study on irrigation water management in … · k. innocent munyentwari. iii abstract ... l...
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REPUBLIC OF RWANDA
MINISTRY OF EDUCATION
HIGHER INSTITUTE OFAGRICULTURE AND ANIMAL HUSBANDRY (I.S.A.E)
FACULTY OF AGRICULTURAL ENGINEERING AND ENVIRONMENTALSCIENCES
DEPARTMENT OF SOIL AND WATER MANAGEMENT
Presented by:
K. Innocent MUNYENTWARI
For fulfilment for the requirement of the
Bachelor's degree (Ao)
Option: Irrigation and Drainage
Research Supervisor:
Er. SURESH Kumar Pande (M. Tech),
Busogo, December 2011
STUDY ON IRRIGATION WATER
MANAGEMENT IN BASE I SWAMP
RUHANGO DISTRICT
REPUBLIC OF RWANDA
MINISTRY OF EDUCATION
HIGHER INSTITUTE OFAGRICULTURE AND ANIMAL HUSBANDRY (I.S.A.E)
FACULTY OF AGRICULTURAL ENGINEERING AND ENVIRONMENTALSCIENCES
DEPARTMENT OF SOIL AND WATER MANAGEMENT
Presented by:
K. Innocent MUNYENTWARI
For fulfilment for the requirement of the
Bachelor's degree (Ao)
Option: Irrigation and Drainage
Research Supervisor:
Er. SURESH Kumar Pande (M. Tech),
Busogo, December 2011
STUDY ON IRRIGATION WATER
MANAGEMENT IN BASE I SWAMP
RUHANGO DISTRICT
REPUBLIC OF RWANDA
MINISTRY OF EDUCATION
HIGHER INSTITUTE OFAGRICULTURE AND ANIMAL HUSBANDRY (I.S.A.E)
FACULTY OF AGRICULTURAL ENGINEERING AND ENVIRONMENTALSCIENCES
DEPARTMENT OF SOIL AND WATER MANAGEMENT
Presented by:
K. Innocent MUNYENTWARI
For fulfilment for the requirement of the
Bachelor's degree (Ao)
Option: Irrigation and Drainage
Research Supervisor:
Er. SURESH Kumar Pande (M. Tech),
Busogo, December 2011
STUDY ON IRRIGATION WATER
MANAGEMENT IN BASE I SWAMP
RUHANGO DISTRICT
ii
ACKNOWLEDGEMENT
First and foremost, I thank the almighty God for his abundant blessings and protection during my
field work.
I feel highly indebted to Dr. Charles KAREMANGINGO, the Rector of Higher Institute of
Agriculture and Animal Husbandry, ISAE-Busogo for making excellent environment for
pursuing our studies in the institute.
I am grateful to Professor Dr. SANKARANARAYANAN, Dean, faculty of Agricultural
engineering and environmental sciences for his permission to take up my project research.
My deep sense of gratitude is due to Mr. Suresh Kumar Pande, Head department of Soil and
Water management at the same time my research supervisor for his valuable guidance,
collaboration and constructive suggestion, encouragement and his dedication which helped me to
come to the successful completion of this project work.
I offer my sincere gratitude to all academic staff of the Department of Soil and Water
management. I am grateful to my class mates and all well wishers who helped me to conduct
some of my activities. My heartfelt thanks goes to my dear parents, aunt Floride CYIZA,
colleagues and friends who have shown great understanding and sympathy while I have been
involved in the preparation of this study report.
K. Innocent MUNYENTWARI
iii
ABSTRACT
Because of increasing population on the earth it has become inevitable to enhance agricultural
food production on limited land resources. Water is one of the most important inputs for crop
production. Irrigation has been found the best option to cope with the climatic vagaries affecting
mankind for food security. In the process of irrigation, water is lost during storage, conveyance,
application and for accomplishing special needs of plots being irrigated. Identifying the various
components/ways of water losses and knowing what improvements can be made is essential for
making the most effective use of irrigation water.
