Negele Borena city administration for their limitless cooperation. We are indebted to the Guji zone and Liben zone water resources office for providing relevant data which were crucial for our hydrogeological mapping The team is grateful to the management of the Geological Survey of Ethiopia, particularly to Director General (GSE) Mr. Masresha G/Selassie and Mr. Yohannes Belete, Head of Groundwater Resources Assessment Department (GSE) and Mr. Muhudin Abdela, Senior Hydrogeologist and Project Coordinator. Special thanks go to the NGOs and private water drilling and consultant companies for providing data from private databases. Our special thanks also go to Oromia Water Works Enterprise and Construction for providing assistance during the field work. Finally, the team acknowledges the untiring support of the local people who assisted the team by all means possible and facilitated the data collection and those who helped us in different ways.

Acknowledgment

ContentsAcknowledgment .........................................................................................................................................................................3Extended Summary....................................................................................................................................................................11Introduction ................................................................................................................................................................................ 151. Basic Characteristics of the Area ..................................................................................................................................171.1 Location and Accessibility .......................................................................................................................................................171.2 Population, Settlements and Health Status ........................................................................................................................181.3 Land Use ......................................................................................................................................................................................252. Selected Physical and Geographical Settings ............................................................................................................272.1 Geomorphology .........................................................................................................................................................................282.2 Soil and Vegetation Cover .......................................................................................................................................................292.3 Climatic Characteristics ...........................................................................................................................................................312.3.1 Climatic Zones and Measurements...................................................................................................................................312.3.2 Precipitation ............................................................................................................................................................................362.4 Hydrography and Hydrology of the Area ............................................................................................................................432.4.1 Surface Water Network Development ..............................................................................................................................442.4.2 Surface Water Regime .........................................................................................................................................................452.4.3 Baseflow ..................................................................................................................................................................................472.5 Water Balance ............................................................................................................................................................................542.6 Drought and Climate Changes...............................................................................................................................................573. Geological Settings ...........................................................................................................................................................613.1 Previous Work ............................................................................................................................................................................613.2 Stratigraphy ................................................................................................................................................................................623.3 Lithology ......................................................................................................................................................................................623.3.1 Crystalline Basement Rocks and Associated Intrusive Rocks ....................................................................................633.3.2 Paleozoic Clastic Sediment .................................................................................................................................................653.3.3 Mesozoic Sedimentary Formations ..................................................................................................................................653.3.4 Tertiary Volcanic Rocks ........................................................................................................................................................673.3.5 Quaternary Sediments (Qa, Qe) .........................................................................................................................................673.4 Structure ......................................................................................................................................................................................673.5 Geological History .....................................................................................................................................................................694. Hydrogeology .....................................................................................................................................................................714.1 Water Point Inventory ...............................................................................................................................................................714.2 Hydrogeological Classification/Characterization ..............................................................................................................734.3 Elements of the Hydrogeological System of the Area (Aquifers) ..................................................................................744.3.1 Local and Moderately Productive Porous Aquifers .......................................................................................................754.3.2 Extensive and Moderately Productive Fissured and Karstic Aquifers ......................................................................754.3.3 Extensive and Low Productive Fissured Aquifers ..........................................................................................................784.4 Hydrogeological Conceptual Model .....................................................................................................................................804.5 Annual Recharge in the Area .................................................................................................................................................835. Hydrogeochemistry ......................................................................................................................................................... 855.1 Sampling and Analysis .............................................................................................................................................................855.2 Classification of Natural Waters ............................................................................................................................................865.2.1 Precipitation ............................................................................................................................................................................895.2.2 Surface Water.........................................................................................................................................................................89

5.2.3 Groundwater in Mesozoic and Quaternary Sediments ................................................................................................895.2.4 Groundwater in Basement Rock ........................................................................................................................................905.3 Water Quality .............................................................................................................................................................................905.3.1 Domestic Use ..........................................................................................................................................................................905.3.2 Irrigation Use ..........................................................................................................................................................................925.3.3 Industrial Use ..........................................................................................................................................................................935.4 Mineral and Thermal Water ....................................................................................................................................................946. Natural Resources of the Area ...................................................................................................................................... 956.1 Economic Geology ....................................................................................................................................................................956.2 Water Resources .......................................................................................................................................................................956.2.1 Surface Water Resources Development ..........................................................................................................................976.2.2 Groundwater Resources Development ............................................................................................................................976.3 Human and Land Use Resources and Development..................................................................................................... 1056.4 Wind and Solar Energy Development ............................................................................................................................... 1056.5 Environmental Problems and their Control / Management ........................................................................................ 1056.6 Touristic Potential of the Area ............................................................................................................................................. 108Conclusions ..............................................................................................................................................................................109References.................................................................................................................................................................................111Annex 1 – Field Inventory Data...................................................................................................................................................113Annex 2 – Water Chemistry........................................................................................................................................................ 123Annex 3 – Well Logs ......................................................................................................................................................................127

List of Figures

Fig. 1.1 Location map ............................................................................................................................................................ 17Fig. 1.2 The main roads and settlements ........................................................................................................................ 18Fig. 1.3 Administrative zones ............................................................................................................................................. 19Fig. 1.4 Malaria risk in Ethiopia .......................................................................................................................................... 21Fig. 1.5 Land use ...................................................................................................................................................................25Fig. 2.1 Generalized physiographic units .........................................................................................................................27Fig. 2.2 Gorge of the Genale River in Donto village .......................................................................................................28Fig. 2.3 Distribution of soil types .......................................................................................................................................29Fig. 2.4 Acacia trees .............................................................................................................................................................30Fig. 2.5 Climatic zones .........................................................................................................................................................33Fig. 2.6 Temperature at Negele meteo-station ...............................................................................................................34Fig. 2.7 Monthly no. of sunshine hours at Kibremengist and Negele Borena stations .........................................35Fig. 2.8 Mean monthly potential evapotranspiration [mm], temperature [°C] and relative humidity [%] .........35Fig. 2.9 Monthly wind speed [km/d] of selected stations ...........................................................................................36Fig. 2.10 Seasonal classification and precipitation regimes of Ethiopia (source: NMSA, 1996) ..........................37Fig. 2.11 The Negele meteo-station precipitation pattern .............................................................................................38Fig. 2.12 The Siru meteo-station precipitation pattern ...................................................................................................39Fig. 2.13 The Zembaba Wiha meteo-station precipitation pattern ..............................................................................39Fig. 2.14 Long-term fluctuation and average of precipitation for the Negele meteo-station ................................. 41Fig. 2.15 Fluctuation and average of precipitation for the Siru meteo-station ..........................................................42Fig. 2.16 Fluctuation and average of precipitation for the Zembaba Wiha meteo-station .....................................43Fig. 2.17 The principal river basins of the area .................................................................................................................44Fig. 2.18 Mean monthly flow of the Genale River at Halowey, Chenemasa and Girja gauging stations [m3/s] ... 47Fig. 2.19 Flow diagram of the Genale River at the Chenamasa river gauging station ............................................ 47Fig. 2.20 An annual variability of the mean annual flow of Genale River at Chenamasa river gauging station .....48Fig. 2.21 Method of Kille baseflow assessment ...............................................................................................................49Fig. 2.22 Kille baseflow separation ......................................................................................................................................50Fig. 2.23 Method of baseflow separation .......................................................................................................................... 51Fig. 2.24 Hydrograph baseflow separation ........................................................................................................................ 51Fig. 2.25 The most drought prone areas of Ethiopia (source: RRC, 1985) .................................................................57Fig. 3.1 Limestone outcrop near Siru village ..................................................................................................................65Fig. 3.2 Recent fault at quarry near Siru village .............................................................................................................68Fig. 3.3 Mesozoic propagation of the Karoo rift to the southeastern part of Ethiopia (modified after

Gani et al., 2008) ....................................................................................................................................................70Fig. 4.1 Extent and location of moderately productive porous aquifers ...................................................................75Fig. 4.2 Extent and location of moderately productive fissured and karst aquifers ............................................... 76Fig. 4.3 Medawelabu spring (CS-2) ................................................................................................................................... 76Fig. 4.4 Frequency of yield of springs and wells in fissured aquifer developed in limestone rocks ...................77Fig. 4.5 Extent and location of the low productive fissured aquifer developed in basement rocks ...................79Fig. 4.6 Frequency of yield of springs and wells in fissured aquifer developed in basement rocks ..................79Fig. 4.7 Digging for groundwater in pockets of weathered basement rock nearby Negele town .....................80

Fig. 4.8 Conceptual hydrogeological model of southeastern highlands and lowlands .............................................. 81Fig. 4.9 Fissures in roof of the Sof Omar cave ...................................................................................................................... 81Fig. 5.1 Level of cation-anion balance .....................................................................................................................................86Fig. 5.2 Piper diagram for classification of natural waters .................................................................................................88Fig. 5.3 Content of nitrate in analysis of water in the study area .....................................................................................92Fig. 6.1 Geoelectric section of Siminto site ...........................................................................................................................99Fig. 6.2 Geoelectric section of the Hadessa site ..................................................................................................................99Fig. 6.3 Geoelectric section of the Dekka site .................................................................................................................... 100Fig. 6.4 Geoelectric section of the Dibi Guchi site .............................................................................................................101Fig. 6.5 Geoelectric section of the Meda Welabu site ......................................................................................................101Fig. 6.6 Geoelectric section of the Shishu 1 site ............................................................................................................... 102Fig. 6.7 Geoelectric section of the Shishu 2 site ............................................................................................................... 103Fig. 6.8 Geoelectric section of the Filo site ......................................................................................................................... 103

List of TablesList of authors and professionals participating in the project ............................................................................................ 16Tab. 1.1 Population in the study area .................................................................................................................................20Tab. 1.2 Mortal diseases in Ethiopia (WHO, 2006) ........................................................................................................22Tab. 1.3 Rural water facilities by Zones ............................................................................................................................22 Tab. 1.4a Leading causes of hospital and health center morbidity 2008/2009 in Oromia Region .....................23Tab. 1.4b Leading causes of hospital and health center morbidity 2008/2009 in Somali Region ......................23Tab. 2.1 Ethiopian climate classification ...........................................................................................................................32Tab 2.2 Climatic stations in the Negele area ..................................................................................................................33Tab. 2.3 Characterization of the precipitation pattern in Ethiopia ..............................................................................36 Tab. 2.4 Monthly long-term average precipitation in selected meteo-stations of the Negele sheet [mm] .......38Tab. 2.5 Long-term monthly rainfall at Negele [mm] (fully recorded years only) ....................................................40Tab. 2.6 Monthly rainfall at Siru [mm] (fully recorded years only) ..............................................................................42Tab. 2.7 Long-term monthly rainfall at Zembaba Wiha [mm] (fully recorded years only) .....................................42Tab. 2.8 Data on the river gauging stations .....................................................................................................................45Tab. 2.9 Runoff data ...............................................................................................................................................................46Tab. 2.10 Baseflow data for the Negele area .....................................................................................................................53Tab. 2.11 Water balance input data .....................................................................................................................................55Tab. 2.12 Water balance of Shaya basin .............................................................................................................................55Tab. 2.13 Comparison of water losses in water balance with estimated deep base flow .......................................56Tab. 3.1 Litho stratigraphy of the mapped area ..............................................................................................................62Tab. 3.2 Summarized review of the Precambrian rocks ................................................................................................63Tab. 3.3 Detailed description of limestone sequence ....................................................................................................66Tab. 4.1 Aquifer classification based on well yield for Genale-Dawa basin ..............................................................72Tab. 4.2 Aquifer classification by Lahmeyer (2005) ......................................................................................................72Tab. 4.3 Summary of field inventory ..................................................................................................................................73Tab. 4.4 Summary of basic data of wells in the Negele sheet ..................................................................................... 74Tab. 4.5 Basic hydraulic characteristics of wells in the Negele sheet ........................................................................ 74Tab. 4.6 Estimated minimum recharge to ground water from stations of the Genale-Dawa basin ....................83Tab. 4.7 Rainfall infiltration factor for Wabe Shebelle basin by WWDST (2003) ...................................................84Tab. 5.1 Level of balance ......................................................................................................................................................86Tab. 5.2 Summary of hydrochemical types ......................................................................................................................87Tab. 5.3 Groundwater descriptive statistics of TDS, EC and Cl values ......................................................................89Tab. 5.4 Chemical composition of rain water ..................................................................................................................89Tab. 5.5 Groundwater chemistry compared to drinking water standards and guidelines .................................... 91Tab. 5.6 Suitability of water for irrigation .........................................................................................................................92Tab. 5.7 Suitability of water for use in industry ...............................................................................................................93Tab. 5.8 Concentration limits for incrustation .................................................................................................................94Tab. 5.9 Concentration limits for corrosion ......................................................................................................................94Tab. 6.1 Aquifers of the area ................................................................................................................................................96Tab. 6.2 Assessment of water resources of the Negele area .......................................................................................96

Under Separate Cover (see attached CD)

Annexes:Annex 1 Field Inventory DataAnnex 2 Water ChemistryAnnex 3 Well Logs

Maps:Hydrogeological Map of Negele NB 37–11 - full size and A3 sizeHydrochemical Map of Negele NB 37–11 - full size and A3 size

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Extended SummaryThe Negele area is located in Eastern Ethiopia on the Negele map sheet (NB 37-11) at the

scale of 1:250,000, covering an area of 18,370 km2. The area is a part of the Oromia and Somali regional states and is inhabited by 0.4 million people and only a small part of the area is cultivated.

The southern and eastern part of the Negele area is below 1,500 m above sea level (a.s.l.) and is represented by Negele hills and the flat Negele plain. This area rises to the northwest to the highlands at about 2,000 m a.s.l. and higher. The area is a part of the Genale-Dawa river basin. The rainy season is bimodal from March to May and from October to November; the annual mean rainfall of 679 mm was adopted for the Negele area. There are several permanent rivers (Awata, Mormora, Dawa, Genale and Welmel) and intermittent rivers particularly in the southeastern part of the area. Specific surface runoff was adopted as being a value of 6.0 l/s.km2. The adopted value of specific baseflow is relatively low i.e. 0.6 l/s.km2 representing 30 mm/year and 4.5 % of precipitation. The Negele area faced severe Kiremt drought in 1969, 1970 and 1987. The years that drought was most serious in the Negele area were 1969, 1973 and 1977. The area shows high Kiremt drought probability (third highest in Ethiopia).

The aquifer system has been defined based on the hydrogeological characteristics of lithological units described by the geological maps and data from the field inventory and desk study. The characterization of the area shows the following aquifer/aquitard systems:

1. Extensive and moderately productive porous aquifers with spring and well yield Q = 0.51–5 l/s developed in Quaternary unconsolidated deposits.

2. Extensive and moderately productive fissured and karst aquifer with spring and well yield Q = 0.51–5 l/s developed in Hamanlei limestone.

3. Extensive and low productive fissured aquifers with spring and well yield Q = 0.051–0.5 l/s developed in basement rocks.

The hydrograph separation and Kille method show that the infiltration coefficient (recharge) is about 4.5 % of the total precipitation. Part of the groundwater infiltrates directly from precipitation and groundwater flows laterally to local and or regional drainage base levels represented by rivers in deep valleys where it emerges as springs or flows vertically recharging deeper aquifers. This type of front recharge is limited because the position of the aquifers in lowlands with low precipitation depth and limited surplus of water for infiltration causes limited direct recharge of the aquifers. Recharge from areas with higher precipitation to the north and west of the Negele sheet is also possible. Large outcrops of basement rocks at Negele are recharged directly by precipitation which is adequate in the area to form good groundwater resources. The intermittent and ephemeral rivers and flood episodes of perennial rivers in the lowlands contribute significantly to the recharge of aquifers along river banks. Bank recharge

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provides a relatively large amount of good quality groundwater with low TDS for development in the alluvial aquifers of the lowlands.

Chemistry of groundwater in the Negele area is highly variable reflecting variability in composition of sedimentary and basement rocks. The dominant hydrochemical type of groundwater in the western and northern part of the Negele area is bicarbonate type but sulphate type dominates in southeastern part of the sheet. The basic Ca–HCO

3 type occurs in the centre of the western

part of the sheet. High TDS and sulphate content in groundwater is caused by its circulation in limestone with higher solubility and its contact with gypsum strata. The high sulphate content in groundwater circulating in the basement can be because of its contact with various sulphidic mineralizations which is typical for basement rocks of this area. In general, the TDS increases from the northwest from area with higher precipitation to the southeast to more arid part of the sheet. The general trend is highly affected by TDS and groundwater hydrochemistry is highly affected by soluble gypsum and even rock salt which is common in some sedimentary units. Groundwater TDS varies from 24 mg/l to 5,492 mg/l and is not convenient for drinking in more than 50 % of sampled points based on drinking water standards. The use of groundwater can be limited by pollution particularly of human and animal origin and some samples show increasing concentrations of nitrates additional to high TDS.

The total amount of water resources of the area has been assessed to be 3,478 Mm3/ year. The use of surface water for irrigation is the most important development factor and 80 % of available surface water resources will be used for irrigation. This portion represents 2,782 Mm3/ year. Considering that about 10,000 m3 of water is needed to irrigate 1 ha of land, the calculated irrigation resources represent an irrigation potential of 278,200 ha (2,782 km2) which is about 15 % of the Negele area.

The total volume of renewable groundwater resources of active aquifers in the area has been assessed to be 347 Mm3/year. Considering the total number of people living within the area is 0.4 million the need for water supply can be nearly 7.7 Mm3/year (20 l/c.d). The figure shows that recent demand represents less than 1 % of renewable groundwater resources of active aquifers, meaning that aquifers can provide adequate drinking water even in the future considering the trends in population growth and can be also used for supply of areas adjacent to the Negele area.

Most of the people within the area live in small towns and villages. Water supply based on wells drilled in limestone or basement rocks represents the most secure water and should be applied for small towns and concentrated village settlements. Technically, it is recommended to drill wells with a depth of about 150–250 m in aquifers developed in limestone. Each of the wells can yield about 2 l/s. Such wells can provide 172,800 l/d and can supply a small town or group of villages with about 8,600 inhabitants considering a daily consumption of 20 l/c.d. The drilling of wells with a depth of about 30–70 m is recommended for areas covered by basement rocks. Each of the wells can yield about 1 l/s. Such wells can provide 86,400 l/d and can supply a small town or group of villages with about 4,320 inhabitants. In this respect it is recommended to drill wells for the water supply in selected sites. Drilling should be done in sites where there is not an adequate water supply and or quality of water at existing water source is not safe for drinking purpose and where groundwater resources are abundant but not effectively utilized. The proposed eight drilling sites were investigated by geophysical measurements (VES) and are shown on the hydrogeological map.

The minimum required distance of water supply wells and potential pollution sources should be maintained during the development of groundwater resources for towns and villages. In addition to priority in development of groundwater for safe drinking water supply it should be possible to select the most fertile soil to be developed by small scale irrigation and livestock watering based

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on groundwater to increase the stability of food supply in prolonged periods of drought in the Negele area.

Soil erosion and protection is one of the limiting factors of sustainable development of agriculture within the area and should be addressed in all development projects, but data about soil erosion are scarce in the area.

The work which is summarized in the presented explanatory notes shows the relatively good water, agricultural, industrial, human potential of the Negele area.

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BackgroundEthiopia is a country affected by environmental degradation including recurrent droughts which

lead to food insecurity, and drought stricken areas have been constantly degraded over the past several decades by improper utilization of natural resources. The eastern dissected highlands and lowlands of Ethiopia are no exception to the above mentioned fact. The rugged topography and high gradient coupled with increasing population and intense deforestation aggravates the problem. It is therefore important to compile a map of water resources to be able to propose and implement appropriate protection measures during development efforts. It is also vital in identifying and tackling existing problems and proposing their solution. In this context the project for hydrogeological investigation of the “Groundwater Resources Assessment of the Southeastern Highlands and Associated Lowlands of Ethiopia” was performed in the Negele sheet in 2010 by the Geological Survey of Ethiopia. The publication of the project results was conducted in the framework of bilateral cooperation between the Czech and Ethiopian governments, where the participation of the Czech experts was financed by the Czech Development Agency in the framework of the Czech Republic Development Assistance Program. Participation of the Ethiopian professionals was financed by the Ethiopian government. This report deals with the assessment of hydrogeological and hydrochemical characteristics and other environmental parameters acquired during the desk and field work and discussion between stakeholders and the joint Czech-Ethiopian team of professionals.

Objective and ScopeWater is a finite resource and must be managed in a sustainable way. For sustainable

development, water resource investigation can play an important role in the efficient and optimal utilization of the water resources available to a country. The main objectives of the study for hydrogeological mapping were to identify water-bearing lithological units and their basic characteristics, to indentify recharge and discharge areas as well as groundwater flow direction, to categorize water quality within water bearing formations, to indicate the suitability of groundwater for different purposes, and to compile hydrogeological and hydrochemical maps with accompanying explanatory notes of the study area based on the information and analysis made. The work covers the interpretation of aerial photos and satellite images, meteorological and hydrological data analysis, quantification of inventoried water points, collection of representative water samples and data for hydrochemical studies, and evaluation of water resource management of the area. The hydrogeological investigation of the Negele map sheet is part of the project entitled “Groundwater Resources Assessment of the Southeastern Lowlands and Associated Highlands” that was conducted between 2009 and 2011 to alleviate water shortage in the area.

IntroductionIntroduction

The desk and field work was carried out by a group of Ethiopian hydrogeologists. Final assessment and publication of the map was carried out by a joint Czech-Ethiopian team of professionals. The names of participating experts are shown in the following list.

List of professionals participating in the project

Name Institution Participation field

Jiri Sima AQUATEST a.s. Editor

Getachew Zewdie Geological Survey of Ethiopia Chief compiler

Muhedin Abdela Geological Survey of Ethiopia Project coordinator

Ondrej Nol AQUATEST a.s. Hydrogeological expert

Antonin Orgon AQUATEST a.s. GIS expert

Romana Suranova AQUATEST a.s. Printing expert

Craig Hampson AQUATEST a.s. Language revision

Tenebit Zecariyas Geological Survey of Ethiopia Data acquisition and evaluation

Aboma Abdissa Geological Survey of EthiopiaGeophysical study and field data interpre-tation

Yielak Alemu Geological Survey of EthiopiaGeophysical study and field data interpre-tation

Samson Hailu Geological Survey of EthiopiaGeophysical study and field data interpre-tation

Aklilu Hailu Geological Survey of Ethiopia Geophysical study and report compilation

Dana Capova Czech Geological SurveyAEGOS project expert – coordination, tech-nical architecture, interoperability

Vladimir Ambrozek Czech Geological SurveyAEGOS project expert data conversion and processing

Petr Coupek Czech Geological SurveyAEGOS project expert – data on-line provi-sion

Shiferaw Ayele Geological Survey of Ethiopia AEGOS project country representative

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1.1 Location and AccessibilityThe study area is located in Eastern Ethiopia, in part of the Eastern Ethiopian highlands (plateau)

but mainly in the adjacent lowlands of the Ogaden plain. Geographically the study area is bounded from north to south by latitudes 5°00‘ N and 6°00‘ N, and from west to east by longitudes 39°00‘ E and 40°30‘ E. The area covers approximately 18,369 km2 of the topographic map sheet of Negele (NB 37-11) at a scale of 1:250,000. The location of the map is illustrated in Fig. 1.1. The sheet is bounded by the Dodola sheet in the north, the Hagere Mariam sheet in the west, the Filtu sheet in the east and the Wachile sheet in the south.

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Basic Characteristics of the Area

1. Basic Characteristics

of the Area1.

Fig. 1.1 Location map

18 Basic Characteristics of the Area

The area of the Negele map sheet can be reached through two routes: from Addis Ababa through Asela–Goba–Delo Mena–Bidire to Negele Borena and from Addis Ababa through Awasa–Aposto–Alta Wondo–Kibremengist to Negele Borena, both being approx. 600 km long. The all weather gravel road, currently being upgraded to an asphalt road, connecting Kibremengist and Negele Borena is the most convenient access and crosses the area from the northwest to the southeast. This road extends to the east to Filtu and then to the southeast to Dolo, outside the map area. All weather roads in the map area connect Negele to Bulbul, Bura Dera, Mugayo, Addadi Oda, Chebbe and Genale villages, and roads leading from Harokalo through Jidola to Mugayo, Bulbul to Alghe, Shakisso through Kenticha to Denni Dama and Kibremengist to Chembe (outside the map area); attempts have also been made to cover the north east part of the Negele sheet via Medawelabu. The main accessible roads and settlements are shown in Fig. 1.2.

1.2 Population, Settlements and Health StatusThe study area is part of the Oromia and Somali regional states. The population density varies

from place to place in the high and lowland areas, however, the population density as well as the number of settlements in the eastern part of the study area is not high due to the lack of sustainable water resources, harsh climatic conditions and the way of living (most people of the area are pastoralists). The density is higher in the highlands in the western part of the study area because of the favorable climatic and living conditions; especially where there is better access to sufficient farmland and a sustainable water supply for the community, as well as the proximity of the villages to roads and markets, etc. Population density varies from place to place in the urban areas and rural villages of the lowlands and highlands. The density in the northwestern part of the

Fig. 1.2 The main roads and settlements

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19Basic Characteristics of the Area

area is 30–80 inhabitants per km2 and there are 4–7 inhabitants per km2 in the southeastern part of the area. The average population density is 30 inhabitants per km2.

The inhabitants are dominantly the Oromo tribe of the Borena, and the Guji and Arsi tribes. The Somali tribe of the Degodi, Merihan, Gurra and Gerri clans is present in small numbers to the west and progressively dominate to the southeast. The Oromos are semi-nomadic cattle breeders, with small plots of farmland. Each family home is situated separately from another by several acres of grazing and they are interspersed in the woody grassland of the western and northern Negele map sheet. They cultivate mainly corn, teff and at some places barley, sorghum and wheat to subsidize their subsistence. The Somalis raise camels, goats and practice farming in some places. Population density in this area is not high manly due to a lack of sustainable water resources and harsh climatic conditions. However, towns in the area, for example Negele Borena, are mixed ethnic centers inhabited by the settlers of different ways of life (mainly trade and agriculture) from different ethnic groups.

There are 4 Zones within the mapped area (see Fig. 1.3), however none of them are located entirely within the boundary of the map sheets. To calculate the total number of people living within the mapped area the number of people living in the Weredas was assessed from the total Weredas population and by the percentage of the area within the map sheets. Tab. 1.1 shows the population in the different Weredas within the mapped area.

Based on the data provided by the Central Statistics Authority, the total population is assumed to be 364,482; however, this figure could in reality be several thousand higher. The urban population

Fig. 1.3 Administrative zones

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comprises only 20 % (Negele Borena is the biggest urban settlement in the area) and the remaining 80 % of the population live in rural areas.

Considering the trends in population growth, access to water will become worse by 2015 in urban areas and 2025 in rural areas, respectively. People in the area could face a water scarcity i.e. less than 1,000 m3/year, and/or even water stress i.e. availability of less than 500 m3/year (Tesfay Tafese, 2001).

The life expectancy at birth is 49 years for males and 51 years for females (WHO, 2006). As in most developing countries, Ethiopia‘s main health problems are communicable diseases caused by poor sanitation and malnutrition. Mortality and morbidity data are based primarily on health facility records which show that the leading causes of hospital deaths are dysentery and gastroenteritis, tuberculosis, pneumonia, malnutrition and anemia, and liver diseases including hepatitis, tetanus, and malaria. The situation is complicated by the fact that Ethiopia’s population mainly lives in rural areas (84 %) where access to healthcare is more complicated than in urban areas.

The country faces chronic problems with malaria (Fig. 1.4) which is endemic over 70 % of the country, and was once a scourge in areas below 1,500 m a.s.l. Practically the entire area of the Negele sheet has a minimal malaria risk. The threat of malaria had declined considerably as a result of government efforts supported by the WHO and AID, but sporadic seasonal outbreaks are common. UNICEF estimated that the number of malaria cases per year is about 9 million and the number of extra cases in an epidemic year is about 6 million. The occurrence of outbreaks is largely a result of heavy rain, unusually high temperatures, and the settling of peasants in new lowland locations. An example of the different diseases in Ethiopia is shown in Tab. 1.2.

Tab. 1.1 Population in the study area

Region Zone Wereda

Werea area in mapped area Total

population

Assessed populationin mapped area[km2] [%]

Oromia Bale Gura Demole 59 1 28,651 287

Oromia Bale Delo Mena 825 16 80,593 12,895

Oromia Bale Meda Welabu 1,361 43 94,826 40,775

Oromia Borena Arero 103 1 45,105 451

Oromia Guji Adola 472 33 113,735 37,533

Oromia Guji Odo Shakiso 1,731 42 175,115 73,548

Oromia Guji Girja 16 2 49,176 984

Oromia Guji Wadera 885 96 45,948 44,110

Oromia Guji Liben 5,473 75 136,311 102,233

Somali Region Liben Filtu 3,066 18 125,952 22,671

Somali Region Afder Qarsasula 4,372 74 39,183 28,995

Total 18,369 364,482

Source: Population by Zone, Central Statistics Authority Statistical Abstract (2007)

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Access to safe drinking water is limited and some statistics suggest that only 15 % of rural inhabitants have access to safe drinking water. The WHO (2006) statistics show that 31 % of the rural population has sustainable access to improved drinking water sources (96 % of the urban population). This low number is alarming because 70 % of contagious diseases are caused by contaminated water. This is a serious problem for Ethiopia in the effort to establish a strong

Fig. 1.4 Malaria risk in Ethiopia

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agricultural community that will be able to safeguard the supply of food for the whole country. One of the priorities of government policy is therefore to provide safe drinking water to rural communities.

The supply of safe water is not equal in all of the Zones of the region. The total number of facilities and the number of inhabitants for a single facility are shown in Tab. 1.3. Preliminarily results of the population and housing census of 2007 show that a particularly large number of

Tab. 1.2 Mortal diseases in Ethiopia (WHO, 2006)

Type of disease Total inhabitants [%] Children under 5

Respiratory 12 22

HIV/AIDS 12 4

Prenatal/neonatal 8 30

Diarrheal 6 17

Tuberculosis 4

Measles 4 4

Cardio-vascular 3

Ischemic heart diseases 3

Malaria/injuries 3 2

Syphilis/others 2 14

Tab. 1.3 Rural water facilities by Zones

Zone/Wereda Number of facilities Number of inhabitants per facility

Bale/Gura Demole 1 bono 28,651

Bale/Delo Mena 2 bono 40,297

Bale/Meda Welabu 5 bono 18,965

Borena/Arero 4 bono, 1 pond 4,100

Guji/Adola 16 bono 7,108

Guji/Odo Shakiso4 bono, 3 tankers, 2 water pump, 2 springs, 1 pond

14,593

Guji/Girja none 49,176

Guji/Wadera 6 bono, 1 pond 6,478

Guji/Liben 33 bono, 2 tankers, 20 ponds 2,478

Liben/Filtu 16 (16 wells) 7,872

Afder/Qarsasula none 39,138

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ponds serving for water supply can provide an adequate volume of water, but do not follow the requirements for safe water supply to inhabitants.

The leading causes of hospital and health center morbidity in 2008/2009 in the Oromia and Somali region are shown in Tab. 1.4a and 1.4b.

Tab. 1.4a Leading causes of hospital and health center morbidity 2008/2009 in Oromia Region

Rank Diagnosis No. of all cases % of all cases

1 Acute upper respiratory tract infection 103,154 5.93

2 Other helminthes 106,620 4.81

3 Other unspecified malaria 103,154 4.65

4 Gastritis and duodenitis 102,252 4.61

5 Homicide and injury purposelyinflicted by another person (not in war)

100,161 4.52

6 All other diseases of gento – urinal system 82,493 3.72

7 Bronchopneumonia 78,189 3.53

8 Infection of skin and subcutaneous tissue 72,881 3.29

9 Muscular rheumatism and rheumatism unspecified 72,868 3.29

10 Pyrexia of unknown origin 70,226 3.17

Total of leading diseases 920,411 41.50

Total of other diseases 1,297,453 58.50

Total of all diseases 2,217,864 100.00

Source: Oromia Regional Health Bureau

Tab. 1.4b Leading causes of hospital and health center morbidity 2008/2009 in Somali Region (Part 1)


1 Gastro – enteritis and colitis 93,380 13.43

2 A.U.R.I 76,449 10.99

3 Other unspecified malaria 74,024 10.64

4 All forms of pneumonia 63,994 9.2

5 Gento – urinal system 54,111 7.78

6 Gastrities and duodenities 52,136 7.5

7 Anemia 34,954 5.03

8 Infection of skin and subcutaneous tissue 31,693 4.56

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Conclusions of a review made by the Ethiopian Health Sector Development Program (HSDP, 2008) show that despite the significant rise in access to water and improved sanitation, there is no data on rates of usage of these services. Ethiopia still suffers from a heavy disease burden that is directly related to poor hygiene practices and sanitation services. Each year, the average Ethiopian child has five to twelve diarrhea episodes and diarrheal illnesses kill between 50,000 to 112,000 children each year. Women and girls are most affected by inadequate sanitation services as they are forced to spend more time fetching water and caring for the sick than participating in income-generating activities or attending school.

During the last few years, there has been an increased level of political commitment to hygiene and environmental health services in Ethiopia leading to the Ministry of Health defining a Hygiene and Environmental Health Program (www.moh.gov.et). The program is based on key policies such as the National Sanitation Strategy and Protocol and the Millennium Sanitation Movement has established a framework that serves to motivate and align relevant actors to speed up sanitation coverage and hygiene behavioral change. In addition, three key ministries–Health, Water Resources and Education–have joined to launch the National WASH program, which provides a strategic framework for achieving a national vision for universal access to hygiene sanitation.

The Ministry of Health has defined the following objectives of the program: • Increase sanitation measures including latrine coverage and ensure facilities are properly

handled, sustained and utilized.• Promote communal solid waste disposal sites, including improvement of medical and other

waste management systems in public and private health institutions.• Increase drinking water quality monitoring; and monitor food safety and food processing

industries.

Health Extension Workers (HWEs) play a significant role in carrying out the key activities of the program throughout communities. HEWs promote personal and environmental hygiene and provide support to the community; increase community awareness and involvement in safe water supply and prevention of water contamination; promote behavioral change to improve food safety and control vector born diseases; build a Healthy House Model and work with the relevant institutions to ensure irrigation development projects and water conservation schemes.

Improving safe water supply to people living in the mapped area basin contributes to an improvement in their health which is one of the fundamental problems for the creation of strong pastoral and farm communities capable of full time engagement in agricultural activity.

Tab. 1.4b Leading causes of hospital and health center morbidity 2008/2009 in Somali Region (Part 2)


9 Parasitic diseases 16,218 2.33

10 Eye diseases 15,580 2.24

Total of leading diseases 512,539 73.7

Total of other diseases 182,865 26.30

Total of all diseases 695,404 100.00

Source: Somali Regional Health Bureau

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1.3 Land UsePoor land use practices, improper management systems and lack of appropriate soil conservation

measures have played a major role in causing land degradation problems in the country. Because of the rugged terrain, the rates of soil erosion and land degradation in Ethiopia are high. Setegn (2010) mentions the soil depth of more than 34 % of the Ethiopian territory is already less than 35 cm, indicating that Ethiopia loses a large volume of fertile soil every year and the degradation of land through soil erosion is increasing at a high rate. The highlands are now so seriously eroded that they will no longer be economically productive in the foreseeable future.

Only a small percentage of the Negele area is classified as intensively and/or moderately cultivated land (Fig. 1.5).

A large part of the area is coverd by bush, wooded or open grassland. Disturbed high forest where coffee is cultivated is found in northwestern highlands. The eastern part of the lowlands is used mainly for pasture and crop cultivation is not common in this area.

Fig. 1.5 Land use

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The land and water resources are in danger due to the rapid growth of the population, deforestation and overgrazing, soil erosion, sediment deposition, storage capacity reduction, drainage and water logging, flooding, and pollutant transport. In recent years, there has been an increased concern over climate change caused by increasing concentrations of CO

2 and other trace gases in the

atmosphere. A major effect of climate change is alterations in the hydrologic cycles and changes in water availability. Increased evaporation combined with changes in precipitation characteristics has the potential to affect runoff, frequency and intensity of floods and droughts, soil moisture, and water supplies for irrigation and generation of hydroelectric power.

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27

The entire study area is located on the eastern shoulder of the southeastern plateau Ethiopian with a general slope to the southeast. The area is predominantly composed of plain and hill domes with summits not greater than 2,000 m a.s.l. (Fig 2.1). The highlands cover the western part of the area and the lowland plains with elevations from 1,100 to 1,500 m a.s.l. are located in the centre and eastern part of the area together with deep valleys of Genale, Welmel and Dumel rivers which have altitudes of below 500 m.

The most distinct physiographic units are as follows:• River gorges–Genale, Dumel and Welmel• Plains–flat plains and gentle slopes east of Negele town (sedimentary rocks)• Highlands–undulating highlands west of Negele town (basement rocks)

2. Selected Physical and Geographical Settings2.

Fig. 2.1 Generalized physiographic units

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Selected Physical and Geographical Settings

28 Selected Physical and Geographical Settings

2.1 GeomorphologyThe geomorphology of the area is variable, and it is generally the result of repeated tectonic

events with the associated erosion of Mesozoic sedimentary, crystalline and igneous rocks and deposition processes. The tectonic activity and lithological variation in the area also partly or wholly control the drainage density and drainage pattern. Most of the river channels follow the young lineaments. The maximum elevation is 2,237 m a.s.l. (no name peak) in the northwestern corner of the Negele sheet and a minimum elevation of 400 m on the bank of the Genale River in the eastern limit of the map sheet. The average elevation is 1,173 m a.s.l.

One of the most important phenomena of the area results from the general slope of the eastern Ethiopian plateau to the southeast, which is a result of global tectonics and influences the recent direction of surface as well as groundwater flow.

The vast occurrence of sedimentary and basement rocks, together with erosion due to the steep terrain and large rivers and streams all play a major role in shaping the present topographic setting of the area. The map area lies at the foot of the south- and southeastward descending slope of the southeast Ethiopian plateau. The majority of the area is characterized by flat to rolling topography. The discontinuous N-S and NW-SE trending chain of ridges formed of Precambrian basement

rocks (often sheared) and erosional remnants of Mesozoic sedimentary rocks are conspicuous topographic features (Tadesse et al., 1998). Moreover, in the western part of Negele map sheet, interspersed hills formed of more resistant basement rocks and granitic intrusions rise from the surrounding gently undulating plain, whereas, a locally incised, tableland topographic configuration

Fig. 2.2 Gorge of the Genale River in Donto village

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29Selected Physical and Geographical Settings

formed of Mesozoic sedimentary rocks becomes a prominent feature to the east. Deep gorges and valleys are typical characteristics of the major rivers and their immediate tributaries.

The gorges are developed along the Genale, Dumal and Welmel rivers (Fig.2.2).

2.2 Soil and Vegetation CoverSoil and vegetation cover reflects the basic climatic condition of the area as well as the regional

and site specific geological, geomorphological and erosion characteristics.

Soil

Soil stores rainwater in its pores before it infiltrates to greater depths and recharges the aquifer system. Water stored in upper layers evaporates directly. Soil water that is stored in deeper layers is absorbed by vegetation roots then transpires to leaves where it is evaporated. The amount of evapotranspiration from soil is controlled by soil attributes such as soil texture, soil structure and soil moisture content therefore the ability of soil to store and transport water is different for every soil type. Deeper soil has a larger soil moisture reserve than thinner soil, which can supply more water to evaporate. Therefore, soils of the same hydrologic conditions are grouped together for the purpose of soil water balance. Different groups of soil are formed in the Negele sheet. The development of soils depends primarily on geological and climatic conditions. The soils were formed from different types of rocks and occurrence is restricted to these parent rocks and along

Fig. 2.3 Distribution of soil types

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the transporting agents. The hydrology of the soil is dependent on the texture of the rocks and the degree of weathering. Soils derived from coarse grained rocks inherit a coarse texture, whereas those derived from fine grained rocks are characterized by a fine texture. They are variable in spatial distribution even under the same climate zone. The soil classes used for the soil water balance and groundwater recharge evaluation are based on the hydrological property of the FAO classification of soil (FAO, 1977).

According to the soil map provided by the Ministry of Agriculture, the study area is mainly covered with five major types of soil classes. These are Cambisols, Rendzinas Lithosols and Vertisols. Distribution of soil types is shown in Fig. 2.3.

Cambisols Most Cambisols are medium-grained and have a good structural stability, a high porosity,

and good water holding capacity and good internal drainage. Most Cambisols also contain at least some weatherable minerals in the silt and sand fractions. Based on these characteristics, Cambisols have a good infiltration capacity to recharge groundwater. Hydrologically, chromic Cambisols are more permeable than Eutric Cambisols.

RendzinasIt is dark, grayish-brown and humus rich. It is one of the soils most closely associated with the

bedrock type and an example of the initial stages of soil development. The soil of this type contains a significant amount of gravel and stones. It is usually developed beneath grassland formed by weathering of soft rock types: usually carbonate rocks (dolomite, limestone, marl, chalk) but occasionally sulfate rocks (gypsum).

Fig. 2.4 Acacia trees

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LithosolsLithosols are mineral soils less than 10 cm thick, developed over hard rock. These soils have

no agricultural value. They are often referred to as “skeletal soils“ because of their extreme shallowness and steepness and consequently their high erosion hazard.

LuvisolsLuvisols have a distinct clay accumulation horizon. Most Luvisols are well-drained but Luvisols

in depression areas with shallow groundwater may develop gleyic soil properties in and below the argic horizon. Stagnant properties are found where a dense illuvial horizon obstructs downward percolation and the surface soil becomes saturated with water for extended periods of time.

VertisolsVertisols are commonly known as black cotton soil containing smectite clay characterized by

their sticky nature and high water holding capacity and low infiltration capacity. Vertisols become very hard in the dry season and are sticky in the wet season.

Vegetation

The vegetations of Negele map sheet varies from small to large trees. The common land covers are shrub and bush, forest, woodland, grassland and cultivated land. The forest coverage is restricted to sub-humid climates and along river courses. One of the parameters that influences the occurrence of sub-surface groundwater is the land cover and land use of the area. The effect of land use/cover is manifested either by reducing runoff or by trapping water on leaves. Water droplets trapped in this way go down to recharge groundwater. Land use/cover may also negatively affect groundwater by evapotranspiration, assuming interception to be constant. Land cover/use is classified according to root depth when estimating the soil water balance.

2.3 Climatic CharacteristicsThe area is mainly characterized by an arid and semiarid climate in which the rainy season

passes from March to May and from October to November. The mean annual rainfall is between 300 mm in the southeastern lowlands and 1,000 mm in the northwestern highlands based on rainfall assessment within the Genale-Dawa Basin. The mean maximum annual temperature is 22 °C and the mean minimum annual temperature is 19 °C based on temperature–elevation relationship for the Genale-Dawa basin.

2.3.1 Climatic Zones and MeasurementsThe climatic conditions of Ethiopia are mostly dominated by altitude. According to Daniel

Gamatchu (1977) there are wide varieties in climatic zones. Climatic zones defined by Javier Gozálbez and Dulce Cebrián (2006) and Tesfaye Chernet (1993) are shown in Tab. 2.1.

A climatic zoning map (Fig. 2.5) has been compiled based on the climatic region classifications given in Tab. 2.1 and the elevation of the study area. A small area in the western corner covering 5 % of the area lies in the Weina Dega (subtropical) zone and covers the summit of the map sheet, 93 % of the area in the western and central part of the lowland area is in the Kolla (tropical) region, and 2 % is in the Bereha (semi-desert) region along the Genale, Dumel and Welmel rivers.

The outstanding modern quantitative climatic classification of Koeppen (1989) defines the climatic types according to the values of temperature and precipitation regardless of the geographic location of the region. Criteria for classification of the principal climatic types in a modified Koppen system are based on the mean annual and mean monthly precipitation and temperature values. The actual application of the Koeppen system to climatological statistics shows that the Ethiopian

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climate is grouped into three main categories, each divided into three or more types making a total of 11 principal climatic types.

The highlands of the northwestern part of the Negele sheet belong to the Aws zone–characterized by a tropical climate. The mean temperature of the coldest month is above 18° C and the mean annual rainfall is 680 –1,200 mm. This type of climate prevails up to an elevation of 1,750 m a.s.l. This climate is characterized by tall grass and usually grass and trees are intermingled.

The lowlands of the southeastern part of the Negele sheet belong to the Bsh zone–characterized by a hot semi-arid climate. The zone is characteristic by mean annual temperatures from 18 °C to 27 °C. Precipitation is highly variable from year to year with a men annual precipitation of 410 –820 mm. The vegetation is of steppe type.

Tab. 2.1 Ethiopian climate classification

Name / Altitude / Mean annual temperature

Precipitationbelow 900 mm

Precipitationbetween 900 and 1,400 mm

Precipitationabove 1,400 mm

High Wurch (Kur)above 3,700 mbelow 5 oC

Afro-alpinemeadows of grazing land and steppes, no farmingHelichrysum, Lobelia

Wurch (Kur)3,700–3,200 m 5–10 oC

Sub-afroalpine barleyErica, Hypericum

Sub-afroalpine barleyErica, Hypericum

Dega3,200–2,300 m10–15 oC

Afro-mountain (tempe-rate)forest – woodlandbarley, wheat, pulsesJuniperus, Hagenia, Pod-ocarpus

Afro-mountain (temperate)bamboo forestbarley, wheat, nug, pulsesJuniperus, Hagenia, Podo-carpu, bamboo

Weina Dega2,300–1,500 m15–20 oC

Savannah (sub-tropical)wheat, teff, some cornacacia savannah

Shrub-savannah(sub-tropical)corn, sorghum, teff, en-set, nug, wheat, barleyAcacia, Cordia, Ficus

Wooded savannah(sub-tropical)corn, teff, nug, enset, barleyAcacia, Cordia, Ficus, bam-boo

Kolla1,500–500 m above 30 oC

Tropicalsorghum and teffacacia bushes

Tropicalsorghum, teff, nug, pea-nutsAcacia, Cordia, Ficus

Wet tropicalmango, sugar cane, corn, coffee, orangesCyathea, Albizia

Berehabelow 500 mabove 40 oC

Semi-desertand desertcrops only with irri-gationthorny acacias,Commiphora

Remark: after Javier Gozálbez and Dulce Cebrián (2006), Tesfaye Chernet (1993)

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Tab 2.2 Climatic stations in the Negele area (Part 1)

Fig. 2.5 Climatic zones

Map ID Station Class X UTM Y UTMAltitude [m a.s.l.]

Data type Sub-basin

RF6 Adola 1 497538 650322 1,680 P, T, H Dawa

RF7 Negele Borena 1 563910 589736 1,425P, T, E ,H, S, W

Genale

RF21 Wadera 4 533821 638600 1,900 P Dawa

RF26 Chenamasa 3 559375 630690 1,250 P Genale

RF29 Siru 4 645063 572902 1,150 P Genale

RF39 Zenbaba Wuha 4 518520 651300 1,820 P Dawa

RF43 Bittata 4 552143 605716 1,500 P Dawa

RF44 Hare Kelo 3 542990 614068 1,600 P Dawa

RF46 Bidre 3 571283 653535 1,620 P Genale

RF48 Genale Donta 3 559201 630819 1,150 P Genale

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The bottom of the river valleys belongs to Bwh zone–characterized by a hot arid climate. The zone is characterized by mean annual temperatures from 27 °C to 30 °C. Precipitation is below 450 mm/year. The area is characterized by strong winds and little cloud cover. Evapotranspiration is twenty or more times in excess of precipitation and the area is barren with little vegetation.

There are 12 meteorological stations operated by the Meteorological Institute within the mapped area and several others in near surroundings. The station is located in Negele town provides basic meteorological characteristics. Stations in Kibre Mengist and Filtu are located nearby and represent 1st class stations in mountain and lowland areas, respectively and their measurements are used for comparison.

The air temperature shows small seasonal changes with an annual average of 19.3 °C. The minimum and maximum temperature ranges from 10.4 °C to 29.4 °C. Fig. 2.6 confirms that the seasonal variation of temperature is more or less constant through the year. The mean temperature recorded in the major stations is relatively similar. The highest mean temperature is recorded in the Genale Donta and Filtu stations and the minimum temperature is recorded in the Kibremengist station. The hot months are from December to March. Temperatures in June and July are relatively low. Temperatures are high in the southern and southeastern part of the sub-basin, whereas low

Tab 2.2 Climatic stations in the Negele area (Part 2)

Map ID Station Class X UTM Y UTMAltitude [m a.s.l.]

Data type Sub-basin

RF82 Haidimtu 4 661441 569740 1,300 P Genale

RF89 Welinso 3 588579 635573 1,505 P Genale

RF23 Filtu* 1 683757 565287 1,159P, T, E ,H, S, W

Genale

RF30 Kibre Mengist** 1 498470 649433 1,680 P, E , H, S Dawa

Remark: * Agere Mariam sheet, ** Filtu sheet

Fig. 2.6 Temperature at Negele meteo-station

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temperatures are observed in the northwestern part of the Negele map sheet. Temperatures are usually the most important factor for evapotranspiration. Evaporation will continue to increase at an increased rate as the temperature rises as long as there is water to evaporate. The temperature correlates negatively with elevation.