Therefore a study was undertaken in Base I swamp in Ruhango district with the objectives of
evaluation of irrigation water distribution efficiency and determination of design discharge for
main and secondary canal/channel for irrigating rice crop. To assess the above objectives
detailed methodology is developed for carrying out the study.
The study revealed that the soils are highly permeable with 17.1 cm/hr hydraulic conductivity
(rapid class) leading to high water losses. Results also indicated that very low irrigation
efficiency with 18% poses high water losses to the tune of 82%. The required discharge of main
and secondary channel found was 0.152 cumecs and 0.076 cumecs respectively. The desired
cross section of main and secondary channel found was 0.47 sq.m and 0.27 sq.m respectively.
The secondary channel bed width as proposed by the project was more (0.1m) than actually
found due to siltation and weed infestation. However due to problems such as lack of water
during dry season, weeding, siltation, canal erosion, low use of fertilizers etc; farmers are not
able to get more production in whole areas. The production level of rice in the study area is 5.6
tons/ ha. It is evident that to produce 5.6 tons of rice from one hectare land, an amount of 22970
cubic meter water will be required. The total water losses per day in the whole swamp are
estimated as 1680 cubic meter. Therefore the water use efficiency was 0.24 kg/cubic meter. The
7 hectares are subjected to water logging due to low elevation with respect to water level of main
stream during rainy season and water stagnation during rest of the year. This leads to the losses
of cultivated crops. It is clear from the results that there is a need for good water management for
getting the benefits of irrigation in 105 ha of Base I swamp.
iv
RESUME
En raison de la population croissante sur le monde il est devenu inévitable pour augmenter la
production agricole de nourriture sur les ressources de terre limitées. L'eau est l'une des entrées
les plus importantes pour la production agricole. L'irrigation a été trouvée la meilleure option à
faire face aux extravagances climatiques affectant l'humanité pour la sécurité de nourriture.
En cours d'irrigation, l'eau est perdue pendant le stockage, transport, application et pour
accomplir les besoins spéciaux des parcelles de terrain étant irriguées. L'identification des divers
composants / manières des pertes d'eau et savoir quelles améliorations peuvent être apportées est
essentielle pour faire l'utilisation la plus effiicace de l'eau d'irrigation. Par conséquent une étude
a eu lieu dans le marais de Base I dans le District de Ruhango avec les objectifs d'evaluer
l’efficacité de distribution d’eau d'irrigation et de la détermination le débit de conception pour
canal principal et secondaire.
Évaluer la méthodologie détaillée ces objectifs est développée pour effectuer l'étude. L'étude a
indiqué que les sols sont fortement perméables avec la conductivité hydraulique de 17.1 cm/hr
menant à d'énormes pertes d'eau. Les résultats ont également indiqué que l'efficacité très basse
d'irrigation avec 18% pose des pertes d'eau élevées pour un montant de 82%. Le débit
exigé par le canal principal et secondaire était 0.152 m3/s et 0.076 m3/s respectivement. La
section voulus pour les canaux primaires et secondaires était 0.47m2 et 0.27m2 respectivement.
La base du canal primaire et secondaire propose par le projet était de (0.1m) plus grand que
celles qui existent sur terrain. Partant des problèmes tels que la manque de l'eau pendant la saison
sèche, le sarclage, l'envasement, l'érosion de canal, mauvaise utilisation des fertilisants, etc. Les
agriculteurs ne peuvent pas obtenir plus de production dans leur exploitation. Le niveau de
production du riz est 5.6 tonne/ha. Il est évident que pour produire 5.6 tonnes/ha, une quantité de
22970 m3 l'eau est exigée. La quantité total d’eau pérdu par jour est estimé à 1680 mètre cube.