The number of sunshine hours is one of the characteristics that affects the rate of evaporation. The maximum number of sunshine hours is recorded in the Filtu station (southeast of the Negele sheet). The minimum number of sunshine hours is observed in the Kibremengist station (northwest of the Negele sheet). The number of sunshine hours decreases from March to June and rises from mid June to August.

Relative humidity and evapotranspiration are closely related because if the relative humidity is close to its holding capacity, the ability of plants to transpire may be inhibited. The higher the

Fig. 2.7 Monthly no. of sunshine hours at Kibremengist and Negele Borena stations

Negele Borena

Kibremengist

9876543210

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Fig. 2.8 Mean monthly potential evapotranspiration [mm], temperature [°C] and relative humidity [%]

RH

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relative humidity, the lower the evaporation rate; the drier the air above the surface, the faster the evaporation. So the seasonal trend of evapotranspiration within a given climatic region follows the seasonal trend of solar radiation and air temperature. Minimum evapotranspiration rates generally occur during the coldest months of the year; maximum rates generally coincide with the winter season when water may be in short supply.

Wind speed plays a role in controlling the evapotranspiration rates by influencing the moisture gradient. Evaporation has a direct relation with the wind speed. As water vaporizes into the atmosphere, the boundary layer between the earth and the air becomes more and more saturated so that the water vapor has to be continuously removed and replaced with drier air. The wind speed plotted in Fig. 2.9 relatively confirms the direct influence of evaporation when water is available. The potential evaporation at Filtu is the largest compared to the other stations. Moisture-laden air is forced to rise over a mountain barrier, producing more rainfall on the windward side than on the leeward side.

2.3.2 Precipitation The Ethiopian territory is divided into four zones marked as A, B, C, and D, each of them with

different precipitation patterns. The seasonal classification and precipitation regimes of Ethiopia (after NMSA, 1996) are characterized in Tab. 2.3 and shown in Fig. 2.10.

Fig. 2.9 Monthly wind speed [km/d] of selected stations

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Negele Borena

Filtu

Tab. 2.3 Characterization of the precipitation pattern in Ethiopia (Part 1)

Zone Precipitation pattern

A

This region mainly covers the central and central eastern part of the country. It is characterized by three distinct seasons, and by bimodal precipitation patterns with small peaks in April and the main rainy season during mid June to mid September with peaks in July.

B

This region covers the western part of the country. It is characterized by a single pre-cipitation peak. Two distinct seasons, one being wet and the other dry, are encoun-tered in this region. The analysis of mean monthly precipitation patterns shows that this zone can be split into southwestern (b1) with the wet season during February/March to October/November, western (b2) with the wet season during April/May to October/November, and northwestern (b3) with the wet season during June to September parts.

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Zone Precipitation pattern

CThis region mainly covers the southern and southeastern parts of the country. It has two distinct precipitation peaks with a dry season between. The first wet season is from March to May and the second is from September to November.

DThe Red Sea region in the extreme northeastern part of the country receives diffused precipitation with no distinct pattern; however precipitation occurs mainly during the winter.

Tab. 2.3 Characterization of the precipitation pattern in Ethiopia (Part 2)

Fig. 2.10 Seasonal classification and precipitation regimes of Ethiopia (source: NMSA, 1996)

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The mapped area belongs to region C which is characterized by four distinct seasons and by bimodal precipitation patterns with peaks in April and October. Hence, region C is similar to region A. In general the annual rainfall depends on the regional altitude variation of the area and precipitation decreases from west to east. The highlands in the west receive mean annual precipitation above 700 mm/year. The mean annual rainfall is less than 400 mm/year, for the arid regions in the Ogaden. These low precipitation regions have higher intensity of precipitation than those areas which have a higher amount of annual precipitation. The intensity of precipitation of more than 100 mm a day in the lowlands and less than 50 mm a day in the highlands are common.

Basic precipitation data from Negele, Siru and Zembaba Wiha (Tab. 2.4) meteo-station represents the typical precipitation pattern the region. A graphical presentation of precipitation pattern is shown in Fig. 2.11–2.13.

Years with a full set of data were extracted from the review of data from the period from 1953 to 2000. Long-term precipitation data is given in Tab. 2.5 and Fig. 2.14. The long-term average annual precipitation from Negele meteo-station is 738 mm for the assessed 41 years.

Years with a full set of data were extracted from the review of data from the period from 1973 to 2000 also for the Siru station which is the most eastern located station of the Negele sheet. Long-term data are represented by the Fitu station located in the Filtu sheet about 45 km east of

Tab. 2.4 Monthly long-term average precipitation in selected meteo-stations of the Negele sheet [mm]

St /M Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Negele 11.6 23.9 62.8 204.6 152.2 10.7 7.3 4.90 38.1 161.3 47.7 10.90

Siru 1.5 9.6 44.3 96.3 61.1 0.0 0.0 0.03 19.6 95.3 7.0 2.10

Zembaba 18.2 24.8 40.0 144.4 132.1 40.8 15.7 21.00 69.2 102.4 48.5 22.14

Fig. 2.11 The Negele meteo-station precipitation pattern

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Siru. Precipitation data is given in Tab. 2.6 and Fig. 2.15. The 3 years average annual precipitation from Siru meteo-station is 311 mm.

Years with a full set of data were extracted from the review of data from the period from 1980 to 1997 for the Zembaba Wiha station. Long-term precipitation data is given in Tab. 2.7 and Fig. 2.16. The long-term average annual precipitation from Zembaba Wiha meteo-station is 725 mm for the assessed 13 years.

Fig. 2.12 The Siru meteo-station precipitation pattern

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Fig. 2.13 The Zembaba Wiha meteo-station precipitation pattern

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Tab. 2.5 Long-term monthly rainfall at Negele [mm] (fully recorded years only) (Part 1)

Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total

1953 0.9 6.9 9.0 225.4 100.6 0.0 2.0 28.4 52.5 193.5 23.1 0.0 642.3

1954 0.0 3.2 46.5 116.5 155.8 0.0 1.8 7.3 1.8 64.1 0.0 1.3 398.3

1955 6.5 0.0 0.0 243.8 37.9 0.0 0.0 0.0 27.7 177.6 62.5 21.4 577.4

1956 0.0 1.0 0.0 145.0 59.3 0.0 0.0 0.0 5.4 81.7 0.0 0.0 292.4

1957 0.0 8.5 37.2 122.6 203.6 27.9 7.2 2.0 55.4 91.0 70.6 46.0 672.0

1958 7.4 25.3 25.0 183.8 43.5 2.3 32.7 0.1 15.0 119.9 21.8 5.7 482.5

1959 69.4 0.5 7.0 167.9 49.8 7.0 12.8 9.0 8.9 227.6 0.0 0.0 559.9

1960 0.0 0.0 166.5 87.9 151.9 0.5 6.7 2.0 7.0 131.4 20.9 18.4 593.2

1964 2.3 0.0 26.8 241.1 72.8 45.6 5.0 14.5 19.9 98.0 11.4 70.8 608.2

1965 0.0 0.0 29.9 181.4 156.4 8.4 1.5 7.4 26.3 312.6 199.3 0.0 923.2

1966 0.0 58.3 145.7 360.8 161.2 23.0 0.0 0.0 39.7 151.8 42.1 0.0 982.6

1967 0.0 0.0 79.2 240.4 333.0 10.3 18.0 5.0 18.6 243.6 115.0 0.0 1,063.1

1968 0.0 154.2 116.5 258.9 198.3 21.7 22.1 0.5 4.0 124.0 36.0 1.0 937.2

1969 7.8 15.1 34.7 190.5 98.6 0.0 0.0 0.0 24.7 396.7 28.0 12.0 808.1

1970 213.7 23.5 138 155.5 161.8 0.0 3.0 3.1 21.0 241.1 2.7 0.0 963.4

1971 4.6 0.0 28.5 251.3 157.5 6.0 0.0 0.0 20.2 235.0 57.9 0.0 761.0

1972 1.9 230.7 2.9 365.7 401.9 21.3 0.0 8.0 54.3 227.0 50.0 18.0 1,381.7

1973 0.0 22.4 0.0 161.0 167.8 5.5 8.6 2.2 90.6 154.4 13.3 0.0 625.8

1974 0.0 0.0 146.8 132.1 268.2 22.2 7.6 30.8 14.9 187 9.4 4.6 823.6

1975 44.3 0.0 14.6 222.5 181.2 13.2 9.2 9.6 80.5 177.1 20.0 0.0 772.2

1976 0.0 2.7 14.8 200.2 421.5 0.1 3.0 0.5 116.0 158.2 117.0 1.5 1,035.2

1978 5.0 180.6 94.5 22.5 143.0 9.7 19.9 0.0 87.0 291.8 83.1 1.5 938.6

1979 48.3 9.7 55.5 264.9 203.0 36.6 1.2 2.0 9.4 123 3.5 9.1 766.2

1980 0.0 0.0 0.0 280.9 349.0 0.0 2.8 15.6 76.1 106.8 69.9 0.0 901.1

1981 0.0 24.8 519.9 368.5 68.6 0.0 3.5 8.4 59.4 220.5 35.0 0.0 1,308.6

1982 0.0 57.2 29.2 323.8 247.0 0.0 2.2 0.0 26.7 233.6 85.0 26.2 1,030.9

1983 0.0 28.1 3.5 157.5 123.3 32.9 3.1 26.5 31.8 128.0 166.5 0.0 701.2

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1984 0.0 0.0 8.8 176.4 121.1 0.0 0.9 0.0 65.5 99.0 27.4 23.3 522.4

1985 0.1 0.0 71.7 192.0 116.8 1.7 0.3 2.5 26.0 168.0 68.3 10.9 658.3

1986 0.0 0.0 16.9 379.3 120.9 39.6 3.0 0.1 63.6 46.7 43.5 4.2 717.8

1987 0.0 14.9 80.3 175.0 296.0 22.0 0.0 1.3 19.0 107.0 98.0 0.0 813.5

1988 0.0 2.6 33.3 295.6 51.8 0.0 7.7 0.0 15.3 119.8 0.0 2.3 528.4

1989 12.4 0.0 202.0 160.0 88.6 0.0 22.8 0.0 58.6 132.0 12.6 12.2 701.2

1993 9.3 61.2 2.2 244.5 191.2 8.8 6.6 0.0 0.0 135.8 3.0 5.9 668.5

1994 0.0 0.0 30.7 146.7 207.6 1.4 16.8 2.2 48.1 131.0 30.3 8.6 623.4

1995 0.0 24.4 98.1 289.8 52.5 10.7 9.1 5.9 54.1 145.6 48.7 0.0 738.9

1996 1.8 0.8 146.8 274.5 75.8 7.5 4.4 0.1 26,2 182.0 54.4 0.6 774.9

1997 0.2 0.0 33.9 96.3 42.2 43.6 3.3 0.0 67.3 203.8 106.1 27.3 624.0

1998 60.7 17.5 11.5 114.0 130.3 22.8 13.4 5.1 1.4 68.4 20.0 6.5 471.6

1999 0.0 0.0 105.5 27.1 64.7 2.0 2.6 1.1 6.6 135.5 12.0 1.7 358.8

2000 0.8 0.0 0.0 105.1 134.8 0.3 2.2 6.0 4.5 169.4 66.1 23.1 512.3

Tab. 2.5 Long-term monthly rainfall at Negele [mm] (fully recorded years only) (Part 2)

Fig. 2.14 Long-term fluctuation and average of precipitation for the Negele meteo-station

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Tab. 2.6 Monthly rainfall at Siru [mm] (fully recorded years only)


1974 0.0 0.0 73.5 69.0 72.0 0.0 0.0 0.0 0.4 33.1 0.0 2.4 250.4

1975 0.0 0.0 0.0 50.7 56.0 0.0 0.0 0.0 6.0 54.1 10.6 0.0 177.4

1976 0.0 0.0 17.9 161.5 113.8 0.0 0.0 0.0 83.8 114.2 6.1 8.0 505.3

Fig. 2.15 Fluctuation and average of precipitation for the Siru meteo-station

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Tab. 2.7 Long-term monthly rainfall at Zembaba Wiha [mm] (fully recorded years only) (Part 1)


1980 0.0 0.0 13.3 91.5 240.4 19.4 0.0 7.1 36.0 116.3 0.0 0.0 524.0

1981 1.9 45.4 23.7 332.1 113.9 9.9 9.9 29.3 69.5 64.4 11.0 0.0 711.0

1983 27.0 45.5 6.0 68.8 44.7 44.4 0.0 15.0 47.2 136.4 108.0 57.4 600.4

1985 5.5 0.0 43.7 370.6 400.6 21.7 15.0 7.9 92.9 113.3 22.2 5.0 1,098.4

1986 0.0 6.6 40.4 251.9 190.1 89.3 26.3 37.5 116.0 239.4 20.9 19.4 1,037.9

1987 11.6 9.0 94.0 122.2 330.4 54.6 19.9 19.9 48.2 54.8 8.3 13.5 786.4

1988 0.0 11.4 9.0 69.1 52.7 86.0 51.9 35.0 62.7 97.1 20.3 12.0 507.2

1989 0.0 21.5 0.0 50.0 78.7 12.8 24.6 15.3 93.4 113.9 113.1 142.0 665.3

1993 70.4 61.3 3.4 107.2 54.7 6.3 0.5 3.7 0.0 28.8 85.8 14.7 436.8

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The graphs show the high fluctuations in precipitation. Differences in precipitation can exceed 100 % in some years.

The Tyson polygon method was used for assessment of average precipitation for the Negele area. The annual precipitation of the area is 679 mm.

2.4 Hydrography and Hydrology of the AreaThe Negele area is found within the Genale-Dawa basin. It is known as the third biggest basin

in Ethiopia that has a size of about 172,880 km2. The basin has a relatively low runoff with a mean flow of 125 m3/s. The minimum flow of the Genale and Dawa is in the period from December to March and the maximum flow is in period of August to November. This is due to the dominant precipitation pattern and arid character of the climate. The general trend of drainage in the area is from the elevated northwestern mountainous area to the southeastern lowland plain area of the Ogaden basin and the Indian Ocean. The principal river basins of the area are shown in Fig. 2.17.


1994 0.0 0.0 0.0 22.1 47.8 81.7 8.1 2.1 15.4 38.7 27.6 15.1 258.6

1995 0.0 47.5 122.7 224.1 64.2 63.9 33.9 82.3 229 69.1 5.2 0.0 942.1

1996 0.9 0.0 195.4 224.9 239.6 81.8 0.0 17.0 53.3 98.5 59.2 2.9 973.5

1997 0.7 0.0 45.5 200.3 37.5 14.2 38.1 32.6 90.8 188.7 199.9 40.9 889.2

Tab. 2.7 Long-term monthly rainfall at Zembaba Wiha [mm] (fully recorded years only) (Part 2)

Fig. 2.16 Fluctuation and average of precipitation for the Zembaba Wiha meteo-station

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2.4.1 Surface Water Network DevelopmentThe drainage pattern in the Negele sheet is mainly characterized by a dendritic to sub parallel

pattern and is controlled by structures. The drainage density depends on the slope, nature and attitude of bedrocks and the existing regional and local fracture patterns. They reflect the lithology and structure of a given area and can be of great value for groundwater resource evaluation. It is regarded as an important landscape characteristic. Parallel types of drainage patterns are indicative of the presence of structures that act as conduits or storage for sub-surface water. The dendritic drainage pattern is the manifestation of lithological and topographic homogeneity. It is a measure of how dissected a basin is, and it is expected that drainage density affects the transformation of rainfall into runoff (Reddy, 2001). Many studies have integrated lineaments and drainage maps to infer the groundwater recharge potential zone (Edet et al., 1998; Shaban et al., 2006).

Four major perennial rivers flow across Negele map sheet. The Genale River is the biggest river in the study area which starts from the adjacent southern Aresi–Bale highlands and flows across the map sheet in the northwest to southeast direction toward the lowland. The Genale River drains volcanic rocks at its source and passes across basement and sedimentary rocks. It formed deep gorge and valley along its way downstream. In the Negele map sheet it fully penetrates sedimentary rocks and flows over the basement rocks. Genale is used for water supply for Filtu town and surrounding villages, respectively. The Welmel River also starts from the highlands of Bale and drains across Negele sheet. It formed deep valley in limestone unit in the north east of the study area. The Awata River flows to the study area from northwest to south east and bends to south direction probably due to the Wadera belt structural background. The Mormora River flows to the Negele sheet in the south direction. It drains the adjacent highlands and escarpment in the western part of the area. It starts from volcanic units and drains across the basement rocks.

Fig. 2.17 The principal river basins of the area

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2.4.2 Surface Water RegimeThere are about 38 river gauging stations within the Genale-Dawa basin. Some of them are

operational but lots of stations have no data. In the Negele sheet there are three registered gauging stations. Unfortunately, only data from RG 11 (Genale–Chenemasa) exists and can be used for assessment of surface and baseflow. Data on other rivers were also calculated for assessment of surface as well as baseflow. The selected river stations are summarized in Tab. 2.8.

Records from all stations reflect the fact that the river discharge is directly proportional to the intensity of rainfall within the basin. There is a high discharge fluctuation between the wet and dry seasons of the year. The highest flow period is from June to October and the peak flow for all rivers is usually recorded in August. Mean monthly flow of the Genale River at Girja, Chenamasa and Halowey gauging stations is shown in Fig. 2.18.

Measured discharge of the Genale River at the Chenemasa river gauging station in the period from 1975 to 2005 is shown in Fig. 2.19. The figure shows that flow is relative regular, however its total value of annual flow and particularly maximal monthly flow can vary within years substantially. The

Tab. 2.8 Data on the river gauging stations

Map ID River Station X UTM Y UTMAltitude[m a.s.l.]

Area[km2]

Sub-basin

Map sheet

RG4 Awata Shakiso 492867 639307 1,640 1,624.1 DawaAgereMariam

RG8 Bura Dera Negele 559406 578429 1,315 72.5 Dawa Negele

RG6 DawaDawaDigati

476711 588603 1,150 2,375.7 DawaAgereMariam

RG1 DawaMelkaGuba

535132 537536 750 20,097.9 Dawa Wachile

RG2 Dawa Siftu 818049 438774 200 48,495.9 DawaSheetborder

RG28 Dumel Dildila 629019 744377 1,160 207.5 Genale Dodola

RG27 Genale Up. Girja 494472 685417 1,360 3,177.4 Genale Dilla

RG11 Genale Chenemasa 559250 630852 1,120 9,190.3 Genale Negele

RG19 Genale Kole bridge 812770 490749 198 56,135.5 Genale Sede

RG10 Genale Halowey 821046 481599 195 56,582.9 Genale Sede

RG21 GenaleWeldia/Donto

173849 472191 181 82,027.0 Genale Dolo

RG5 Mormora Megado 478289 627275 1,660 1,321.3 DawaAgereMariam

RG7 Negele Cr. Negele 561684 590426 1,425 10.0 Dawa Negele

RG16 WelmelMelka Amana

586880 689892 1,060 1,395.8 Genale Dodola

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lowest daily discharge of 0.007 m3/s (14.2.1991) and the highest daily discharge 924.795 m3/s (26.11.1997) were recorded at the river gauging station. The calculated mean annual flow of 92.12 m3/s represents flow generated mainly in the eastern highlands where the Genale River rises (originates) and which receives the highest precipitation within the catchment.

The annual variability of the mean annual flow of the Genale River at the Chenamasa river gauging station is shown in Fig. 2.20.

The assessment of specific runoff is based on data from flow measurements and calculated specific runoff in the gauging stations is shown in Tab. 2.9 and the appropriate area of the pertinent river basin within the Negele sheet considering the altitude and rock composition of the area. The specific runoff for Genale-Dawa is 0.72 l/km2. The specific runoff is assessed for basaltic rocks basement and sedimentary rocks separately. The specific runoff for volcanic rocks was assessed to be 13.2 l/s.km2 based on data from the Awata, Upper Genale, Genale Chenemasa, Mormora and Welmel river gauging stations. The specific runoff for sedimentary rocks was assessed to be 0.14 l/s.km2 based on the difference between river gauging stations at the Genala Kola bridge and Halowey. The small value of specific runoff is given mainly by the arid character of the area covered with sedimentary rocks in the lower reaches of the Genale River. The specific runoff for basement rocks was assessed to be 1 l/s.km2 based on data from the Dawa River at Digati, Melka Guba and Shiftu river gauging stations. This assessment is a general one and specific runoff will be highly variable based on location of basement rocks.

Tab. 2.9 Runoff data

Map ID

River StationMean flow [m3/s]

Annual flow[mm]

Area [km2]

Specific runoff[l/s.km2]

Sub-basin

Aquifer

RG4 Awata Shakiso 16.56 321.8 1,624.1 10.20 DawaBasalt/basement

RG8 Bura Dera Negele No data 72.5 Dawa Limestone

RG6 Dawa Dawa Digati 2.54 33.7 2,375.7 1.07 DawaBasal/basement

RG1 Dawa Melka Guba 27.67 43.4 20,097.9 1.38 Dawa Basement

RG2 Dawa Siftu 25.61 16.7 48,495.9 0.53 DawaBasement/limestone

RG27 Genale Up. Girja 74.10 736.0 3,177.4 23.31 Genale Basalt

RG11 Genale Chenemasa 92.12 152.7 9,190.3 10.02 GenaleBasalt/basement

RG19 Genale Kole bridge 148.22 41.1 56,135.5 2.64 Genale Sediment

RG10 Genale Halowey 157.52 43.6 56,582.9 2.78 Genale Sediment

RG5 Mormora Megado 13.09 312.7 1,321.3 9.91 DawaBasalt/basement

RG7 Negele Cr. Negele No data 10.0 Dawa Limestone

RG16 Welmel Melka Amana 17.34 391.1 1,395.8 12.42 Genale Basalt

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Evaporation and usage of surface water for water supply and irrigation has not been considered in the assessment.

2.4.3 BaseflowThe same gauging stations were used for calculation of baseflow, because these stations have

provided flow data for several years.

Fig. 2.18 Mean monthly flow of the Genale River at Halowey, Chenemasa and Girja gauging stations [m3/s]

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Fig. 2.19 Flow diagram of the Genale River at the Chenamasa river gauging station

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1975 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007

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Baseflow represents one of the most important types of information on groundwater resources in the basin. The methods were analyzed by Bogena et al. (2005) and it was found by means of a correlation analysis that the appropriate baseflow values can be determined on the basis of daily river discharge data. The baseflow can be identified from a series of observed monthly low-water runoff values (MoLR) as the simplest assessment method. It has been shown that a long-term average of MoLR of a 20-year period is a good approximation for groundwater recharge in unconsolidated rock areas. However, in consolidated rock areas the MoLR values are often affected by interflow leading to a significant overestimation of groundwater recharge. Hence, a more sophisticated hydrograph separation method based on the Kille method is recommended in these areas.

The Kille method (see Fig. 2.21) for calculation of baseflow was used in the study together with separation of hydrographs where baseflow data is deduced from the discharge record of a stream by separating the baseflow component from the total discharge.

The application of the method can be summarized as follows:1. For each month in a year the minimum daily discharge rate (Q in m3/s) was selected. In

total, the number of Q values is n = 12 × length of the record set in years.2. Sort the n rates into ascending order and plot them against the corresponding orders (i). In

general, a subset of points of low discharge in the scatter plot fits on a straight line.3. The linear zone of the distribution curve represents the baseflow. The MoLR is calculated by

means of the gradient m, the number of values n and the axis intercept y0: MoLR = m × n / 2 + y0. If the hydrographic basin is closed (i.e. there is no water flowing in/out from/to an adjacent basin) and the aquifer is in steady state with respect to storage on an annual basis, then the average groundwater recharge rate R = MoLR.

Fig. 2.20 An annual variability of the mean annual flow of Genale River at Chenamasa river gauging station

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2004

2005

Avg

aver

age

disc

harg

e [m

3 /s]

year

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4. Convert R into a value in mm/y, i.e. multiply the value in m3/s by 60 × 60 × 24 × 365 ×× 1,000 and subsequently divide the result by the drainage area of the basin in m2.

Data on baseflow assessed by the Kille method is shown in Fig. 2.22 and in Tab. 2.10 together with baseflow data assessed by the hydrograph separation method.

Separation of the hydrograph (see Fig. 2.23) is another method that was used for assessment of baseflow. Baseflow separation techniques use the time-series record of stream flow to derive the baseflow signature. The common separation methods are either graphical which tend to focus on defining the points where baseflow intersects the rising and falling limbs of the quickflow response, or involve filtering where data processing of the entire stream hydrograph derives a baseflow hydrograph.

The graphical method was used for assessment of baseflow for rivers of the area. The daily flow data were used to plot the baseflow component of a flood hydrograph event, including the point where the baseflow intersects the falling limb. Stream flow subsequent to this point was assumed to be entirely baseflow, until the start of the hydrographic response to the next significant rainfall event. These graphical approaches (Fig. 2.23) to partitioning baseflow vary in complexity and include (Linsley, 1958):

a) the constant discharge method (green line on the chart) assuming that baseflow is constant during the storm hydrograph; the minimum streamflow immediately prior to the rising limb is used as the constant value;

b) the constant slope method (blue line on the chart) connecting the start of the rising limb with the inflection point on the receding limb; this assumes an instant response in baseflow to the rainfall event;

c) the concave method (violet line on the chart) attempting to represent the assumed initial decrease in baseflow during the climbing limb by projecting the declining hydrographic trend evident prior to the rainfall event to directly under the crest of the flood hydrograph; this minimum is then connected to the inflection point on the receding limb of storm hydrograph to model the delayed increase in baseflow.

Separation of hydrograph and results of separation are shown in Fig. 2.24.

Fig. 2.21 Method of Kille baseflow assessment

MoLR = 0.00060 n/2 + 0.02296

0

0,1

0,2

0,3

0,4

0,5

0,6

0 100 200 300 400 500 600 i

MoL

R [m

3 /s]

n

linear zone of the distribution curve

n/2y0

interflow

baseflow

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Fig. 2.22 Kille baseflow separation

24022020018016014012010080604020

log

Q

1,5

1,25

1

0,75

0,5

0,25

0

-0,25

-0,5

-0,75

-1

-1,25

-1,5

-1,75

Awata3.5 m3/s

36343230282624222018161412108642

log

Q

1,75

1,5

1,25

1

0,75

0,5

0,25

Genale - Upper24.64 m3/s

28026024022020018016014012010080604020

log

Q

1,25

1

0,75

0,5

0,25

0

-0,25

-0,5

-0,75

-1

-1,25

-1,5

-1,75

Mormora5.5 m3/s

323130292827262524232221201918171615141312111098765432

log

Q

2

1,75

1,5

1,25

1

0,75

0,5

0,25

0

-0,25

-0,5

-0,75

Genale - Chenemasa44.46 m3/s

75706560555045403530252015105

log

Q

2,5

2,25

2

1,75

1,5

1,25

1

0,75

0,5

0,25

0

Genale - Kole73.09 m3/s

170160150140130120110100908070605040302010

log

Q

2,75

2,5

2,25

2

1,75

1,5

1,25

1

0,75

0,5

0,25

0

-0,25

Genale - Halowey78.16 m3/s

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Comparison of the assessment of baseflow using the Kille method and hydrograph separation is shown in Tab. 2.10. Results show very small differences between assessment of baseflow using the Kille method and hydrograph separation.

The assessment of specific baseflow is based on data from flow measurements and using the Kille method. The specific runoff is assessed for volcanic rocks basement and sedimentary rocks separately. The specific baseflow for aquifers developed in volcanic rocks was assessed to be 4.75 l/s.km2 based on data from the Awata, Upper Genale, Genale Chenemasa, Mormora and Welmel river gauging stations. The specific baseflow for aquifers developed in sedimentary rocks

Fig. 2.23 Method of baseflow separation

b

ca

crest

inflexion point

flow

time

day360350340330320310300290280270260250240230220210200190180170160150140130120110100908070605040302010

Q

80

75

70

65

60

55

50

45

40

35

30

25

20

15

10

5

0

Awata (2001)2.84 m3/s

Fig. 2.24 Hydrograph baseflow separation (Part 1)

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day360350340330320310300290280270260250240230220210200190180170160150140130120110100908070605040302010

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45

40

35

30

25

20

15

10

5

0

Mormora (2002)8.75 m3/s

day360350340330320310300290280270260250240230220210200190180170160150140130120110100908070605040302010

Q

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340

320

300

280

260

240

220

200

180

160

140

120

100

80

60

40

20

0

Upper Genale - Girgi (2004)39.34 m3/s

day360350340330320310300290280270260250240230220210200190180170160150140130120110100908070605040302010

Q

50048046044042040038036034032030028026024022020018016014012010080604020

0

Genale - Chenemasa (1988)45.56 m3/s


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day360350340330320310300290280270260250240230220210200190180170160150140130120110100908070605040302010

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550

500

450

400

350

300

250

200

150

100

50

0

Genale - Kole bridge (2001)95.42 m3/s

day360350340330320310300290280270260250240230220210200190180170160150140130120110100908070605040302010

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1 1001 050

1 000950900850

800750700650

600550500

450400350300

250200150100

500

Genale - Halowey (1992)85.46 m3/s


Tab. 2.10 Baseflow data for the Negele area (Part 1)

Ma

p I

D

Riv

er

Are

a [k

m2]

Sp

eci

fic

run

off

[l/s

.km

2]

Kill

e m

eth

od

[m

3/s

]

Hyd

rog

rap

h

sep

arat

ion

[m

3/s

]

Sp

ecif

ic

bas

eflo

w[l

/s.k

m2]

Aq

uif

er

RG4 Awata 1,624.1 10.20 3.50 2.80 2.16/1.70Basalt/basement

RG8 Bura Dera 72.5 No data Limestone

RG6 Dawa (Digati) 2,375.7 1.07 1.86 2.21 0.78/0.93Basalt/basement

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was assessed to be 0.14 l/s.km2 based on difference between the river gauging stations at Genala Kola bridge and Halowey. The small value of specific runoff is given mainly by arid character of the area covered with sedimentary rocks in lower reaches of the Genale River. The specific baseflow for aquifers developed in basement rocks was assessed to be 1 l/s.km2 based on data from the Dawa River at Digati, Melka Guba and Shiftu river gauging stations. This assessment is a general one and specific runoff will be highly variable based on location of basement rocks.

The use of groundwater for water supply has not been considered in the assessment.

2.5 Water BalancePrecipitation is partly evaporated, partly transpired and part of the water flows to rivers as runoff

(surface runoff and baseflow). The rest of the water infiltrates into aquifers. The balance was studied for Goro meteo-station, which is located on the volcanic plateau and river gauging stations on the surrounding rivers. The upper part of the aquifer developed in volcanic rocks is drained as shallow local baseflow on the plateau which is represented by the Robe river gauging stations on the Shaya River or the Goro river gauging station on the Tegona River. The aquifer developed in volcanic rocks is totally drained by deeper local drainage occurring below the escarpment and is represented either by the Delo Mena river gauging stations on the Yadot River, the Deyu Harewa river gauging station on the Deyiu River, or the Melke Amana river gauging station on the Welmel River. Deep regional drainage aquifers developed in volcanic and sedimentary rocks are totally drained by deep regional drainage which is measured between the river gauging stations at Chenemasa and Haloway. Data for assessment of water balance are shown in Tab. 2.11.

Tab. 2.10 Baseflow data for the Negele area (Part 2)M

ap

ID

Riv

er

Are

a [k

m2]

Sp

eci

fic

run

off

[l/s

.km

2]

Kill

e m

eth

od

[m

3/s

]

Hyd

rog

rap

h

sep

arat

ion

[m

3/s

]

Sp

ecif

ic

bas

eflo

w[l

/s.k

m2]

Aq

uif

er

RG1Dawa(Melka Guba)

20,097.9 1.38 14.63 13.36 0.73/0.66 Basement

RG2 Dawa (Shiftu) 48,495.9 0.53 16.30 15.53 0.34/0.32Basement/limestone

RG27 Genale (Girja) 3,177.4 23.31 24.64 39.34 7.75/12.38 Basalt

RG11Genale(Chenemasa)

9,190.3 10.02 44.46 45.56 4.84/4.96Basalt/basement

RG19Genale(Kole bridge)

56,135.5 2.64 73.09 95.42 1.30/1.70 Sediment

RG10Genale(Haloway)

56,582.9 2.78 78.16 85.46 1.38/1.51 Sediment

RG5 Mormora 1,321.3 9.91 5.50 8.75 4.16/6.62Basalt/basement

RG7 Negele Cr. 10.0 No data Limestone

RG16 Welmel 1,395.8 12.42 6.80 8.63 4.87/6.18 Basalt

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The water balance assessment is based on the following considerations:• The average monthly precipitation from Goro meteo-station (Tab. 2.12) represents the input

recharge for the whole plateau.• The average monthly evapotranspiration in Goba meteo-station is shown in Tab. 2.12.• Infiltration into the shallow local aquifer is represented by Shaya and Tegona baseflows.• The deficit in the water balance of Shaya or Tegona basins represents infiltration into deeper

aquifers and its value is manifested as deeper local and/or regional baseflow. Infiltration into deeper aquifers can be expressed by the equation I

deeper = P

precipitation – Et

evapotranspiration – TR

total runoff.

• Infiltration and formation of deep local baseflow was computed for the sub-basins of Yadot, Deyiu, Welmel and Halgol rivers from catchments located on the plateau and escarpment. The

Tab. 2.11 Water balance input data

Riv

er

Ga

ug

ing

st

ati

on

Are

a[k

m2]

Ba

se f

low

[m3/s

]

Ba

se f

low

Infi

ltra

tio

n[m

m/y

ea

r]

Sp

eci

fic

run

off

[l/s

.km

2]

Me

an

flo

w[m

3/s

]

Mea

n b

asef

low

ra

te

Genale Chenemasa 9,190.3 44.46 deep local 152.7 4.8 92.12 0.48

Yadot Delo Mena 451.9 2.88 deep local 201.1 6.4 6.67 0.43

Deyiu Deyu Harewa 111.1 0.42 deep local 119.3 3.8 0.97 0.43

Welmel Melka Amana 1,395.8 6.80 deep local 153.7 4.9 17.34 0.39

Shaya Robe 450.9 0.74shallow local

51.8 1.6 4.31 0.17

Tegona Goba 84.4 0.21shallow local

78.5 2.5 1.40 0.15

Genale

between Chene-masa and Halo-way minus its si-nistral tributaries

47,392.6 13.30deepregional

8.9 0.3 65.40

Tab. 2.12 Water balance of Shaya basin

Month/parametr

Units Station Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total

Precipita-tion

mm/year

Goro 20 28 67 201 155 37 26 48 86 173 58 16 915

Evapo-transpira-tion

mm/year

Goba 101 100 116 98 101 98 94 95 89 80 83 95 1,149

Total runoff

mm/year

Shaya 7 7 5 27 26 11 30 51 38 61 28 13 302

Deep local and regional flow

mm/year

77 28 32 137

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rate between surface and baseflow, which is higher compared to the river gauging stations represents shallow local baseflow and deep regional flow. This condition shows that the groundwater catchments of these rivers are possibly bigger than their surface catchments and groundwater infiltration on the highest parts of the plateau also participates in deep local and regional baseflows. This condition is also common in relatively homogeneous aquifers (e.g. aquifers in volcanic rocks) where the groundwater level gradient is not uniform in both directions from the groundwater divide which is caused by the steep gradient of the erosion escarpment (Harenna escarpment).

• Groundwater of deep regional baseflow is drained by the Genale River particularly in segment between river gauging stations Chenemasa and Haloway and the difference in baseflows between both river gauging stations represents deep regional base flow (calculated deep local baseflow of Welmel River was subtracted from this difference as well as deep local baseflow of Dumal, Wabera and Wabe Mena rivers which was assessed by analogy with Welmel).

• Not all components of base flow fluctuate and are stable during the year.

The highest monthly precipitation occurs in April, May and October. During these months not all water volume is consumed either by evapotranspiration or by runoff and rest of the water can infiltrate into the first aquifer developed in volcanic rocks (shallow part of the aquifer). Assessment of the total volume of infiltration for the Shaya basin is 137 for mm/year (Tab. 2.13) and 207 mm/ year for the Tegona basin.

Comparison of infiltration into deeper aquifers from the Tegona and Shaya basins with calculated deep local baseflow of 157 mm/year and deep regional base flow of 14 mm/year revealed a difference of -34 mm/year for the Shaya basin and 36 mm/year for Tegona basin (Tab. 2.13). Calculated deeper baseflows are more or less in equivalence with balanced infiltration from the Tegona and Shaya basins into deeper aquifers.

Tab. 2.13 Comparison of water losses in water balance with estimated deep base flow

Base flow Source of data Balanced value [mm/year]

Shallow local base flow (included in total run off of Tegona)

Tegona 78.5

Deep local and regional flow Water balance for Shaya 137

Deep local and regional flow Water balance for Tegona 207

Deep local base flow Yadot, Welmel, Deyiu 157

Deep regional base flowGenale between Chenemasa and Haloway minus its sinistral tribu-taries

14

Difference between water balance of Shaya and deep base flows

Shaya -34

Difference between water balance of Tegona and deep base flows

Tegona 36

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The presented water balance is calculated based on available data and demonstrates a system approach to assessment of hydrological and hydrogeological data and is in conformity with the conceptual hydrogeological model presented in Chapter 4.

2.6 Drought and Climate ChangesThe whole Ethiopian territory is often affected by reoccurring droughts causing famine. The

impact of drought is severe in both the arid lowlands as well as the highlands of Ethiopia. The

Fig. 2.25 The most drought prone areas of Ethiopia (source: RRC, 1985)

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existence of drought and desertification is well known from geological and archeological evidence as well as from historical documents and on-going measurements. It is matter of fact that the centre of the Ethiopian civilization was shifted about 1,000 km from Axum in the dry north to Addis Ababa located in the more humid centre of the current (modern) Ethiopia over the last 2,000 years. The northern and eastern parts of the country appeared to be highly vulnerable to reoccurring drought and famine. The most drought-prone regions of Ethiopia are shown in Fig. 2.25.

There are many causes of drought, starting with a local deficit of vapor and condensation nuclei and changes in land use causing changes in soil reflectivity etc., to global changes related to the greenhouse effect with the warming of the surface water of tropical seas. Climate change is dangerous because it can accelerate irregularities in the behavior of synoptic weather systems over the country which is one of the main reasons for the failure of the seasonal rains. Geological and historical evidence was described in detail by Brooks (draft, 2005) and Sima (2009).

The study of NMSA (1996) considers an occurrence of meteorological drought when seasonal rainfall over a region is less than 19 % of its mean. In addition, a drought is classified as moderate and severe if seasonal rainfall deficiency is between 21–25 % and more than 25 %, respectively. A year is considered to be a drought year for the country as a whole in the case the area affected by one of the above criterion for drought, either individually or collectively, is more than 20 % of the total area of the country. The study of drought incidence, intensity and frequency within the whole Ethiopian territory takes into consideration data from the period 1969 to 1987 resulting in the following:

1. Occurrence of drought in the Belg season in 1971, 1973, 1975, 1977, 1984 and 1986 affected more than half the regions. The year 1975 was the most serious, including in the Bale region. The impact is considered to be catastrophic if drought occurs continuously for three or more years.

2. Occurrence of drought in the Kiremt season has more of an effect because 95 % of crop production relies on these rains. Drought occurred in 1972, 1984 and 1987 of which the latter affected about 70 % of the country, including a part of the Sidamo region.

3. Occurrence of drought in both the Belg and Kiremt seasons (drought year) in 1973 and 1984 with failure of rain in 6 out of 14 regions.

The study revealed that Belg drought was serious in the Bale and Sidamo areas in (severe drought in bold italics) 1970, 1971, 1972, 1973, 1974, 1975, 1976, 1977, 1978, and 1984 (Sidamo), which puts the Bale area in the third place in drought probability in Ethiopia after Tigray and Wollo araes. The Kiremt drought was serious in the Bale and Sidamo areas in 1969, 1970, 1971, 1972, 1974, 1976, 1977,1979, 1980, 1984 (Sidamo) and 1987 (Sidamo), which puts the Bale area in first place in drought probability in Ethiopia during the Kiremt season in front of the areas of Gonder and Haraghe. Drought was serious in the Bale and Sidamo areas in 1969 (Sidamo), 1973, and 1977 showing the Bale region gas the highest probability of drought during the whole year.

Climate ChangeCurrent climate change poses a significant challenge to Ethiopia by affecting food security,

water and energy supply, poverty reduction and sustainable development efforts, as well as by causing natural resource degradation and natural disasters. For example the impacts of past droughts such as those of 1972/73, 1984 and 2002/03 are still fresh in the memories of many Ethiopians. Floods in 2006 caused substantial loss to human life and property in many parts of the country. In this context, planning and implementing climate change adaptation polices, measures and strategies in Ethiopia will be necessary.

The agricultural sector is the most vulnerable to climate variability and change. In terms of livelihoods, small scale rain-fed subsistence farmers and pastoralists are the most vulnerable.

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The annual minimum temperature is expressed in terms of temperature differences from the mean and averaged for 40 stations. There has been a warming trend in the annual minimum temperature over the past 55 years. It has increasing by about 0.37 °C every ten years. The trend analysis of annual rainfall shows that precipitation remained more or less constant when averaged over the whole country.

For the IPCC mid-range (A1B) emission scenario, the mean annual temperature will increase in the range of 0.9–1.1 °C by 2030, in the range of 1.7–2.1 °C by 2050 and in the range of 2.7 –3.4 °C by 2080 over Ethiopia compared to the 1961 –1990 normal. A small increase in annual precipitation is also expected over the country.

The other climate related hazard that affects Ethiopia from time to time is flooding. Major floods occurred in different parts of the country in 1988, 1993, 1994, 1995, 1996 and 2006. All of them caused loss of life and property.

In recent years the environment has become a key issue in Ethiopia. The main environmental problems in the country include land degradation, soil erosion, and deforestation, loss of biodiversity, desertification, recurrent drought, flood and water and air pollution.

A large part of the country is dry sub-humid, semi-arid and arid, which is prone to desertification and drought. The country has also fragile highland ecosystems that are currently under stress due to population pressure and associated socio-economic practices. Ethiopia s history is associated–more often than not–with major natural and manmade hazards that affect the population from time to time. Drought and famine, flood, malaria, land degradation, livestock disease, insect pests and earthquakes have been the main sources of risk and vulnerability in most parts of the country. Especially, recurrent drought, famine and recently floods are the main problems that affect millions of the country s population almost every year. While the causes of most disasters are climate related, the deterioration of the natural environment due to unchecked human activities and poverty has further exacerbated the situation.

The major adverse impacts of climate variability in Ethiopia include: • Food insecurity arising from the occurrence of droughts and floods. • Outbreaks of diseases such as malaria, dengue fever, water borne diseases (such as cholera,

dysentery) associated with floods and respiratory diseases associated with droughts. • Heavy rainfalls which tend to accelerate land degradation. • Damage to communication, road and other infrastructure by floods.

For example in 2006 flooding in the main rainy season (June–September) caused the following disasters (NMA, 2006): • More than 250 fatalities and about 250 people unaccounted for in Dire Dawa flood. • More than 10,000 people in Dire Dawa became homeless. • More than 364 fatalities in Southern Omo and more than 6,000 (updated to 8,350 after August

15) people were displaced over Southern Omo, where around 14 villages were flooded. • More than 16,000 people over West Shewa were been displaced. • Similar situations also occurred over Afar, Western Tigray, Gambella Zuria and over the low

lying areas of Lake Tana.

In terms of loss in property and livestock • The DPPA estimate is about 199,000 critically affected people due to the flood in the

country. • More than 900 livestock drowned over South Omo. In addition, 2,700 heads of cattle and 760

traditional silos were washed away (WFP).

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• About 10,000 livestock encircled by river floods in Afar. • Over Dire Dawa, the loss in property is estimated in the order of millions of dollars.

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61

The geology of the Negele area is part of the Ogaden basin. The sheet area is a small portion of the southeastern Ethiopian plateau. The first geological cycle is of the Precambrian age and includes metamorphic and ultrametamorphic rocks. The second cycle ranges from the Jurassic to the Cretaceous and is represented by a sedimentary rocks which rest non-conformably on Precambrian. This cycle is mainly composed of a clastic to carbonate series with evaporates intercalations deposited in a marine and lagoon environment. The third cycle is mainly composed by volcanic rocks connected to Tertiary volcanism. The Negele sheet is covered by a Precambrian basement, Mesozoic sediments, Tertiary and Quaternary volcanic rocks, and sedimentary deposits.

3.1 Previous WorkThe earliest geological investigation started in the early 1940 s after the discovery of placer

gold by local people in the area of Adola. Some of the earlier works include: Jelenc (1966) in his compiled work, during his investigation for economic mineral deposits of Ethiopia, divided the rocks of the Adola region into high grade and low grade series. Subsequent works (Gilboy, 1970; Chater, 1971) modified this concept and proposed a threefold classification for the rocks of the Adola region and its surroundings: i.e. Lower, Middle and Upper Groups.

Lithostratigraphic sequences of the various rocks in the gneissic terrain are tentatively established, mainly based on structural configuration of presumable non-overturned sequences, field evidence e.g. indication and intensity of analectic products (granitic layers and pods) and migmatization to some extent using tectono-metamorphic evolution and available geochronological data (Tadesse and Melaku, 1998). Tadesse and Melaku (1998) classified the metamorphic rocks into two lithostratigraphic groups i.e. high grade (gneisses, migmatite and schists) and low grade (metavolcano-sedimentary rocks and mafic-ultramafic complexes) and provided a detailed description of other formations on the Negele map sheet 1:250,000. The volcanic rocks are presented as Tertiary basaltic flows of an alkaline character and are described by Kazmin on the Geological map of Ethiopia at a scale of 1:2,000,000 (1972, 1979), Merla et. al. (1979) and Mengesha et al. (1996).

Bosellini (1989) has exhaustively dealt with the lithostratigraphic sequence of the continental margin of Somalia and the surrounding regions. In his work an intermittent basin evolution is proposed to be caused by subsequent episodic subsidence and uplifts associated with the major catastrophic events in the Horn of Africa during Mesozoic era Hambisa et al. (1997), mapped four uninterrupted Mesozoic sedimentary successions with gradational contacts in the Dodola area, to the north of the Negele map sheet.

3.3. Geological Settings

Geological Settings

Ge

olo

gic

al S

ett

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62 Geological Settings

Genale-Dawa river basin integrated resource development master plan study (Lahmeyer international, 2005). Report on The Sothern Rangelands Livestock Development Project by Agrotec-C.R.G.–S.E.D.E.S. Ass. (1974) also described basic data on the geology of the area.

3.2 StratigraphyThe geology of the Negele area consists of a variety of litho-startgraphical units ranging from

the Precambrian metamorphic basement to Mesozoic sedimentary sequences and Cenozoic volcanic rocks. Quaternary sediments also cover large areas of the sheet. Paleozoic basal clastic sediments, exposed in the deepest sections cut by the Genale River form a package of mainly continental clastic sediments between the underlying late Proterozoic basement rocks and the overlying Jurassic to Cretaceous sedimentary sequence. The Jurassic to Cretaceous limestone gradationally overlies the lower lying basal clastics succession and forms a sequence of limestone, calcareous sandstone and intraformational conglomeratic breccia horizons in excess of 700 m thick. The Cenozoic volcanic flows overlie non-conformably both the Proterozoic basement rocks and Jurassic limestone successions.

A general stratigraphy scheme of the area with the age of the formations is shown in Tab. 3.1. The thickness of formation is based on data published by Agrotec-C.R.G.–S.E.D.E.S. Ass. (1974).

3.3 LithologyThe description of the lithological units is mainly taken from 1:250,000 geological mapping

of the Genale-Dawa river basin integrated resource development master plan study (Lahmeyer international, 2005) and the geological map 1:250,000 by Tadesse and Melaku (1998).

Tab. 3.1 Litho stratigraphy of the mapped area

Age FormationAverage thickness [m]

Generalized lithological description

Quaternary

Alluvial and Eluvial deposits

Gravel sand and clay

Volcanic rocks (V)Scoracious basalts and associa-ted minor volcanic units

TertiaryPlateau volcanic rocks

various Olivine basalt

Mesozoic

Jura

ssic

Upper Hamanlei Jh2 Melmel limestone

100–300

Pelletal oolitic grainstone chalky limestone

Lower Hamanlei Jh1 Jerder limestone

Bioclastic limestone and dolomite

Paleozoic ? Glacial deposits ? Sandstone

Precam-brian La

te

Pro

tero

zoic

Low grade Metavolcano -sedimentary rocks and mafic-ultramaficcomplexes

High grade Gneisses, migmatite and schist

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63Geological Settings

3.3.1 Crystalline Basement Rocks and Associated Intrusive RocksThe crystalline basement rocks are the prominent rocks that cover a wide area of the northern,

western, south and southwestern part of the map sheet. These rocks are well exposed along large perennial river valleys. The crystalline basement rocks of the area comprise the Protorezoic age high grade and low grade belts in association with pre-, post- and syn- tectonic intrusives. The high-grade belt consists of gneisses; schists, migmatite, and it occupies the lower litho-stratigraphic position, while the low grade belts are mainly metavolcano-sedimentary rocks and mafic-ultramafic complexes.

3.3.1.1 High Grade Rocks

The high-grade basement rocks are mainly exposed in the western, northwestern and southeastern parts of the map sheet. Beside these, the deep cut of the Genale River also exposed the units. These rocks are dated as Archean to mid Proterozoic and they are separated from each other by marked unconformities which adapted for classification.

Algae Group

The group consists of biotite-hornbelnde gneiss with subordinate biotite, hornblende-biotite, quartzofeldspathic gneiss, biotite, deformed and/or undeformed biotite granite as well as migmatite. In addition to the gneiss rocks, talc-tremolite schist occurs as minor lenses. The rocks of this group are the most prominent rock units in terms of aerial coverage. It is extensively exposed as a wide belt running from north to south in the western part of the map sheet and it is exposed along the valley of the Genale River. It is often cut by discordant and concordant pegmatitic and quartz vein and veinlets. In geo-chronological order, it represents the oldest rock of the area.

Awata Group

The group is constituted mainly of biotite-plagioclase microcline-quartz mylonite with a presence of migmatized hornblende-biotite gneiss and hornblende gneiss. It is exposed in

Tab. 3.2 Summarized review of the Precambrian rocks

Group Lithology

Mormora GroupQuartz-graphite schist with intercalated marble and quartz-sericite schist (Pgs) Biotite-hornblende gneiss (pbhg)

Quartz-biotite, quartz-sericite and gamet-staurolite-quartz-biotite schist (Pqbs)

Adola Group Actinolite schist and actinolite-quartz-epidote schist (pcas)

Talc, chlorite, tremolite-chlorite-talc, chlorite-actinolite and actinolite schists (Ptts)

Metagabbro (pmg)

Serpentinite (psrp)

Wadera Group Quartzofeldspathic mylonite (pqfm)

Awata group Biotite-plagioclase-microcline-quartz mylonite (Pqkg)

Migmatite (pmgt)

Alghe Group Biotite gneiss (pbg)

Biotite-hornblende gneiss (pbhg)

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the western part of the map sheet inside the Wadera shear zone. Contact with adjoining units such as Algae and Wadera groups is not distinct. In some places, it grades into the Wadera unit and displays a reduced grain size and mafic mineral proportion. The consistent presence of asymmetric quartz and feldspar porphyroblasts in the unit indicates the abundance of sinisterial movement in the unit.

Wadera Group

The group consists of dominantly quartzofeldspathic mylonite, with a minor presence of quartzofeldspathic gneiss, biotite plagioclase- microcline-quartz gneiss and biotite granite. It exposed as a discontinuous N-S running belt in the western margin of the map sheet, juxtaposed with a belt of low-grade Kenticha rock. This unit uniformly dips to the west and it is interpreted to have been gently underthrust beneath a Kenticha low-grade rock assemblage. Morphologically it forms a prominent sharp crested ridge that runs from north to south and this morphological feature is peculiar to the Wadera group in the Wadera shear zone. Apart from this the unit occupies more elevated topography than the other high grade mapable units.

3.3.1.2 Low Grade Rocks

The low-grade include meta-volcano sedimentary rocks and mafic-ultramafic complexes. These low-grade assemblages are classified as the upper complex of the late Proterozoic age. The rocks of this group occur as small patches in the high-grade unit in the northern part of the map sheet. While in the southwestern part of the map sheet they become more prominent especially along the Bulbul thrust belt southwest of Negele.

Adola Group

The group consists of talc schist, chlorite-tremolite-talc schist, chlorite-actinolite schist, actinolite schist and meta-gabbro. Around Bulbul the unit becomes extensive and frequently truncated by deformed and often altered biotite granite and it is covered non-conformably by a Jurassic-cretaceous limestone succession in the east. This unit thins out to the north of Negele town and widens southwestward. In addition, it appears as lenses inside high grade rocks. In the area near to the Bulbul thrust contact it shows a pencil-like structure especially north of the road near Bura Dera.

Mormora Group

The group consists of quartz-biotite schist, quartz-sericite schist and garnet-staurolite-quartz-biotite schist with minor intercalated actinolite and chlorite schist. It is extensively exposed in the Bulbul low-grade belt and as minor intercalation in the Wadera group. It is strongly sheared and foliation is often crenulated. This group is dominantly composed of quartz-biotite schist.

3.3.1.3 Associated Intrusive Rocks

The intrusive rocks in the Negele map sheet are dominantly granitic in composition, with occasional variation in the relative abundance of quartz, K-feldspar and plagioclase crystals. These granitic rocks of variable dimensions intruded various gneissic and migmatitic rocks of the gneissic terrain, as well as mafic-ultramafic and volcano-sedimentary rocks of the low-grade belts.

Syn-tectonic Biotite Granite (pfgt)

The granite is exposed forming N-S trending elliptical, but mostly isolated ridges. Pfgt is pinkish grey, medium to coarse grained (3–6 mm) and weakly foliated. The foliation is pronounced towards the contact with the gneissic units, such as Pbhg, Pbg and Pqfm.

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Post-tectonic Biotite Granite (Pgt)

The variably sized biotite granite pluton is found interspersed in the map area, forming elevated topography and prominent hills. Pgt is generally pinkish grey, medium to coarse grained (3–6 mm) inequigranular and massive.

Post-tectonic Pegmatoidal Granite (Ppgt)

Post-tectonic pegmatoidal granite characterized by large grains of microcline 2–3 cm in diameter, sometimes exceeding 3 cm. The randomly and abundantly distributed coarse microcline, plagioclase and quartz phenocrystals impart a pegmatitic appearance to the rock. Ppgt is light pink to pinkish grey, coarsely grained (>5 mm) inequigranular and massive.

3.3.2 Paleozoic Clastic SedimentThe sediment is composed of cross-bedded sandstone, which is the representative of continental

clastic sedimentation of Paleozoic. The exposure of this layer was recorded near to Negele town at Gobicha and at Mene Kubsa (Bura Dhera) to the south west of Negele. It occurs as a discontinuous patch, often missing and can be considered as a minor unit in terms of thickness and coverage. It is cross-bedded with massive reddish brown to buff white beds. It is well sorted, medium-grained and its thickness never exceeds 20 m. Usually it rests non-conformably on the basement rocks and a loosely packed conglomerate layer composed of rounded and granules of quartz grains and fragments of basement rock are common. The cross bedding nature of the deposit and the absence of marine fossils indicate that this sandstone may have been deposited in a high-energy terrigeneous and fluvial depositional environment.

3.3.3 Mesozoic Sedimentary FormationsIn the literature, the Mesozoic sedimentary rocks have been divided in to two different successions

on the basis of their presumable age (e.g. Kazmin, 1972). The lower carbonate succession is referred to as Jurassic while the upper carbonate succession is considered to be Jurassic to Cretaceous. The Negele sheet consists of sedimentary rocks of the lower carbonate succession only. They are represented by the Hamanlei formation which has organogenic and oolitic limestone with shale and sandstone and grades southward (the present Somalia coast) into deeper water shale and limestone

Fig. 3.1 Limestone outcrop near Siru village

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(BEICIP, 1985). According to Yihunie and Tesfaye (1997), this unit is further classified in to two major sequences of limestone based on a recognizable angular unconformity. These are Melmel limestone (Jh2) and Jerder limestone (Jh1) (Fig. 3.1).

3.3.3.1 Melmel Limestone

Melmel limestone (the Upper Hamanlei) covers an extensive area in the eastern part of the sheet. It contains pelletal oolitic grainstone, mudstone, alternate beds of wackstone to packstone to grainstone and conglomerates. Melmel succession differentiates from the underlain Jerder by clear angular unconformity. Melmel limestone is further subdivided into two-sub groups based on the conglomerate horizon that appears between the upper and lower parts. The lower part of Melmel comprises pelletal oolitic grainstone, mudstone, alternate beds of wackstone to packstone and packstone to grainstone and conglomerates. The grainstone is dull grey to reddish yellow and forms parallel and planar beds, whose thickness ranges from micrite to dismicrite and then to fossiliferouse micrite. The mudstone is reddish yellow in color and forms horizontally layered massive beds. Texturally they are dominantly micrite, dismicrite and fossiliferouse. The alternating beds of wackstone-packstone-grainstone consist of horizontally bedded dull white to reddish yellow packstone to grainstone at the bottom and fine to medium grained yellowish white wackstone in the middle as well as yellowish brown, fine grained massive, hard, horizontally bedded packstone to grainstone at the top part. The top most part of the lower Melmel succession is characterized by coarse to very coarse grained, reddish brown to pink, poorly sorted, well lithified conglomeratic limestone. The upper part of Melmel limestone is formed by a thick succession of yellowish grey and fine laminated massively splitting thick oolitic packstone to grainstone. The base of the Upper Melmel is inter-bedded with several levels of conglomeratic breccia horizons. It is fossiliferouse and the fossils are strongly aligned indicating an environment of high energy, possibly shallow marine. Moreover, the presence of angular interclasts of quartz silt further supports their proximity to the erosional surface.

3.3.3.2 Jerder Limestone

The Jerder limestone (the Lower Hamanlei) forms the lowermost succession of Jurassic sedimentary rocks within the map sheet. It is exposed in the central part of the sheet around Negele town and in the deep valleys of the Genala and Welmel rivers. It mainly contains

Sequence Sub sequence Lithology Texture Thickness Environment

Me

lme

l lim

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Ell MedoMenalimestone

Chalky limestone Fine

Shallow marine

Oolitic packstoneto grainstones

Fine 60 m

Unconformi-ty surface

Conglomerates Very coarse 5–10 m

Bera kabye limestones

Packstoneto grainstone

Alternate beds of wackstone to packsto-ne to packstone to and packstone to grainstone

Micrit and dismicrit

60 m

Mudstones Massive 15 m

Pelletal ooliticgrainstone

Micrite >130 m

Tab. 3.3 Detailed description of limestone sequence (Part 1)

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a succession of mudstones, fossil reef limestone, mudstone, black shale, dolomitic wackstone, pelletal grainstone, sandstone and conglomerates. In the geological report of the Negele area, by Tadesse and Melaku (1998), Jerder limestone is further divided in two based on the existence of a thirty-meter thick layer of laminated, fine-grained and fissile black shale. This Balck shale layer consists of index fossils of ammonites, belemnites and foraminifera specious of the late Calovian to later Oxfordian. The presence of the black shale layer indicates deposition in the deep-sea environment and possibly the maximum depth of the sea record in the area.

The sedimentary sequence of the limestone unit is described in detail in Tab. 3.3.

3.3.4 Tertiary Volcanic Rocks Cenozoic volcanic flows and unconsolidated sediments in the Negele map sheet include Tertiary

basaltic lava flows consisting of olivine basalt of Oligocene-Miocene age. The volcanic rocks outcrop in patches and cover a very small area of the sheet.

3.3.5 Quaternary Sediments (Qa, Qe)The general distribution of the Quaternary sediments in the study area is localized to flat terrains

and Welmel, Genale, Dawa, Awata Mormora, Meda, and Sera river channels. The thickness of these deposits varies up to 3 m. The alluvial sediments range in size from fine sands to silty, clayey silty soil. The eluvial sediment is formed by weathering of bed rocks, mainly limestone. It forms a flat plain to the east of Negele town.

3.4 StructureThe Phanesozoic marine record of East Africa and the surrounding region is mainly governed by

extensional deformation related to the break up of Gondawana land, starting at Permian. It produced a northeasterly trending rift and northwesterly trending transverse fault system. The main rift gave

Tab. 3.3 Detailed description of limestone sequence (Part 2)

Sequence Sub sequence Lithology Texture Thickness Environment

Jerd

er

lime

sto

ne

Angularunconformi-ty surface

Sandstone and conglomerate

Medium tocoarse

1 mFluvial

Ell Kabye limestone

Pelletal grainstone Unsorted 4–10 m

Shallow marine

Dolomitic wackstone Micrite 20 m

Fossil reefs/ Bioherms Micrite

Mudstones

Black Shale Fine grained 30 m Deep sea

Dulu Libehlimestone

Mudstones to wackstone

Micrite 1–4 m

Shallow marine

Fossil reefs/ bioherms Micrite 40 m

Slightly dolomitic mudstone

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rise to the present Indian Ocean whose major faults run along the eastern coast of Africa (Kenya and Somalia). In association with this, a triradial system of E-W, NE-SW and NW-SE trending grabbers developed. As a result of the opening of the North Atlantic and Proto-Indian oceans the triple junction of these grabens has been identified in the Southern Ogaden, Calub area.

The Ethiopian Paleozoic is known by extensive peneplanation. However, a few older continental types of sediment of fluvio-lacustrine and glacial origin reported from Eastern and Northern Ethiopia may suggest that the Paleozoic era was not merely a time of denudation.

The early phases of folding, subsequent thrusting and later shearing deformational events, are recognized to impart N-S oriented regional and local structural fabrics, to the late Proterozoic rocks of the Negele area and Southern Ethiopia at large (Tadesse and Melaku, 1998). The various structures, such as folds, foliations, lineations, shear zones, faults and lineaments, displayed in the uncovered crystalline basement area of the Negele map sheet were developed throughout the progressive syn-orogenic (folding and thrusting) and post-orogenic (shearing) deformations. These shear zones are very important in controlling the fracture density of the basement rocks in the Negele sheet. Their relevancy is revealed by boreholes drilled in these shear zones.

Faults have been described by Tadesse and Melaku (1998) as ductile, brittle-ductile generated throughout different deformational episodes of deformation of the basement rocks. The trends of the fault and lineaments are prominently N-S, NW-SE, E-W, and NNW-SSE. N-S faults are abundant in the basement and their occurrence increases westward. Most of them are restricted to the Wadera

Fig. 3.2 Recent fault at quarry near Siru village

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Shear Zone, within which the N-S faults can reach a length of 10 km. NW-SE and E-W faults are also present in the Wadera Shear Zone with a length of up to 40 km. N-S, E-W, NW-SE, and less frequently NE-SW faults increase in intensity towards the west; their length goes from a few hundred meters up to 15 km. The Alghe fault extends southeast onto the Wachile map sheet. The Hada Tedecha fault extends from the Kenticha thrust contact to the east of Bupo village up to the north of Harokalo. The lineaments are well recognizable through aerial photographs. They are associated to the last deformation phase of the East African Orogenesis. Faulting continued up to the recent era (Fig. 3.2).

Sedimentary structures are identified to be contemporaneous and pencontemporaneous to the diagenesis and lithification processes of the unconsolidated marine sediments (Tadesse and Melaku, 1998). These structures include: bedding, cross bedding, and slump folds. Bedding planes with a subhorizontal attitude are observed in almost all parts of the sedimentary area of the Negele map sheet. The bedding planes have an initial flat lying attitude but slightly curved bedding surfaces which resemble convolent bedding and suprataneous folds are occasionally observed locally. These structures could be the result of different forms of compaction of water saturated sediments and unconsolidated sediments in an uneven topography, respectively. Laterally, the bedding thickness of individual limestone beds is noted to increase towards the east and southeast. The general dip of the sedimentary bedding tilts by about 3–50 towards the southeast. Within bedding internal structures are also commonly observed. Generally, bedding thickness ranges from 1–4 m. In the Jerder limestone, within bedding splitting or parting is usually noted to be slabby <0.3 m thick. However, the black shale layers show very fine, less than 1 cm within bedding lamination. The within bedding splitting thickness is noted to increase to massive beds 1 m thick in the Melmel limestone. Minor slump folds have been noted in Dulu Libeh sub-sequence of Jerder limestone in southeast slumped masses. These folds may have resulted from submarine slumping and/or gravity-driving slides of unconsolidated sediments down the slope.

3.5 Geological HistoryConstruction of the Proterozoic complex basement assemblage located west of Negele reflects

a reworking of existing materials; accretion and collision events, and the addition of a new lithosphere via magmatism associated with sea-floor spreading and continental rifting. The approximate N-S structural grain elements imposed by this evolution has controlled the deposition of phanerozoic materials and most importantly the location of widespread Cenozoic volcanism which is almost exclusively restricted to areas affected by pan-African events (Kazmin et al., 1978).

The study area is underlain by Proterozoic crystalline basement complexes, covered by marine Jurassic successions and Tertiary volcanic flows west of Negele, Hadessa, Qurale, and Gobicha. The basement complexes (gneissic terrain and narrow low grade belts) are designated as parts of the Mozambique belt and the Arabian-Nubian Shield, respectively. These rocks are then intruded by syn- and post-tectonic orogenic plutons. The crystalline basement rocks are suggested to be affected by the late Proterozoic (pan-African) deformation, metamorphism, and magmatism, and are contemporaneously intruded by syn and post tectonic basic to acid intrusive rocks. Thrust contacts between the gneissic terrain and low-grade belts, are often marked by N-S trending regional lineaments and associated shear zones accompanied with an easterly and westerly dipping mylonitic foliation and reverse drag folds (Tadesse and Melaku, 1998). Adola, Bulbul, and Moyale are also interpreted to be dismembered ophiolite sequences (Kazmin, 1975), accreted, folded and chaotically assembled either at final collisional suture along which several terrains are welded together or could possibly be allocthonous nappes obducted for several hundred kilometres from the main collisional suture zone. The presence of extensive ultramylonite proximal to Kenticha and Bulbul low grade belts, thrust related shear zones at thrust contacts and moderate to steeply dipping thrust sheets in the low grade belts, pop-up structure, tectonic melange in the Kenticha belt, suggest a possible rooted suture zone. Gichile (1991, 1992) recognized that these

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rocks are late Proterozoic regionally metamorphosed Mozambique granulites, representing the root zone of the supracrustal sequence in the Pan-African collision zone, which was consecutively thrusted/ uplifted to a higher level during late tectonic stage. Following this line of interpretation Haimanot and Behrmann (1995) and Worku (1996), suggested the Precambrian rocks of southern Ethiopia to constitute a transitional zone between the low grade volcano-sedimentary rocks and the mafic-ultramafic complexes of the Arabian-Nubian Shield, and high grade gneisses, migmatite and intercalated schists of the Mozambique Belt.

The late Triassic is a time of regional subsidence during which rifting begins. During this period the progression in the Karroo rifting allowed the deposition of thick clastic rocks of continental origin which become thicker towards the central part of the rift. Sea floor spreading (separation of east Gondwana from west Gondwana) began after a long period of subsidence in the Callovian and early Oxfordian. The floor spreading ended in the early Hauterivian (121 –120 Ma). The Jurassic transgression came from the southeast, reaching its maximum limit in Western Ethiopia and Eritrea during the Kimmeridgian. This transgression deposited a sandy formation (Adigrat sandstone), followed by neritic sediments composed mainly of thick limestone. From the Hauterivian to the early Tertiary, is a time of crustal uplifting and consecutive formation of the Upper Sandstone due to a forced regression of the sea. Tertiary uplift of the Arabian-Ethiopian swell was accompanied by laterization processes and followed by eruption of volcanic trap rocks.

Fig. 3.3 Mesozoic propagation of the Karoo rift to the southeastern part of Ethiopia (modified after Gani et al., 2008)

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71

4. Hydrogeology4.Hydrogeology of the Negele area is based on the assessment of a large amount of data collected

from existing reports and maps and during field work. There is no previous hydrogeological work at a scale of 1:250,000 and full data sets required for geometrical aquifer configuration are scarce. Analogy is used to assess the groundwater potential of units in the study area where no data was found in the field mapping because the map sheets of Ginnir, Megalo, Filtu, Dodola and Sede were compiled by GSE at the same time and findings in other areas were also used for hydrogeological assessment of the Negele area.

4.1 Water Point InventoryThe field water point inventory was based on a desk study, during which the relevant

materials like geological and drilling reports and maps and aerial photographs were collected from the regional geology department of GSE. Important climatic and gauging station data and topographic maps were obtained from various offices. The desk study also included preliminary data interpretation and preparation of field maps using satellite images, aerial photographs and a digital elevation model (DEM) of the terrain with the geology as a background.

The hydrogeological map of Ethiopia at a scale of 1:2,000,000 was published by Tesfaye Chernet (1993). He classified the geological units of Ethiopia into four major groups depending on the type of permeability and the extent of the aquifer. This hydrogeological map was the basic document for preparation of the field work. Tesfaye (1993) identified the following units:• Mesozoic limestone (Hamanlei limestone) with fissured and/or karst permeability was classified

as a highly productive aquifer; the specific yield of wells was estimated to be in the interval of 0.2 –7.6 l/s.m and the total yield of wells with 20 m of drawdown varies in the interval of 1.8– 68.4 l/s in highly productive aquifers.

• Basement rocks are described as localized aquifers with fracture and intergranular porosity and are characterized as a regional low productivity aquiclude.

• Recharge characteristics were assessed for 50–150 mm/year.• The highlands (in the west) were classified as an area with major water resources. These were

assessed to be widespread and moderate to large in quantity. Groundwater and surface water are of good chemical quality (TDS less than 500 mg/l). Most of the streams are perennial; there are many cold springs, and the groundwater level is between 0 and 100 m and can be exploited in low relief areas (valleys).

• The lowlands (in the east) were classified as an area with major water resources. These were assessed to be widespread and moderate to large in quantity. Groundwater and surface water are of variable chemical quality with TDS 500 –1,500 mg/l in the northwest and 1,500– 3,000 mg/l in the southeast, with most of the perennial streams, and with a groundwater level of below 150 m.

• Groundwater chemistry is characterized as being bicarbonate (HCO3) in the highlands and

sulphate in the lowlands.

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72 Hydrogeology

A complex assessment of hydrogeological data, including water point inventory, hydrological and climatic characterization was performed by Lahmeyer (2005) in “Genale-Dawa River Basin Integrated Master Water Plan Study Project” providing statistic assessment of borehole yield (Tab. 4.1).

The authors classified geological formations of the Genale-Dawa basin based on observation made in the field and existing data. Examples of the different levels of productivity are given in Tab. 4.2.

Topographic maps of 1:50,000 scale were used during the field work as a base map in addition to 1:60,000 aerial photographs. Existing reports about borehole data were collected from regional water bureaus, NGOs and private drilling companies as well as from direct contact with drillers and geologists in the field. A compass and a GPS were used for navigation and locating the water points. The water points were characterized by location, lithology, topography and field measurements of pH, temperature and EC were taken. Pictures and video sequences were captured for documentation and interpretation. Discharge of springs and rivers was measured by the floating, volumetric method and by visual assessment. The static water levels of boreholes with piezometers and open hand dug wells were measured using an electrical sounding deeper. A summary of the field inventory is shown in Tab. 4.3 and an extract from the water point inventory database is shown in Annex 1. Groundwater from water points representing important parts of the area’s hydrogeological system was sampled for chemical analysis (see Chapter 5 and Annex 2). Well logs of selected borehole are shown in Annex 3.

Formation (symbol)

Yield [l/s] Specific capacity [l/s.m] Number of wellsRange Mean Median Range Mean Median

Alluvium 0.50–3.75 2.01 1.47 0.02–8.92 1.657 0.23 7

Basalt (Q) 1.70 2.39 2.00 0.05–0.38 0.160 0.12 7

Basalt (T) 1.50–4.40 3.15 3.15 0.01–1.22 0.339 0.12 10

Ju + Jh 0.83–7.00 2.58 1.50 0.01–35.00 0.04 7

Gt + Qa 0.13–6.50 2.18 1.76 0.02–0.87 0.268 0.10 6

Hm + Qa 0.20–4.67 1.56 0.93 0.02–1.33 0.232 0.08 9

Lm + Qa 1.40–5.00 2.80 2.00 0.11–36.00 0.253 0.29 3

Tab. 4.1 Aquifer classification based on well yield for Genale-Dawa basin

Remark: Gt–granite, Hm–gneiss, migmatite, Ju–Urandab f., Jh–Hamanlei f., Qa–Quaternary alluvium, Lm–limestone

Tab. 4.2 Aquifer classification by Lahmeyer (2005)

Classification Formation name

Low Kohare, basement

Moderate/Low Urandab/Hamanlei

Moderate Gabredare

High/Moderate Volcanic rocks (Basalt)

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73Hydrogeology

Data Assessment was mainly dedicated to data organization, processing, and interpretation in the form of maps and the text of the presented explanatory notes. Aquifers are classified according to their productivity based on the yield measured in the field and hydraulic properties like transmissivity obtained from pumping test data together with topographic settings and recharge conditions. The geographic information system (GIS) ArcGis was used for compilation of the maps.

4.2 Hydrogeological Classification/CharacterizationThe qualitative division of lithological units is based on the hydrogeological characteristics

of various rock types using water point inventory data. The lithological units were divided into groups with dominant porous and fissured permeability. This division served for definition of the aquifer system of the sheet. Since quantitative data such as permeability, aquifer thickness and yield are not adequate or evenly distributed enough to make a detailed quantitative potential classification; analogy was used for characterization of rocks without the adequate number of water points. Hence, the hydrogeological characterization of the study area reveals the following aquifer systems:

Units with porous permeability where the groundwater is flowing through and is accumulated in pores of an unconsolidated or semi-consolidated material. Porous materials of Quaternary age are represented by fluvial and colluvial sediments developed in depressions and/or along valleys of former and existing rivers. The porous aquifers are only locally developed and scattered over the study area. The aquifer with porous permeability forming aquifers is expressed on the hydrogeological map in blue.

Units with fissured and karst permeability where the groundwater is flowing through and is stored in fissures developed in limestone and the permeability can be enhanced by karstification along some fissures. Solution phenomena and karstification in the underground drainage of carbonate rocks are controlled by the drainage base level, which may be represented by a perennially draining stream and/or an impervious formation inside the limestone (marlstone, gypsum) and/or by rocks underlying the carbonate aquifer. A carbonate rock surface, with soil or a relatively permeable, less soluble cover is more favorable for initiation of karstification than bare rock. The rock is presumably dissolved most rapidly in the zone between the highest and lowest positions occupied by the watertable. The units with fissured and karst permeability forming productive aquifers are expressed in the hydrogeological map in green.

Basement rocks represent fissured aquifers of low potential. The groundwater in the hard rock is practically all stored in the fractured zones and the weathered mantle called overburden or regolith. The depth of fractured aquifer zones is generally no more than 50–70 m below the surface.

Tab. 4.3 Summary of field inventory

Water point type Number of inventory Sampled

Borehole (BH) 31 29

Cold spring (CS), hot spring (HS) 5 5

Dug well (DW) 24 24

River water (RV) 6 5

Rain water (RW) 3 3

Total 69 66

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The fractures will tend to close at depth. The faults and joints in igneous rocks are nearly vertical, except for narrow fractures, which are more or less parallel to the rock surface, sheeting and exfoliation. The greatest permeability is found in the sub-soil zone within the partly decomposed rock. Wells tapping this zone have yields roughly an order of magnitude greater than in the fresh rock. The aquifers are expressed in the model of the hydrogeological map in brown/red.

4.3 Elements of the Hydrogeological System of the Area (Aquifers)

Geological description and qualitative division of various geological units together with their topographical position within the area lead to a definition of elements of the hydrogeological system and its conceptual hydrogeological model. The system consists of the following elements:• Porous aquifer developed in alluvial and colluvial sediments of Quaternary age on the plateau,

along rivers and plains of the lowlands.• Fissured and karst aquifer developed in Mesozoic limestone.• An aquifer developed in fractured zones and the weathered mantle of basement rocks.

The hydrogeological map shows aquifers and aquitards defined based on the character of the groundwater flow (pores, fissures), the yield of springs and the hydraulic characteristics of boreholes. The following aquifers and aquitards were defined:

1. Extensive (173 km2) and moderately productive or locally developed and highly productive porous aquifers (T = 1.1–10 m2/d, q = 0.011–0.1 l/s.m, with spring and well yield Q = 0.51–5 l/s). The aquifers are shown in light blue.

The aquifers consist of Quaternary unconsolidated deposits.2. Extensive (9,665 km2) and moderately productive fissured / karst aquifer (T = 1.1–10 m2/d,

q = 0.011–0.1 l/s.m, with spring and well yield Q = 0.51–5 l/s). The aquifers are shown in light green.

The aquifers consist of Hamanlei limestone.3. Extensive (8,170 km2) low productive fissured aquifers (T = 0.11–1 m2/d, q = 0.0011–0.01 l/s.m,

with spring and well yield Q = 0.051–0.5 l/s). The aquifers are shown in brown/red. The aquifers consist of basement rocks.

Tab. 4.4 Summary of basic data of wells in the Negele sheet

Aquifer BoreholesAverage SWL [m]

Average yield [l/s]

Average depth [m]

Max. depth [m]

Min. depth [m]

Basement 24 17 1.50 49 64.4 18

Limestone 34 40 3.14 96 248.0 24

Tab. 4.5 Basic hydraulic characteristics of wells in the Negele sheet

Well ID Depth [m] Aquifer Specific yield [l/s.m] T [m2/d]

Bidre 1 30.00 Limestone ? 0.27 17.50

Bidre 3 44.00 Limestone ? 0.11 7.50

BH-1 (Aba Sirba 1) 41.00 Limestone 1.00 364.80

Aba Sirba 2 50.00 Limestone 5.45

Gobicha 8.62 Alluvium 0.70

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The following detailed hydrogeological characteristics of the aquifers and hydrogeological characteristics of the individual lithological units are described based on field observation in which 99 water points consisting of boreholes, springs and dug wells were inventoried during field seasons of 2010. Basic data (Tab. 4.4) from 22 boreholes are available from aquifers developed in basement rocks and 33 boreholes were available from aquifers developed in sedimentary rocks.

Unfortunately only a limited number of wells have drilling reports with hydraulic data (Tab. 4.5).

4.3.1 Local and Moderately Productive Porous AquifersThe porous aquifers make up 533 km2, accounting for 3 % of the area and consist of alluvial,

colluvial and elluvial sediments of the Quaternary age. These aquifers are shown on the map in light blue. The extent and location of the porous aquifers are shown in Fig. 4.1.

The mappable alluvial and elluvial deposits were observed on various parts of the map sheet. Small unmappable deposits occur throughout the area, often along stream and river courses. No water point with quantitative data was inventoried from elluvial and/or alluvial deposits.

The Quaternary sediments of the sheet are classified as a moderately productive aquifer considering their position at the bottom of valleys along stream channels, flat lands and narrow valleys which are convenient for water storage and water well siting. The thicker cover of Quaternary sediments can be located using simple geophysical measurements, e.g. VES.

4.3.2 Extensive and Moderately Productive Fissured and Karstic AquifersThe fissured and karst aquifers make up 9,665 km2 accounting for 53 % of the area and consist

of fissured and karstic aquifers developed in the Lower and Upper Hamanlei limestone of the

Fig. 4.1 Extent and location of moderately productive porous aquifers

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Fig. 4.2 Extent and location of moderately productive fissured and karst aquifers

Fig. 4.3 Medawelabu spring (CS-2)

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Mesozoic age. These aquifers are shown on the map in light green. The extent and location of the fissured and karstic aquifers are shown in Fig. 4.2.

The Hamanlei limestone is exposed in the eastern part of the map sheet. Topographically, it forms flat plains, cliffs, and canyons of rivers. The limestone is characterized by horizontal bedding which dissected vertically into blocks of different diameters, which facilitate vertical percolation of infiltrated water. The productivity of wells in Negele town is controlled by a N-S running lineament and local weathered layers. The Medawelabu springs and highly to moderately productive boreholes are found in the valley between Madawelabu village and Bidre town. The Roba Jiru spring emerges also along a fault line. The aquifer yield of limestone ranges from 0.02 to 13.24 l/s.

The Korofe (NCS-1) spring emerges from a travertine. It is found in the valley running into the Genale River in a N-S direction. The Jerder limestone in the vicinity of the spring is highly bedded and is cut by vertical fractures. The Medawelabu (CS-2) spring in Fig. 4.3 appears from the foot of the extended ridges in an NW-SE direction. There are series of broad leaved trees along the valley of the ridges which may reflect that the ridges are probably lineaments. The discharge of the spring is about 2.5 l/s.

Discharge of water points yielding groundwater from aquifers developed in limestone varies from 0.02 l/s to 14 l/s with an average discharge of 3.2 l/s.

Direct infiltration, as discussed in the conceptual hydrogeological model of the area, into fissured and karst aquifers developed in limestone is limited in Negele area because of the relatively small volume of precipitation in the eastern part of the Negele sheet.

Data about the yield of various water points from aquifers developed in sedimentary rocks from the Negele sheet were combined with data from the neighboring Ginnir, Dodola and Filtu sheets and the frequency of water point yield was plotted in Fig. 4.4.

Fig. 4.4 Frequency of yield of springs and wells in fissured aquifer developed in limestone rocks

0

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The springs and wells have an average discharge of 9.9 l/s and a median discharge of 2.7 l/s for limestone of the Ginnir, Dodola, Negele and Filtu sheets. The yield in general decreases from the northwest to the southeast. The lowest discharge was measured to be 0.02 l/s and the highest discharge was measured to be 90 l/s. The large differences in the discharge of springs are given by the character of the aquifers developed in limestone whose permeability can be increased by karstification.

Travertine occurrence has been described in Liben (Korofe spring CS-1) and its presence nearby small springs indicates high discharge in the past. The decrease in discharge of these springs can relate to climate change (decrease in precipitation), tectonic uplift and/or development of a karst system whose circulation becomes deeper, or most probably by a combination of all these phenomena.

Tertiary and Quaternary volcanic rocks outcrop in patches and cover a very small area of the sheet. The cross-bedded sandstone represents continental clastic sedimentation of Paleozoic. The exposures of the sandstone are found nearby Negele town at Gobicha and at Mene Kubsa (Bura Dhera) which is to the southwest of Negele where it occurs in discontinuous patches.

The Mesozoic limestone of the sheet is classified as moderately productive fissured and karstic aquifers considering their position at the bottom of valleys along stream channels, and flat lands. These aquifers are convenient for groundwater storage and their groundwater resources can be developed by wells. The well sites can be located using simple geophysical measurements, e.g. VES. The measurement is important in karst aquifers developed in limestone for estimation of depth to groundwater level (which is usually deep).

4.3.3 Extensive and Low Productive Fissured AquifersThe basement rocks are classified as a low productive fissured aquifer which altogether makes

up 8,170 km2, accounting for 44 % of the area and consists of various crystalline (metamorphosed and igneous) rocks of Precambrian age. The basement rocks occupy large areas in the western part of the sheet. Aquifers with fissured permeability are shown on the hydrogeological map in brown/red. The extent and location of the low productive fissured aquifers developed in basement rocks are shown in Fig. 4.5.

The biotite-hornblende gneiss sometimes appears massive while quartzo feldsphatic gneiss are highly sheared, fractured, and weathered, but weathering of resistance quartz and feldspars leads to the development of coarse residual material. Biotite-hornblende gneiss is often cut by discordant and concordant pegmatitic and quartz veins and vein lets. The geophysical survey conducted in Mucho and Dolcha indicates that the biotite-hornblende gneiss is highly fractured and these fractures are the water bearing zone. The biotite gneiss within the Waduma shear zone is highly fractured and weathered. Mylonite while forming intermountain valleys with deposits of weathered material, fractured and weathered zones plays an important role in the accumulation of groundwater which can be developed by dug wells and shallow wells. The groundwater emerging from high grade metamorphosed gneisses has a low discharge and the water points are mostly observed along streams where this unit subjected to local fractures. Discharge of springs varies from 0.018 l/s up to 4 l/s during the dry period when they were measured in the field.

Post-tectonic biotite granite is highly fractured and weathered. The geophysical survey conducted at Shish and Bittata indicated that the weathered and fracture thickness of post tectonic biotite granite reaches 80 m. The weathered and fractured layer in addition to its weathered product of coarse grained sand forms a water bearing zone for dug well and shallow well development.

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Data about yield of water points from aquifers developed in basement rocks from the Negele sheet were combined with data from the neighboring Dodola sheet and the frequency of their yield was plotted in Fig. 4.6.

Fig. 4.5 Extent and location of the low productive fissured aquifer developed in basement rocks

Fig. 4.6 Frequency of yield of springs and wells in fissured aquifer developed in basement rocks

0,0

0,5

1,0

1,5

2,0

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The weathered surface of basement rocks forms pockets where groundwater is accumulated and can be developed by dug wells. Local people dig for groundwater accumulated in pockets of basement rocks particularly along stream beds of dry intermittent rivers.

The aquifer developed in basement rocks of the Negele sheet represent important source of groundwater for the area. There are a lot of water points with relative good discharge providing groundwater with good quality. The area benefits from the fact that western part of the sheet receives an adequate volume of precipitation, rocks are affected by tectonic events and high grade crystalline rocks with quartz veins tend to weather into permeable soils (regolith). Groundwater can be developed by relatively cheap shallow wells with yields of about 1 l/s that are a good source of water for supplying rural communities.

The basement rocks of the sheet are classified as low productive fissured aquifers considering their position at the bottom of valleys along stream channels, and flat lands. These aquifers are convenient for groundwater storage and their groundwater resources can be developed by wells. The well sites can be located using simple geophysical measurements, e.g. VES. The measurement is important in fissured aquifers developed in crystalline rocks for the location of zones with higher frequency and openness of fissures additional to the estimation of the depth of the groundwater.

4.4 Hydrogeological Conceptual Model The general concept of infiltration and groundwater circulation in the southeastern highlands

and adjacent lowlands is shown in Fig. 4.8.

Fig. 4.7 Digging for groundwater in pockets of weathered basement rock nearby Negele town

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Precipitation infiltrates in the highlands into aquifers developed in outcropping volcanic rocks. Infiltrated groundwater forms shallow local groundwater flow which is drained by the local perennial and/or intermittent rivers of the plateau area. Some of the groundwater infiltrates to

Fig. 4.8 Conceptual hydrogeological model of southeastern highlands and lowlands

Fig. 4.9 Fissures in roof of the Sof Omar cave

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deeper aquifers developed by deeper volcanic as well as sedimentary rocks. This deep groundwater flows northwest to the rift valley and to the southeast to the adjacent lowlands. The groundwater that forms deep local groundwater flow is drained by large springs in deep valleys and/or the foot of the erosional (Harenna) escarpment as big springs and feeds perennial rivers (e.g. the Yadot River). The remaining groundwater penetrates even deeper and forms deep regional groundwater flow that recharges aquifers in sedimentary rocks of the eastern lowlands. The deep regional groundwater flow is drained by the main perennial rivers of the lowlands (Genale, Dawa, Wabe Shebelle rivers) and their main tributaries. Direct infiltration into aquifers developed in sedimentary rocks in the lowlands is limited because of limited precipitation in this area; however it contributes to the development of shallow local groundwater flow that is drained by intermittent rivers of the lowlands and also contributes to deep local and deep regional circulation that is drained by the rivers of the lowlands mentioned above. Limited direct infiltration into the aquifers of lowlands was confirmed during inspection of the Sof Omar cave, whose roof is without any stalactical features. Local people also reported that no water is dripping from the clearly visible and relatively open fractures developed in the roof of the cave during the rainy season (Fig. 4.9). Cave-like structures found on the Dodola sheet have openings 50 to 100 cm in height and 1 to 2 m in width, including the development of small stalagmites and stalactites from the floor and roofs of these caves. These karst features confirm the existence of direct infiltration from precipitation in areas receiving an adequate volume of precipitation.

Basement rocks outcropping in valleys of the Genale, Welmel and Dawa rivers form the total drainage level of the area. Large outcrops of basement rocks on the Negele and Dodola sheets form separate low productive fissured aquifers recharged directly by enough precipitation to form good groundwater resources. This direct infiltration forms shallow local groundwater flow which is drained by local permanent rivers (e.g. the Upper Genale, Awata and Mormora rivers). The aquifer developed in basement rocks is also recharged by deep regional groundwater flow. This deep groundwater circulation leads to the formation of Wora Kora hot springs.

The groundwater divide between the main Genale-Dawa and Wabe Shebelle catchments is difficult to define because there is not enough data and the surface water divide should not conform to the groundwater divide. It is necessary to consider that the deep regional groundwater flow in the Ginnir area follows the general dip of the whole Ogaden basin to the east (southeast).

The principles of the general conceptual model of the southeastern highlands and adjacent lowlands can be applied to the area of the Negele sheet. There can be three main mechanism of recharge, in the Negele area, as follows• direct recharge to outcropping aquifers, • recharge from rivers during high waters,• transfer of groundwater by deep regional groundwater flow from areas northwest of the Negele

sheet i.e. from the Dodola sheet with better infiltration potential (Sanetti Plateau).

Recharge to aquifers in the western part of the sheet is mainly direct through overlying soil and elluvial cover; however, the position of the aquifer in the lowlands in the eastern part of the sheet with low precipitation depth and limited surplus of water for infiltration also causes limited direct recharge of aquifers. Recharge from areas with higher precipitation in the Dodola sheet area is also possible. Infiltrated water is flowing form that area through aquifer developed in limestone from northwest to southeast following the general dip of the basin and the hydraulic gradient towards the east. Local recharge of aquifers (particularly porous aquifers) is also possible from rivers during high waters. Infiltrated groundwater forms both deep regional as well as local groundwater flows. Deep regional flow is drained in dry periods directly by the Welmel and Genale rivers and their main tributaries. Discharge of groundwater by springs is not common in the lowlands. Intermittent rivers receive a small amount of groundwater from shallow local groundwater flow in short periods after rainy seasons.

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Infiltrated groundwater can also contribute to the deep regional groundwater flow which flows from the Negele sheet area to the southeast to area of the Filtu sheet and is drained by the Genale River, but can flow to more distant areas in the centre of the Ogaden basin. Deep groundwater in the southeast can be under artesian conditions because of less permeable or impermeable lithological units.

The groundwater flow direction in general coincides with the topography following the surface water flow direction because small intermittent and particularly perennial rivers form local drainage levels for shallow and deep aquifers, and because the main rivers of the area form deep valleys (canyons) and cut overlying aquifers. The flow is partly controlled by the structure, but mainly by the topography and geomorphology of the area. Most of the springs are topographically controlled and others emerge along structures indicating that the groundwater flow is controlled by both factors. Local groundwater flow directions vary from place to place according to the local topography. An important phenomenon for both surface and groundwater flow direction is the inclination of the whole eastern plateau to the southeast.

4.5 Annual Recharge in the Area There is not enough data for direct assessment of recharge. The regional mechanism of recharge

of aquifers in the area was described above. Like in other areas the groundwater recharge is mainly from precipitation depending on its intensity and annual distribution, topographical gradient of the area, lithological composition of aquifers and their tectonic disturbance. The groundwater of the highlands is generally recharged from direct precipitation. There is also a seasonal but significant amount of recharge to localized aquifers from most of the permanent as well as intermittent streams after the Kiremt rains when the level of rivers is above the groundwater level. Aquifers along the rivers are recharged by the surface water of streams. This type of recharge is important in the lowlands where evapotranspiration is higher and precipitation is lower than in the highlands. This type of bank infiltration is very important for local alluvial aquifers and where most of the water well sites are located.

Lahmeyer (2005) in the study of the Genale-Dawa basin considered the infiltration depths shown in Tab. 4.6.

Recharge assessment is based on rainfall infiltration (recharge from rainfall) according to the rainfall infiltration factor (RIF). The criteria used by WWDST (2003) are shown in Tab. 4.7. The recharge area of outcrops of lithological units was considered only if the slope of terrain is less than 20 %.

WWDST (2003) described the recharge of the whole Wabe Shebelle basin to be 1,500 Mm3/ year. This estimation was done based on different approaches that are described as follows:

Tab. 4.6 Estimated minimum recharge to ground water from stations of the Genale-Dawa basin

StationRecorded period

Area [km2]Min[m3/s]

Approximate minimum recharge [mm/year]

Welmel at Melka Amana 1988–1996 1,396 6.25 141.2

Weyb at Sof Omar 1973–2002 4,546 1.86 12.9

Genale at Chenemasa 1989–1997 9,190 10.80 37.1

Genale at Kole* 1989–1998 56,234 439.90 246.7

Remark: * it seems that mean flow has an erroneous value in the order of magnitude–the total runoff is referred in the report to be 84 mm/year

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• Subsurface drainage approach 2,046 Mm3/year• Recharge area approach 397 Mm3/year• Baseflow approach 1,128.6 Mm3/year• Rainfall infiltration 2,311 Mm3/year

The baseflow approach was based on the baseflow separation method when separation of the direct runoff and groundwater components was performed using a runoff hydrograph. However, baseflow separation can only be considered when the groundwater level is always above the water level of the surface water. The method can be applied for the upper Wabe Shebelle catchment where all of the groundwater is discharged to rivers. In the lower part from Imi to Mustahil, water level monitoring for one hydrologic year showed that the water level is always lower that the Wabe Shebelle river bed. When WWDST (2003) calculated renewable groundwater resources of the Wabe Shebelle river basin the safe yield (dynamic groundwater resources) was assessed to be 2,294.7 Mm3/year and 2,228 Mm3/year after subtracting the present use which represents 56.34 mm/year.

Tesfaye (1993) characterized recharge to be 50–150 mm for the Negele sheet.

Recharge calculated from mean values of baseflow shows recharge variability from 0 mm/year to about 150 mm/year depending on the depth variation of precipitation in different years.

Separation of baseflow and water balance presented in Chapter 2 revealed a value of recharge of 14–50 mm/year for the lowland area.

Adopted recharge is 50 mm/year for the eastern part of the Negele sheet and 100 mm/year for the western part of Negele sheet.

Tab. 4.7 Rainfall infiltration factor for Wabe Shebelle basin by WWDST (2003)

Lithostratigrapfical unit Rainfall infiltration factor [%]

Alluvium 6

Basement rocks 5

Basaltic rocks 6

Sandstone and siltstone 5

Limestone 6

Gypsum beds 3

Shale / siltstone 2

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One of the important tasks of the water point inventory and data collection was to survey the groundwater chemistry and to assess the groundwater quality for its use within the mapped area. Therefore, a study of the groundwater quality was carried on the different aquifers (geological formations) of the area as well as various parts of the water circulation system. The results of the hydrochemical study can help to understand the groundwater circulation within the aquifers in addition to comparing the water quality with various standards.

Tesfaye Chernet (1993) identified the hydrochemical characteristics of the natural waters which were collected from different sources and the recharge/discharge conditions of the groundwater. According to Tesfaye Chernet:• the water resources in the lowlands are classified as being water with variable chemical quality

with TDS less than 500 mg/l in the northwestern part, 500–1,500 mg/l in the central part, and 1,500– 3,000 mg/l in the northeastern part of the sheet,

• the groundwater chemistry is characterized as being bicarbonate (HCO3) in the northwestern

part and chloride type (Cl) in the central and eastern part of the sheet.

Lahmeyer (2005) performed an assessment of water quality and described the area where groundwater TDS is above 2,000 mg/l in general. It is between 1,500 mg/l and 3,000 mg/l when groundwater circulates in the limestone and below 1,500 mg/l when the groundwater circulates in the volcanic and basement rocks of the western part of the sheet. The study concluded that the general increase in TDS is from the northwest to the southeast in the Genale-Dawa basin.

Results of chemical analyses were interpreted graphically and are shown on the hydrochemical map of the area.

5.1 Sampling and Analysis A total of sixty six (66) water samples were collected from boreholes, dug wells, springs, river

water, and precipitation water in the study area. All of the water samples collected for laboratory analysis were submitted to the central laboratory of GSE and analyzed for chemical composition. The chemistry of the groundwater obtained from the samples is shown in Annex 2. Chemical analysis of the major constituents (Mg, Ca, Na, HCO

3, SO

4, Cl) and secondary constituents (K, NO

3,

F, HBO2, CO

2, SiO

2), and measurements of electrical conductivity (EC) and pH at room temperature

were performed in the laboratory. Field measurements of pH, temperature and electrical conductivity were made at the time of sampling. The analytical results were presented graphically on a hydrochemical map to facilitate visualization of the water type and their relationship. Suitability of groundwater for drinking, industrial and agricultural purposes is assessed based on the pertinent quality standards.

5. Hydrogeochemistry5.

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Reliability of the analyses was assessed using the cation-anion balance. The assessment showed that only 1 out of 66 (1.5 %) significantly exceeded the reliability level of 10 %. The frequency of the level of balance is shown in Fig. 5.1 and Tab. 5.1.

5.2 Classification of Natural WatersClassification of natural water was used to express the groundwater chemistry on the

hydrochemical map. Hydrochemical types are classified based on the Meq% representation of the main cations and anions by implementing the following scheme: • Basic hydrochemical type, where the content of the main cation and anion is higher than

50 Meq%. This chemical type is expressed on the hydrochemical map by a solid color.• Transitional hydrochemical type, where the content of the main cation and anion ranges

between 35 and 50 Meq%, or exceeds 50 % for one ion only. A dominant ion combination is expressed on the hydrogeological map by the relevant colored horizontal hatching. The secondary ion within the type is expressed by an index (e.g. Mg2+).

• Mixed hydrochemical type, where the content of cations and anions is not above 50 Meq% and only one ion has a concentration over 35 Meq%. This type is expressed on the hydrogeological map by the relevant colored vertical hatching.

Chemistry of groundwater in the Negele area is variable reflecting variability in the geology and hydrogeology of the area consisting of basement and sedimentary rocks. The dominant hydrochemical type of groundwater in the western and northern part of the Negele area is the bicarbonate type. The transitional Ca– HCO

3 type dominates in the northwestern part of the Negele

Tab. 5.1 Level of balance

Level of balance [%] Frequency Cumulative frequency [%]

5 60 90.9

10 5 98.5

15 and more 1 100.0

Fig. 5.1 Level of cation-anion balance

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5 10 15 and more

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sheet along with some basic Ca–HCO3 and Na –HCO

3 types. The transitional Ca –SO

4 dominates the

eastern (southeastern) part of the sheet. The high TDS and sulphate content in the groundwater is caused by its circulation in limestone with higher solubility and contact of circulating groundwater with gypsum strata which is a part of the sedimentary sequence or gypsum material present inside the rock matrix of other sedimentary rocks (sandstone, shale, and limestone).

Tab. 5.2 Summary of hydrochemical types

Hydrochemistry Type Number of cases Percentage

Ca–HCO3

Basic 17 25.8

Ca–Mg–HCO3

Basic 1 1.5

Mg–HCO3

Basic 3 4.5

Na–HCO3

Basic 4 6.1

Ca–HCO3

Trans 13 19.7

Ca–Mg–HCO3

Trans 2 3.0

Ca–Na–HCO3

Trans 1 1.5

Mg–Ca–HCO3

Trans 1 1.5

Mg–HCO3

Trans 1 1.5

Na–HCO3

Trans 2 3.0

Na–Cl–HCO3

Trans 1 1.5

Ca–Cl Mixed 1 1.5

Ca–HCO3

Mixed 1 1.5

Ca–SO4–HCO

3Trans 1 1.5

Ca–SO4

Basic 2 3.0

Ca–HCO3–SO

4Basic 1 1.5

Na–Ca–HCO3

Trans 1 1.5

Na–SO4

Trans 4 6.1

Ca–Mg–SO4

Trans 1 1.5

Na–HCO3–Cl Basic 1 1.5

Ca–Cl–HCO3

Trans 2 3.0

Na–Mg–HCO3

Trans 1 1.5

Ca–HCO3–SO

4Trans 1 1.5

Na–Cl Basic 1 1.5

Ca–Mg–SO4

Basic 1 1.5

Na–Cl–SO4

Basic 1 1.5

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The secondary ion constituent in transitional types of groundwater in the Negele area can be any of actions (Ca, Mg, Na) and/or anions (SO

4, HCO

3, Cl).

The high variability in TDS and in hydrochemistry of groundwater and dominant bicarbonate groundwater in the western part and sulphate groundwater in the eastern part of the sheet indicates the dynamic hydrogeological regime. Groundwater mainly infiltrates into aquifers in the highlands in the west and northwest and flows to the east and southeast to the lowlands with an arid climate receiving a small volume of precipitation and where groundwater flows in lithologicaly inhomogeneous fissured aquifers developed in various Mesozoic sedimentary rocks (mainly limestone) and unconsolidated Quaternary sediments. In general, TDS increases from the northwest to the southeast to the drainage area formed by the valleys of the Genale River and its tributaries. This trend is shown by idealized isosalinity lines on the hydrochemical map. The general trend in TDS as well as in groundwater hydrochemistry is highly affected by soluble gypsum and even rock salt which is common in some sedimentary units. It may be also affected by various sulphidic mineralizations inside basement rocks.

The hydrochemistry of groundwater of the area is expressed on the hydrochemical map by the relevant solid colors (for basic types) or colored hatching (for transitional and mixed types).

A general overview of the hydrochemistry of the natural water of the study area is shown in Tab. 5.2. To facilitate visualization of the classification of water types, the percentage of major cations and anions of the analyzed samples is plotted on the Piper diagram as shown in Fig. 5.2.

The basic statistical data for values of electric conductivity (EC), total dissolved solids (TDS) and concentration of chloride (Cl) are shown in Tab. 5.3.

Fig. 5.2 Piper diagram for classification of natural waters

80 60 40 20 20 40 60 80

20

40

60

80 80

60

40

20

20

40

60

80

20

40

60

80

Ca Na HCO3 Cl

Mg SO4

LimestoneSoilBasementPrecipitationRiver water

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89Hydrogeochemistry

5.2.1 PrecipitationHydrochemistry of rain water of the area is not known in detail; however, chemical composition

of three samples taken in Negele town is shown in Tab. 5.4. A difference in the ion (cation and anion) balance of 6 –10 % shows the analysis to be reliable. The water chemistry can be classified as basic Ca– HCO

3 type.

The hydrochemistry of rain water is shown on the hydrochemical map by a pie chart.

5.2.2 Surface Water Hydrochemistry of surface water is represented by 5 samples from Genale, Awata and Bulbul

rivers. The samples from the Genale River (NRV-3 Sora and NRV-1 Filtu) were taken along the water course from upstream to downstream. The chemistry of the water in Genale River does not change in this part of the river and is of basic Ca –HCO

3 type with TDS of about 90 mg/l. Samples

taken from the Awata River shows transitional Ca –HCO3 type with TDS about 70 mg/l. Chemistry

in samples taken from intermittent Bulbul River shows similar transitional Ca –HCO3 type but with

a different TDS value of 516 mg/l.

The hydrochemistry of surface water is shown on the hydrochemical map by a pie chart.

5.2.3 Groundwater in Mesozoic and Quaternary SedimentsGroundwater from aquifers hosted in Mesozoic sediments represented by limestone and Quaternary

sediments represented by alluvial and elluvial sediments occurs over about 50 % of the area. Rain water infiltrates in outcrops of sedimentary rocks and flows through pores, fissures and karst opening aquifers from recharge areas into discharge areas and appears as springs particularly in deeper valleys along perennial rivers or is directly drained by the Genale and Welmel rivers. The groundwater is also developed by dug wells and boreholes. The aquifers in limestone can be also recharged from highland areas in the located in the northwest of the Negele sheet and from rivers during high waters.

Tab. 5.3 Groundwater descriptive statistics of TDS, EC and Cl values

TDS [mg/l] EC [μS/cm] Cl [mg/l]

Average 920 1,107 87

Median 752 941 59

Minimum 65 63 1

Maximum 5,492 6,280 371

Count* 63 63 63

Remark: * analyses of precipitation are not considered in statistic assessment

Tab. 5.4 Chemical composition of rain water

EC

[μS

/cm

]

HC

O3

[mg

/l]

Cl

[mg

/l]

SO

4

[mg

/l]

F [mg

/l]

NO

3

[mg

/l]

Na

[mg

/l]

K [mg

/l]

Ca

[mg

/l]

Mg

[mg

/l]

SiO

2

[mg

/l]

pH

TD

S[m

g/l

]

74.0 45.0 6.00 4.00 0.18 0.9 1.0 0.3 16.0 0.5 1.0 7.17 74.88

53.0 29.0 0.99 2.00 0.06 0.4 0.5 0.6 8.0 0.3 1.0 7.13 42.85

26.0 15.0 0.99 0.99 0.02 0.9 0.4 0.6 4.0 0.2 1.0 6.36 24.10

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The groundwater of sedimentary units in the Negele map sheet is represented by 18 water samples. These samples were collected form 12 boreholes, 3 dug wells and 5 springs. The dominant chemistry is mainly of Ca –HCO

3 and Ca–SO

4 types but Na, Mg, HCO

3 and Cl ions are also present as

secondary constituents in transitional types of groundwater. Groundwater TDS in limestone varies from 702 mg/l to 3,455 mg/l with an average value of 1,360 mg/l and the sulphate content varies from 24 mg/l to 2,045 mg/l with an average concentration of 469 mg/l. The extreme content of sulphate in the groundwater indicates that groundwater circulates in gypsum containing limestone. The chemical type of groundwater in the alluvial aquifer is bicarbonate with TDS of 175 to 345 mg/l.

The hydrochemistry of groundwater discharged from sedimentary rocks is expressed on the hydrochemical map by the relevant colors.

5.2.4 Groundwater in Basement RockGroundwater from aquifers hosted in basement rock represented by low and high metamorphosed

rocks occurs over about 50 % of the area. Rain water infiltrates in outcrops of crystalline rocks and form a shallow local groundwater flow and circulates through pores, fissures of aquifers from recharge areas into discharge areas and appears as springs particularly in depressions and in valleys along perennial and intermittent rivers or is directly drained by bigger rivers like the Awata, Mormora. The groundwater is also developed by dug wells and boreholes. The aquifers in the basement rock can be also recharged by deep regional groundwater circulation (Wora Kora hot spring) from an area located in highlands northwest of the Negele sheet and from rivers during high waters.

The groundwater of accumulated in basement rocks of the Negele map sheet is represented by 38 water samples. These samples were collected form 15 boreholes, 21 dug wells and 2 springs. The chemistry of groundwater is highly variable reflecting local characteristics of a shallow aquifer. The dominant chemistry is mainly of basic and transitional Ca–HCO

3 and Na– HCO

3 types but

sulphate and even chloride type occurs in the area covered with basement rocks. Na, Mg, HCO3,

SO4 and Cl ions are also present as secondary constituents in transitional types of groundwater.

The basic Na –HCO3 type of groundwater is developed in the central part of area covered with

crystalline rocks. Groundwater TDS in basement varies from 180 mg/l to 5,492 mg/l with average value of 871 mg/l. A relatively high TDS for shallow local groundwater flow typical for water resources developed in weathered and fissured part of crystalline rocks shows to mixing processes of infiltrating water with some highly mineralized water that can originate when groundwater is in contact with various sulphidic mineralizations which is typical for basement rock in this area. Hydrochemistry of the hot spring is described in Chapter 5.4.

The hydrochemistry of groundwater discharged from basement rocks is expressed on the hydrochemical map by the relevant colors.

5.3 Water QualityWater quality of the mapped area was assessed from the point of view of drinking, agriculture

and industrial use.

5.3.1 Domestic UseTo assess the suitability of water for drinking purposes, the results of the chemical analyses were

compared with the Ethiopian standards for drinking water (see Tab. 5.5.) published in the Negarit Gazeta No. 12/1990 and The Guidelines of Ministry of Water Resources (MoWR, 2002).

Tab. 5.5 shows that groundwater of the mapped area is not convenient for drinking in more than 50 % of the sampled points. This situation reflects of the fact that the majority of groundwater dissolutes gypsum and even the rock salt occurring within sedimentary formations.

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91Hydrogeochemistry

The content of calcium, sulphate and nitrate and particularly TDS in nearly 50 % of cases exceeds the highest desirable level and represents the main threats to the groundwater quality. Deterioration of groundwater quality by a high content of calcium, TDS and sulphate is caused by the natural character of the aquifers and results from dissolution of gypsum and possible sulphidic minerals through which the groundwater is circulating. The high content of nitrates is caused by human factors (pollution) that add allochthonous material into the groundwater in the aquifer (human and animal waste).

Particular interest was paid to the content of nitrates in groundwater. The content of nitrates is not related to the rock composition (type) but it reflects pollution of groundwater by human and/or animal waste. The background content of nitrates in groundwater is about 5 to 10 mg/l depending on the relevant land cover. In forest areas it can be even higher because of decomposition of various plants

Tab. 5.5 Groundwater chemistry compared to drinking water standards and guidelines

PropertyRange (min–max)[mg/l]

Ethiopian standards (1) and MoWR Guidelines (2) [mg/l]

Number of exceeding values

Highest desirable level

Maximum permissible level

Highest desirable level

Maximum permissible level

Na (2) 3–844 358 2

Ca (1) 4–423 75 200 32 7

Cl (1) 0.99–750 200 600 5 1

Cl (2) 0.99–750 533 1

HBO2

0–0 0.3 0

(free) ammonia

0.05 0.1

Fe (1) 0.1 1

Fe (2) 0.4

Mg (1) 0.1–280 50 150 17 3

Mn (1) 0.05 0.5

Mn (2) 0.5

SO4 (1) 0.99–2,531 200 400 11 7

SO4 (2) 0.99–2,531 483 7

TDS (1) 24.1–5,492 500 1,500 44 9

pH (1) 6.36–8.18 7.0–8.5 6.5–9.2 12 1

pH (2) 6.36–8.18 6.5–8.5 1

NO3 (1) 0.4–84 10 45 18 6

NO3 (2) 0.4–84 50 6

F (1) 0.02–7.4 1 1.5 17 8

F (2) 0.02–7.4 3 2

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and other organic material. The nitrate content varies in the Negele area from 0.4 mg/l to 84 mg/l with a mean value of 4 mg/l (Fig. 5.3).

Water samples (19 out of 66 or less than 30 %) with a nitrate content of above 10 mg/l show that the first (shallow) aquifers are polluted by human activity. The value of 10 mg/l is considered as the natural content of nitrates in the groundwater. The more alarming finding is that in 6 of the 66 samples the concentration of nitrates exceeds the maximum permissible level. This pollution is an important factor particularly in highly vulnerable groundwater resources in shallow aquifers developed in crystalline rocks and karst aquifers developed in limestone. This fact also has to be considered when planning for the future development and protection of groundwater resources in the area. Proper location of water points and suitable protective measures should be applied to boreholes, springs and dug wells used for human water supply. Fig. 5.3 shows the content of nitrates in the analysis of water in the study area.

5.3.2 Irrigation UseAgricultural standards for the quality of groundwater used for irrigation purposes are determined

based on the Sodium Adsorption Ratio (SAR), total dissolved solids and United States Salinity Criteria (USSC). The Sodium Adsorption Ratio (SAR) is used to study the suitability of groundwater for irrigation purposes. It is defined by SAR = Na/[(Ca+Mg)/2] where all concentrations are expressed in mg/l.

Fig. 5.3 Content of nitrate in analysis of water in the study area

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60 70

water points

nitr

ates

[mg/

l]BH-26

DW-2, DW-4

Tab. 5.6 Suitability of water for irrigation

Value of SAR Water class Number of samples in the range

<10 Excellent 50

10–18 Good 8

18–26 Fair 3

>26 Poor 5

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93Hydrogeochemistry

Most of the water samples (see Tab. 5.6) from the study area are found to be suitable for irrigation since they show the SAR value within the water quality class of excellent for agricultural purposes. Groundwater classified as fair and/or poor quality water for irrigation corresponds with water points yielding Na –HCO

3 type of water.

5.3.3 Industrial UseIndustrial water criteria establish the requirements of water quality to be used for different

industrial processes that vary widely. Thus, the composition water for high pressure boilers must

Tab. 5.7 Suitability of water for use in industry (Part 1)

Industry or useSolids (TDS)[mg/l]

pHChlorides as Cl [mg/l]

Sulfates as SO

4 [mg/l]

Number of samples in the range

Brewing 500–1,500 6.5–7.0 60–100 1

Carbonated beverages

< 850 < 250 < 250 39

Confectionary 50–100 > 7.0 4

Dairy < 500 < 30 < 60 19

Food canning and freezing

< 850 > 7.0 27

Food equipment washing

< 850 < 250 39

Food processing general

< 850 39

Ice manufacture 170–1,300 47

Laundering 6.0–6.5 1

Paper and pulp fine < 200 9

Paper groundwood < 500 < 75 21

Paper bleached cardboard

< 300 < 200 10

Paper unbleached cardboard

< 500 < 200 21

Paper soda and sulfate pulps

< 250 < 75 10

Rayon and acetate fiber pulp production

< 100 6

Rayon manufacture 7.8–8.3 5

Sugar < 100 < 20 < 20 6

Tanning 6.0–8.0 63

Textile < 100 < 100 37

Remark: Sugar requirements for TDS are in general low

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meet extremely strict criteria whereas water of low quality can be used for cooling of condensers. The suitability of water for use in industry is shown in Tab. 5.7.

Of almost equal importance for industry as quality of used water is the relative time constancy in concentration of various components. As a result, an adequate groundwater quality often becomes a primary consideration in selecting a new industrial plant location. Groundwater from the mapped area can be used for industry in general, but some specific technologies require water treatment before the water is used in the technology.

Incrustation hazard is important for the design of various pipes as well as technological processes. Incrustation occurs if concentrations exceed the limits shown in Tab. 5.8. Corrosion hazard occurs if concentrations exceed the limits shown in Tab. 5.9.

There is threat of incrustation for about 40 % of the samples because most of the groundwater circulates in carbonate rocks with gypsum and rock salt intercalations or corrosion when groundwater of the area is used in pipes for public water supply or for delivery of water for industry or agriculture.

5.4 Mineral and Thermal WaterThermal water was encountered during the water point inventory.

Tab. 5.8 Concentration limits for incrustation

Component Concentration [mg/l] Number of sample in the range

Bicarbonates (HCO–3) > 400 45

Sulfates (SO–4) > 100 48

Silicon (Si) > 40 39

Iron (total) > 2 Not analyzed

Manganese (total) > 1 Not analyzed

Hydrogen sulfide (H2S) > 1 Not analyzed

Total hardness (TH as CaCO3) > 200 Not calculated

Tab. 5.9 Concentration limits for corrosion

Component Concentration and/or value Number of sample in the range

pH < 7 52

EC > 1,500 μS/cm 52

Chloride (Cl–) > 500 mg/l 65

Hydrogen sulfide (H2S) > 1 mg/l Not analyzed

CO2

> 50 mg/l Not analyzed

Dissolved oxygen (O2) > 2 mg/l Not analyzed

Total hardness (TH as CaCO3) < 100 mg/l Not analyzed

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95

Natural resources of the Negele area vary in origin relating to the geological composition, soil conditions, water, wind and solar radiation, as well as human resources.

6.1 Economic GeologyVolcanic and basement rocks are not very common in the map area. Volcanic rocks outcrop

in small patches in the western part of the area covering the basement rocks. Coarse crushing to the size of gravel is needed for raw materials for road construction. Larger sizes are also important for foundations. The basement rocks cover a large area in the western part of the sheet and are frequently used as road construction material where volcanic rocks do not exist.

The reddish brown, silty, clayey residual soil and fluvial deposits can be a good source of burrow material. The soil (vertisol) occurs on rounded, low relief hills composed of limestone. The soil can serve as an impervious blanket in the construction of dams and other water retaining structures. As it can be seen from field observation the local people make pottery products from the residual soil.

Sand and gravel naturally occurring along the main rivers can be used for preparation of concrete and water well development (gravel packing). There are many existing quarries in the project area especially in the river valleys mentioned.

Limestone, gypsum and mudstone provide potential resources for development of cement and lime as well as for the development of various products in the chemical industry (paint production, plaster of Paris, dimension stones, etc.). There are no cement factories in the area and the potential has yet to be developed on the Negele sheet. There are many existing quarries of limestone especially along roads and crushed limestone is used for road construction. Gypsum sites are developed along the Genale River valley north from Sirru town.

Metallic mineral resources, particularly the Kenticha low grade belt has a good potential for primary gold but it is also well known for mining of gold placers, particularly along the Awata River where a lot of gold panning sites exist. The belt is also known for containing of tantalum minerals and for pegmatite with quartz crystals of optical quality.

6.2 Water ResourcesWater resources of the area depend mainly on rainfall and other climatic characteristics, as well

as the hydrological, geological and topographical settings of the study area. Detailed assessment of water resources in the area is difficult because both climatic and water flow data are scarce and the existing data series are short and incomplete or inaccessible.

6. Natural Resources

of the Area

6.

Natural Resources of the Area

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96 Natural Resources of the Area

There are 10 meteorological stations in the Negele area including a 1st class meteo-station in Negele town operated by the Meteorological Institute. The stations have long-term measurements. The long-term mean annual rainfall of the area has been assessed by the Tyson method and an average of about 679 mm/year has been adopted for the sheet.

The area of the map was calculated from the 1:250,000 hydrogeological map and an area of 18,370 km2 is used for further calculation.

The area of active aquifers that store and transmit water was calculated based on the hydrogeological map. The active aquifers (Tab. 6.1) of porous, karst and fissured permeability cover the whole sheet area of 18,370 km2.

The runoff characteristics vary widely because of variability in climatic conditions and hydrogeological characteristics between different observation points. The Dumal and other smaller rivers flow only in wet years. Surface river flow measurements are only performed on the Genale River at the Chenamasa gauging station and other data are taken from measurements taken outside of the Negele sheet. The surface flow–baseflow assessment is highly affected by the short and/or incomplete series of data and the intermittent character of rivers in some years. Data can also be highly influenced by the effect of bank groundwater storage, difficulties in flow measurements in wide and unstable river channels and unknown groundwater flow beneath gauging stations. For further calculations, the value of specific surface runoff of 10.0 l/s.km2 for areas covered by basement rocks and 2.0 l/s.km2 for areas covered by sedimentary rocks, and specific baseflow of 1.0 l/s.km2 for areas consisting of basement rocks and 0.14 l/s.km2 for areas covered by sedimentary rocks were adopted for the Negele area. The assessed water

Tab. 6.1 Aquifers of the area

Aquifers Area [km2/%]

Porous 534 / 3

Fissured and karst in sedimentary rocks 9,665 /53

Fissured basement rocks 8,171 / 44

Total of the area 18,370

Tab. 6.2 Assessment of water resources of the Negele area

Input Area [km2] Resources total Remark

Precipitation 679 mm 18,370 12,473 Mm3/year

Total water resources – map 6.0 l/s.km2 18,370 3,478 Mm3/year 45 % rainfall

Renewable groundwater resources active aquifers

0.6 l/s.km2 18,370 347 Mm3/year 4,5 % rainfall

Static groundwater resources karst and fissured aquifers

5 % porosity100 m saturatedthickness

9,665 48,325 Mm3

Static groundwater resources porous aquifers

15 % porosity30 m saturatedthickness

534 2,403 Mm3

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97Natural Resources of the Area

resources of the Negele area are shown in Tab. 6.2. Based on the map area, values of specific runoff (6 l/s.km2) and specific baseflow (0.6 l/s.km2) are adopted.

6.2.1 Surface Water Resources DevelopmentGauge measurements show that 45 % of the precipitation is drained as total runoff from the area

and there are good water resources to be used for irrigation, electricity generation as well as for drinking water supply of people living within the area. The total water resources of the area have been assessed to be 3,478 Mm3/year.

The surface water of the area should be used for irrigation, drinking supply (as practiced for the area along the road from Negele to Filtu) as well as for electricity generation; however, construction of dams should not be easy in areas covered by karstified limestone. The irrigation should be preferably applied on perennial rivers like the Genale, Welmen, Dumal, Awata, Mormora and Dawa. Irrigation dams on other rivers should be designed in a different way respecting the intermittent character of the rivers and the topography of the area.

Dams for electricity generation should be constructed on the Ganale and possibly the Welmel where the gorge of the river is represented by less permeable basement rocks.

Considering the fact that the use of surface water for irrigation is the most important development factor for food security in the area, we can recommend about 80 % of available surface water resources to be used for irrigation. This portion represents 2,782 Mm3/year. Considering about 10,000 m3 of water is needed to irrigate 1 ha of land, the calculated irrigation resources represent an irrigation potential of 278,200 ha. This area represents 2,782 km2 which is about 15 % of the Negele area and exceeds the recently moderately and/or intensively cultivated areas of the sheet.

It is known that the area can often be affected by drought periods and during some years irrigation dams will not be refilled by rainfall. When this will happen over several years irrigation cannot be practiced in drought stricken areas. The meteorological observations and experience from the Genale-Dawa basin area as well as other areas shows that the occurrence of drought periods is not uniformly distributed over large areas and in the case of drought in one part of the area (sheet) other areas (or adjacent sheets) can gain a volume of precipitation sufficient for filling irrigation dams. This analysis results in the recommendation that irrigation dams are highly important for agricultural development of the area. Drought periods and their spatial distribution show that agricultural production in areas of adequate rainfall can support areas stricken by drought within the region without the requirement for long distance transport of food aid. It also shows that basic decisions can be made on a regional level. This decision will be quicker than one adopted at a federal level.

The irrigation as well as energy potential of the area has been known for a long time. It was assessed in the framework of the Lahmeyer (2005) Genale-Dawa Water Master Plans and by various specific studies.

6.2.2 Groundwater Resources DevelopmentDespite the fact that river gauge measurements show high evapotranspiration and total runoff of

45 % and only 4.5 % of precipitation infiltrates and appears as baseflow, there are good groundwater resources to be used for the supply of drinking water to people living within the area. There is the potential to use the groundwater to support irrigation as well as water for livestock, particularly in arid eastern part of the sheet. There is also the chance to support people living outside the mapped area by transferring drinking water. The total volume of renewable groundwater resources of active aquifers in the area has been assessed to be 347 Mm3/year.

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Considering the total number of people living within the area is 364,482 (Tab. 1.1) the need for water supply can be nearly 7.7 Mm3/year. Assessment of drinking water demand was based on a calculation of 20 l/c.d (15 l/c.d rural and 22.5 l/c.d for towns with less than 15,000 inhabitants). The figure shows that recent demand represents about 1 % of renewable groundwater resources of active aquifers i.e. aquifers can provide adequate drinking water even in the future considering the trends in population growth.

Tesfay (2001) describes water supply issues and predicts that a large number of areas fall into the category of “water scarcity” areas because of an increase in population and in demands for more water for agriculture, industry and the community. This situation will be even worse in 2025 based on the trends in population growth. He defined “water scarcity” and “water stress” as cases where less than 1,000 m3/year and less than 500 m3/year are available annually per capita, respectively. These limits represent about 133 and 66 Mm3/year; however, they are not supposed to be covered only from groundwater. Comparing these limits to the overall water resources of the area of 3,478 Mm3/year, the scarcity limit represents about 4 % and the stress limit about 2 % of the overall water resources of the sheet. It is necessary to state that the limits are based on the idea of massive human, agriculture and industrial development of the area in the next 15 years.

Most of the people within the area live in small towns and villages. Water supply based on drilled wells represents the most secure water and should be applied for small towns and concentrated village settlements. Technically, it is recommended to drill wells as follows:

a) In aquifers developed in Hamanlei limestone with a depth of about 150– 250 m. Each of the wells can yield about 2 l/s (recent average). The recent average depth of wells is 130 m with an average groundwater level at 100 m below the surface (maximum depth to groundwater level is 40 m below the surface). Each of these wells can provide 172,800 l/d and can supply a small town or group of villages with about 8,640 inhabitants considering a daily consumption of 20 l/c.d.

b) In aquifers developed in basement rocks with a depth of about 30 –70 m. Each of the wells can yield about 1 l/s (recent average). The recent average depth of wells is 30 m with an average groundwater level at 10 m below the surface. Each of these wells can provide 86,400 l/d and can supply a small town or group of villages with about 4,320 inhabitants considering a daily consumption of 20 l/c.d.

The first step in groundwater development should be to provide a safe water supply to people living within the area. In this respect it is recommended to drill wells for the water supply in selected areas. The proposed areas in total number of 8 were checked by hydro-geophysical measurements. Vertical electrical sounding (VES) using the Schlumberger array was employed on the selected sites on the Negele map sheet. The sites are located on the hydrogeological map.

Most of the electrical responses collected during the measurements are very small and the differences in resistivity between consecutive layers are very low. These conditions, therefore, made it very difficult to interprete the geoelectric layers and identify water bearing horizons. An additional problem is the lack of good quality borehole logs causing increased uncertainties in geological and hydrogeological interpretation of the geoelectric layers. Using the existing information about geology and hydrogeology of the area the geophysical and hydrogeological interpretations of geoelectric sections were prepared and the results of VES measurements are discussed in the following text.

Siminto - site 1In this area four VES surveys were conducted and a geoelectric section which basically has

four layers was constructed and shown in Fig. 6.1. The layers are probably clay, wet sandy clay, fractured sandstone, poorly cemented sandstone. The wet sandy clay is found only under VES-1 and VES-4 with resistivity of 10 .m and 12 .m and thicknesses of 77.7 m and 8.2 m, respectively

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and the fractured sandstone which is found throughout the section with 3 to 8 .m resistivity and 79.5 to 240 m could probably be saturated with water and could be a source of groundwater. The sandy clay is thicker under VES-1. The top clay layer has a thickness of 4.5 to 13.7 m which could hinder recharge of groundwater to the fractured sandstone.

Fig. 6.1 Geoelectric section of Siminto site

Fig. 6.2 Geoelectric section of the Hadessa site

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Hadessa - site 2Five VES surveys were carried out in this area. The interpreted resistivity curve and thicknesses

were used to produce a geoelectric section (Fig. 6.2) which constitutes clay, gravelly soil (found under VES-4 and VES-5), saturated sand, shale sandstone, and shale. The gravelly soil which has a resistivity range of 50 to 68 .m and thickness of 3 to 4 m is probably a shallow source, the saturated sand which has a resistivity range of 9 to 23 .m and thickness of 4.2 to 94.2 m, and poorly cemented sandstone (less likely compared to the previous two) could probably contain groundwater. The saturated sand is thick under VES-4 and quickly attenuates towards either end. Therefore, conducting VES surveys in small intervals near and to the left and right of VES-4 may give valuable information for recommending drilling with the best result.

Dekka - site 3A geoelectric section (Fig. 6.3) was produced using the outcomes of five VES which show three

layers in VES-2 to VES-5 and four layers under VES-1. The layers below VES-1 are probably clay, gravelly clay, shale limestone, and shale whereas under the remaining soundings the resistivity layers are probably clay, fractured limestone, and shale limestone. The gravelly soil (much thinner than the layers below it and not visible in this section) is found under VES-1 with a resistivity of 40 .m and thickness of 7.6 m, and the fractured shale limestone with a resistivity of 5 to 9 .m and thickness of 146 to 155 m are probably sources of groundwater. The shale limestone gets deeper starting from VES-2 towards the SE as compared to its vertical position under VES-1 which probably indicates the presence of a fault between VES-1 and VES-2.

Dibi Guchi (Airport) - site 4Four VES surveys were carried out in this area. The results of these soundings were used to

produce a geoelectric section (Fig. 6.4) which basically contains five layers among which three of them are probably clay with varying amounts of sand and moisture, fractured sand and poorly cemented sandstone. Therefore, the gravelly soil which is found under VES-1 and VES-5 with resistivity of 9 and 27 .m and thickness of and 5 and 3.9 m, respectively and fractured sand with resistivity of 7 to 15 .m and thickness of more than 234 m could be saturated and are a probably sources of groundwater. The clay layers are thicker and broader towards the ends of this section so that recharge to the fractured sand is probably easy near the middle.

Meda Welabu - site 5Nine VES surveys were carried out in this area and a geoelectric section was prepared to show

geoelectric units. The section basically constitutes three geoelectric layers. These are probably clay, fractured shale limestone, shale limestone and fractured/poorly cemented sandstone. So, the fractured limestone with 5 to 7 .m resistivity and 4.9 to 52.6 m thickness range, and fractured/poorly cemented sandstone which is found under VES-1 and VES-9 with resistivity of 42 .m and

Fig. 6.3 Geoelectric section of the Dekka site

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37 .m and thickness of more than 125 m could probably be sources of groundwater. The top clay layer gets thinner towards the SEE so groundwater recharge is probably facilitated in this part of the section.

Fig. 6.4 Geoelectric section of the Dibi Guchi site

Fig. 6.5 Geoelectric section of the Meda Welabu site

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Shishu 1 – site 6Four VES surveys were carried out in this area. The VES curves were interpreted as thicknesses

and resistivity of layers and these layers were used to construct a geoelectric section which basically constitutes five geoelectric layers. These are probably clay, gravelly soil, deeply weathered basement rock, fractured basement rock, and fresh basement rock. The gravelly soil which is found under VES-1 and VES-2 with resistivity of 43 to 48 .m and thickness of 1.7 to 2.8 m, the deeply weathered basement rock with resistivity of 3 to 19 .m and thickness of 14.5 to 35 m, and the fractured basement which is found only under VES-1 with resistivity of 58 .m and thickness of about 88.5 m could probably be sources to groundwater. The degree of weathering may increase or sand content decrease towards the SSW or there is probably a fault between VES-2 and VES-3 and the basement could be fresh towards the SSW and the layer above it could be made up of clay material. The reason for this may be the resistivity value of the basement rock gets smaller towards the NNE which indirectly indicates the degree of weathering and fracturing of the basement gets stronger in this direction.

Shishu 2 – site 7Four VES geosounding results were used to construct the geoelectric section which basically

constitutes three to four layers. These geoelectric layers are probably clay, gravelly soil, weathered basement, and fresh basement. The shallow gravelly soil which exists under VES-1 with resistivity of 14 .m and thickness of 2.9 m and VES-3 with resistivity of 16 .m and thickness of 4 m, and the weathered part of the basement which has resistivity of 2 to 5 .m and thickness of 11 to 20.3 m could probably be sources of groundwater.

Fig. 6.6 Geoelectric section of the Shishu 1 site

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Filo – site 8Twelve VES surveys were carried out in this area. The interpreted results were used to

construct a geoelectric section that basically constitutes four geoelectric layers. These layers are probably clay, sandy clay, fractured sandstone/or saturated sand and poorly cemented sandstone. Therefore, the sandy clay which is found throughout the section with resistivity of 8 to 32 .m and thickness of 3.9 to 22.9 m as well as the fractured sandstone/or saturated sand which is also found under all soundings with resistivity of 1 to 3 .m and thickness of 28.5 to 161 m could probably be sources of groundwater. The fractured/or saturated sand gets thicker towards the south from the north with nearly undulating bottom topography which probably indicates that the depression and elevation of the bottom layer is the poorly cemented sandstone. The sandy clay is thick under VES-3 and VES-9. The top clay layer has a varying thickness throughout the section from 0.4 to 2.9 m which probably has an effect on groundwater recharge.

The most difficult question will be how to supply the rural areas with a widely spread population. This should be done from local centers where water wells will be drilled and connected to places of water use with relatively long distribution pipes. Effectiveness and cost of water supply systems for the rural population should be studied as a site specific problem in the future.

Fig. 6.7 Geoelectric section of the Shishu 2 site

Fig. 6.8 Geoelectric section of the Filo site

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Most rural schemes, especially gravity schemes, do not have water levies. The tariff rates of schemes with water charges range from 0.10 Birr/family/month to 6 Birr/m3 of water. Schemes with a motorized borehole source have higher rates ranging from 3 to 6 Birr/m3 of water.

Potential groundwater resources developed in the area surpass the current needs of people living in the area. It even surpasses the potential demand of water when agriculture, living standards and industry will be developed in the future in the area. Groundwater is generally of good quality without harmful substances and can be used for drinking purposes after the supply system is secured by chlorination. The groundwater development potential can be used for development of human resources and agriculture resources not only within the area, but also in other areas with high demand.

Some of the existing water points do not represent safe water supplies as they show an increasing content of nitrates in shallow water supply systems. Deeper wells currently represent a safe type of water supply; however, they have to be protected against pollution from local sources like human and animal waste (sources of pathogens and nitrates) as well as from potential industry (tanneries, textile industry, flower plantations, etc.). The minimum required distance of water supply wells and potential pollution sources should be maintained during water resources development in towns and villages. The same level of interest should also be applied to the development and protection of groundwater resources for rural communities. It should be necessary to start with relatively concentrated communities where the feasibility and impact of developed schemes will be most significant. This problem is accelerated by the fact that the main aquifers of the area are highly vulnerable karstified aquifers developed in limestone in the eastern part of the sheet.

In addition to priority in development of groundwater for safe drinking water supply it should be possible to select the most yielding wells for livestock watering and possibly small scale irrigation to increase the stability of food supply in prolonged periods of drought. The problem was discussed by Tsur and Issar (1998) who stated that if, as it commonly found in reality, the supply of surface water is uncertain then groundwater plays a role in addition to that of increased water supply: the role of a buffer that mitigates the undesired effects of uncertainty in supply of surface water.

Development and protection of the water resources of the area and the environment as a whole have a principal importance for the development of the infrastructure with subsequent impacts upon the eradication of poverty (development of irrigated agriculture, maintaining livestock during drought). Access to drinking water changes the life of women, when a shorter distance for fetching water provides more time for family care and improves the health level of the population (statistics show that 40 % of child death rates is related to water born diseases). About 15 % of the rural population has access to safe drinking water in the area and about 70 % of infections are related to contaminated water resources. This is a serious problem for the creation of strong farm and pastoral communities capable of full time engagement in agricultural activity. It is therefore important to provide safe drinking water to rural communities. Protection of the environment, particularly prevention of soil erosion and degradation leading to food and water scarcity, is an important development aspect for rural communities within the area. This aspect is based on the importance of water retention which is of primary importance with regard to the increase in population numbers, bringing with it an increase in demands on soil use.

Another important task for the future development of knowledge about the groundwater resources of the area is the monitoring of fluctuations in groundwater levels and quality. It would be necessary to drill several monitoring wells within the aquifers for this purpose. It is recommended to drill these wells as additional monitoring equipment for climatic stations and

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conduct groundwater monitoring together with measurements of climate characteristics (Negele, Siru, Genale Donto, Zembaba Wiha, etc.). Selection of monitoring points for observation of groundwater level (quantity) and quality fluctuations in aquifers should be discussed with the Wereda Water Offices.

Results of water resources assessment show that the area is rich in both surface water and groundwater providing a good potential for future development. From the point of view of food security it is highly recommended to make the use of surface water for irrigation and subsequent increase in agriculture production a priority. Several rivers can be used for small hydropower schemes. Considering the surface and groundwater potential of the Negele area:

1. Surface water is sufficient for irrigation of 15 % of the Negele map sheet area (dominantly along Genale and also Dawa, Awata, Mormora rivers) considering the use of 10,000 m3/ha annually.

2. In the case of the groundwater consumption of 20 l/d by the recent population the demand will represents less than 1 % of the assessed groundwater resources with the potential to supply people with relatively good quality drinking water (50 % of developed water points meet requirements of drinking water standards).

The potential of the area provides feasible and environmentally sound water management.

6.3 Human and Land Use Resources and DevelopmentThere is a large human resource potential within the area. The total assessed population is

about 0.4 million and average urban and rural population growth in the Oromia region is 2.9 % and Somali 2.6 %. Taking this into account the population of the area will double in the next 20–25 years. This represents a large potential of manpower for agricultural production as well as for developing industry using the area s natural resources. Agricultural irrigation should be practiced on the arable land and part of the area cover classified as pasture should also be used for arable land, and livestock husbandry should use more effective methods of livestock breeding.

Improvement of the health status of inhabitants using safe water supply systems and utilization of the remaining water resources for agricultural irrigation and the possibly for small hydropower schemes and industrial development (using other natural resources of the area) will improve the standard of life and help to eradicate poverty within this part of Ethiopia.

6.4 Wind and Solar Energy DevelopmentThe area has a good potential for the development of solar and wind energy. It should be feasible

to use the produced energy for local supply e.g. running pumps for groundwater development or for distribution of irrigation water. It could also be feasible to use this electricity for running local small businesses as grain mills, food processing and conservation industry etc.

6.5 Environmental Problems and their Control / ManagementAttention is paid to the eradication of poverty, protection of the environment and natural

resources as well as the increase in education in this field. The explanatory notes provide information for planning in sustainable economical development, other sectorial planning, management in the use of natural and human resources and protection against natural hazards. The study concentrates on the identification and protection of water resources, soil (particularly protection of soil against erosion), protection against natural hazards and wastewater and solid waste management.

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Protection of water resources should be concentrated on better practices in sanitation within towns, villages and rural settlements. About 50 % of surface and groundwater is good in quality and can be used directly for drinking, agricultural and industrial purposes (see Chapter 5). Indication of improper sanitation practices is reflected in the increase of nitrates from human and animal wastes in the shallow groundwater that is used by drilled and dug wells. Water development practices should be based on basic principles of protection as follow:

1. The source of groundwater should not be drilled and/or dug directly in the center of the village/town.

2. The final design of the well and distribution system should prevent direct percolation of water from the surroundings of the well along its casing to the groundwater.

3. A well should be designed upstream from the groundwater flow direction in respect to existing and potential pollution sources.

4. The required minimal protection zones should be respected by land use development in the vicinity of wells/well fields.

5. Regular monitoring of water levels and quality should be performed.6. There should be improvements in the general application of sanitation and waste

management practices.

Soil erosion and protection is one of the limiting factors of sustainable development of agriculture within the area. The Vice-Minister (ENA) of Agriculture disclosed that Ethiopia is losing 1,900 million tons of soil through erosion every year. In the opening of a three-day workshop on soil fertility management, the Vice-Minister Ato Getachew Tekelemedhin said the country is losing 600 million Birr per annum due to reduced agricultural production triggered by the effects of soil erosion. If the current trend continues unabated, a sizeable farming community in the country would be forced to earn their livelihood from sources other than farming. The prominent factors for soil degradation in Ethiopia, according to the Vice-Minister, were population pressure, deforestation, poor agricultural techniques, overgrazing and drought. He noted that the Soil Fertility Initiative (SFI) launched by the World Bank and the UN Food and Agriculture Organization played an important role in preventing soil degradation in sub-Sahran countries including Ethiopia. Addressing the workshop, Mr. Ismail Serageldin, the Vice-President of a World Bank special program, expressed the bank‘s readiness to support Ethiopia‘s soil fertility initiative.

Data about soil erosion in the area are scare. The human causes of soil erosion relate mainly to ploughing, and harvesting seasons and its coincidence with the season with the heaviest rainfall when crop cover is limited. Another human factor which contributes to soil erosion is the short fallow period (one to four years). Soil burning which destroys the organic matter content of the soil is another adverse factor.

Traditional soil cultivation and conservation techniques use ditches for drainage. The ditches run diagonally across the slope, usually with a gradient of more than 5 %. These ditches are made by ploughing deep into the ground. The spacing of the drainage ditches in a field depends on the steepness of the slope, the steeper fields having more drainage ditches than fields on gentler slopes.

Anti-erosion measures consist of several techniques. Some of the most frequent techniques can be defined as follows:

1. The steep slopes of the highlands should be reforested.2. This area as well as parts of gorges, where reforestation is not possible, can be terraced

(similar to the Konso area and/or on the slopes at the northern part of the country).3. Retention of water in the countryside–construction of small dams (even on intermittent

rivers) for irrigation can help not only for the accumulation of water for irrigation, but also to slow down runoff after heavy rains and the accumulation of suspended material (eroded

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soil) in small dams. The accumulated material can be subsequently excavated and used as a fertilizer for arable land.

4. Wicker fascine–is a cheap and very simple anti-erosion measure that can be practiced in all parts of the area either separating agricultural fields of individual owners or implemented inside the field when the fields are big enough and highly prone to erosion.

5. Creation of shrubs/tree rows preventing wind erosion and slowing down surface runoff.6. Covering artificial cuts (along roads and other constructions) by nets or geo-textile.7. Other technical measures and agricultural practices.

A focus on soil conservation is one of the most important factors for environmentally sound land use. Soil conservation contributes significantly to food security in the area.

Natural hazard and protection against the consequences of earthquakes, land slides, rock falls and other hazards is important for the preservation of human lives, property and arable land.

Susceptibility to exogenous risks differs both in quantity and quality between the valley and plain engineering geological provinces. The following natural hazard potentials have been identified:• Slopes of the deep erosion valleys and mountain slopes with repeated rockslides of all sizes and

small to medium sized rockfalls. • Repeated rock falls along the upper rims of the deeply cut valley sides. • River flood plains have been included into risk susceptible units because of the possibility of

floods which can be very severe in arid areas. The observed lithological-structural changes in cuts of alluvial soils indicate the occurrence of catastrophic floods carrying substantially increased volumes of coarse materials in sub-historical times.

• Generally, the clay rich soils covering sedimentary rocks are prone to high plasticity and swelling when wet. That makes them rather problematic not only for building but also as material for earth roads especially during the rainy season.

• Soil erosion and protection has been addressed above so we can say that areas especially susceptible to erosion are medium energy relief in residual and colluvial soil units. An intensive deforestation in these areas will result in a further increase in the erosion susceptibility.

Susceptibility to endogenous risks has to be taken seriously also. Earthquakes are common in Ethiopia, but there is not enough information to assess the hazard potential of the Negele area.

Waste water and solid waste management is important for environmentally sound development of the area. Appropriate management in this field protects not only the environment and soil and water resources but also human health against exposure to harmful pathogens and chemicals.

Recent practice is to release wastewater from households directly to the environment. Wastewater is discharged directly to rivers without appropriate treatment where it is mixed with surface water and is used for drinking by people living downstream from wastewater discharge. People use this polluted water from the river without any knowledge about the potential harm to their health. There is little chance to educate a large number of people about the possible adverse health impact of using polluted water and that is why the waste water producers have the responsibility to treat the water to remove substances harmful for human health.

Infiltration of polluted water to groundwater threatens the groundwater resources of the area. It is very well documented by the increasing content of nitrates in groundwater.

Solid waste management is not practiced in any of the sites visited within the area. Increasing environmental care and protection of natural resources will contribute to better living standards

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of the people living within the area and also to an increase in their working output leading to an increase in food security.

6.6 Touristic Potential of the AreaThe Negele sheet has a low touristic potential because of the climate and remoteness of the

area.

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Over the past 40 years natural disasters on the Ethiopian territory have increased both in frequency and intensity and have led to severe social impacts, particularly in the southeastern part of the country. Evidence has long suggested that disaster risk reduction has a high cost-benefit ratio. Disasters also divert a substantial amount of national resources from development to relief, recovery and reconstruction, depriving the poor of the resources needed to escape poverty. Disasters cannot be avoided but there are ways to reduce risks and to limit their impacts. The action comprises preparedness, mitigation and prevention. It aims to enhance resilience to disasters and is underpinned by knowledge on how to manage risk, build capacity, and make use of information and communication technology as well as earth observation tools. Ethiopia is prone to natural risks like landslides, rock falls, flooding and particularly drought as reflected in geological, historical as well as recent records. Two or three subsequent periods of intense drought can cause severe crop losses, famine and population displacement in the country. The country also faces an increased risk due to climate change and more extreme weather which can be accelerated particularly in the vulnerable semi-arid part of the country. The insufficient quality of drinking water, the natural risks and the overall degradation of the environment are all fundamental problems and contribute to an increase in the rate of migration to urban areas.

These explanatory notes to the hydrogeological and hydrochemical map of the Negele area provide the results of the joint Czech Ethiopian projects. The mapping activity was carried out by field groups of hydrogeologists of the GSE in framework of the project “Groundwater Resources Assessment of the Southeastern Highlands and Associated Lowlands of Ethiopia” in 2010. The mapped area covers 18,370 km2 and is inhabited by 0.4 million people.

Groundwater accumulates in porous aquifers of alluvial and elluvial origin and in fissured and karst aquifers hosted in sedimentary (particularly limestone), volcanic and basement rocks.

There is a relatively good potential for development of surface water for small-scale irrigation and electricity generation in the area because the Genale, Dawa, Awata and Mormora rivers and several intermittent rivers drain groundwater of volcanic, limestone and basement aquifers. It is necessary to consider that the groundwater level in the aquifers will fall to greater depths during periods with inadequate precipitation and river flow fed by groundwater will disappear during periods of drought in most of rivers of the area.

Groundwater is relatively of good quality and about 50 % of the groundwater resources can be directly used for drinking, industrial as well as agricultural purposes. Groundwater should be primarily used for drinking water supply; it should be also used for irrigation should there be clear evidence that pumping for irrigation does not lead to over pumping of the aquifer, undermining of groundwater resources and causing degradation of the aquifer. Should the aquifer be used for irrigation, monitoring wells are recommended to be drilled together with the production

ConclusionsConclusions

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wells for systematic observation of changes in groundwater levels, quality of pumped water and optimization of the pumping system.

Local pollution of groundwater by nitrates is common in rural as well as in urban areas. In the case of developed springs their surroundings should be protected against pollution because most of the springs have shallow groundwater circulation and human as well as animal waste (problem of watering animals directly from the spring) can easily and quickly penetrate the groundwater resources. This is also a problem in karst aquifers which are highly vulnerable to pollution because of their high permeability. The spring should be developed by a solid concrete box and it is preferable that the water will flow from the spring by a tube and distributed to people 10–20 m from the spring (lower position of water distribution point). The area of the protection box should be protected against the entry of people and animals; in particular animals should be completely prevented entry.

It is advisable to use geophysical investigation to select locations where the regolith is thick and volcanic, sedimentary and basement rocks are deeply fractured, weathered and soft for siting wells. Groundwater can be totally missing when the regional groundwater table is not reached in cases where the drilled part of the basalt or basement is massive without joints and fissures. It is also true for aquifers in limestone where groundwater is deep and its level is controlled by the level of surface water or the level of principal springs representing the regional drainage of the area.

The water distribution well should preferably be equipped with a system minimizing discharge of water when it is filled into containers. In the case that water is used for animal watering it should be transported by a tube and distributed to the animals about 20–30 m from the well (lower position of water distribution point – cattle bin). The area of the well head should be protected against accumulation of surface water by drainage ditches and the entrance of animals to the well’s surroundings should be completely eliminated.

The proposed development should take into consideration the protection and conservation of the natural resources of the area. Particular interest should be paid to soil conservation and groundwater protection using the appropriate agricultural methods to decrease soil erosion and to the implementation of water resource protection to protect groundwater against pollution and over pumping, particularly in rural and urban settlements where pollution by nitrates is increasing. Monitoring of environmental components, particularly surface water flow and sediment load, in gauging stations in the lower reaches of the river should be enhanced. Recent inappropriate wastewater and waste management has to be considerably improved.

Despite some local and regional environmental problems the Negele area provides the potential for feasible and environmentally sound natural and human resource management.

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