Par conséquent l'efficacité d'utilisation de l'eau s'est avéré 0.24 kg/m3. Les 7 hectares
connaissaient les problèmes d’inondation dans les saisons pluvieuse et de stagnation dans le reste
de l’an. Ce la mène a la perte des cultures. Elle est claire des résultats qu'il y a un besoin de
gestion appropriée de l'eau pour retirer les avantages de l'irrigation dans le marais de Base I.
v
TABLE OF CONTENT
DEDICATION ............................................................................................................................................... i
ACKNOWLEDGEMENT ............................................................................................................................ ii
ABSTRACT................................................................................................................................................. iii
TABLE OF CONTENT ................................................................................................................................ v
LIST OF FIGURES ..................................................................................................................................... ix
LIST OF TABLES ........................................................................................................................................ x
LIST OF APPENDICES .............................................................................................................................. xi
LIST OF ABREVIATIONS ....................................................................................................................... xii
CHAPTER-1 INTRODUCTION .................................................................................................................. 1
I. 1 Problem statement .................................................................................................................................. 1
I. 2 Principal objective .................................................................................................................................. 2
I. 3 Specific objectives .................................................................................................................................. 2
I. 4 Hypotheses.............................................................................................................................................. 2
I.5 Justification of the study .......................................................................................................................... 2
CHAPTER-2 LITERATURE REVIEW ....................................................................................................... 3
2.1 Importance of irrigation .......................................................................................................................... 3
2.2 Harmful effects of over-irrigation........................................................................................................... 3
2.3 Classification of irrigation methods ........................................................................................................ 4
2.4 Irrigation requirement of rice .................................................................................................................. 5
2.4.1 Good water management practices for rice cultivation........................................................................ 5
2.4.3 Land levelling ...................................................................................................................................... 5
2.5 Swamps development in Rwanda ........................................................................................................... 7
2.5.1 Classification of the swamps in Rwanda ............................................................................................. 7
2.5.2 Management and use of swamps ......................................................................................................... 8
vi
2.5.3 Legal aspects of swamps ...................................................................................................................... 8
2.6 Size of the Basin ..................................................................................................................................... 9
2.7 Shape of the Basin .................................................................................................................................. 9
2.8 Elevation of the watershed ....................................................................................................................10
2.9 The type of arrangement of stream channels ........................................................................................10
2.9.1 Order of stream ..................................................................................................................................10
2.9.2 The length of tributaries .....................................................................................................................11
2.9.3 Stream density....................................................................................................................................11
2.9.4 Drainage density ................................................................................................................................11
2.9.5 Other factors.......................................................................................................................................12
2.10 Various formulas to compute the discharge ........................................................................................12
2.11 Movement of water into the soil .........................................................................................................14
2.11.1 Infiltration ........................................................................................................................................14
2.11.3 Measurement of infiltration .............................................................................................................15
2.11.4 Permeability .....................................................................................................................................15
2.12 The technical aspects in irrigation network ........................................................................................16
2.12.1 Irrigation efficiency .........................................................................................................................16
2.12.2 Water requirement of crops..............................................................................................................16
2.12.3 Available Water (AW) .....................................................................................................................17
2.12.4 Project irrigation efficiency .............................................................................................................18
2.12 .4.1 Water Conveyance Efficiency .....................................................................................................18
2.12.4.2 Water Application Efficiency .......................................................................................................19
2.12.5 Efficiency of irrigation practices, water use and operation of irrigation system .............................19
2.12.5.1 Water Storage Efficiency ..............................................................................................................19
2.12.5.2 Water Distribution Efficiency .......................................................................................................20
2.12.5.3 Water Use Efficiency ....................................................................................................................20
vii
2.12.6 Economic (irrigation) efficiency irrigation system ..........................................................................21
CHAPTER-3 MATERIALS AND METHODS .........................................................................................22
3.1 Study zone description ..........................................................................................................................22
3.1.1 Climate ...............................................................................................................................................22
3.1.2 Soil .....................................................................................................................................................22
3.1.3 Crop ...................................................................................................................................................22
3.2 Materials ...............................................................................................................................................23
3.3 Methodology .........................................................................................................................................23
3.3.1 Discharge measurements procedure...................................................................................................23
3.3.2 Estimated irrigation water conveyance efficiency in the perimeter ...................................................24
3.3.3 Hydraulic conductivity tests...............................................................................................................25
3.3.4 Calculation of evapo-transpiration, infiltration, percolation ..............................................................25
3.3.4.1 Water losses at plots level ...............................................................................................................25
3.3.4.2 Irrigation water requirement using cropwat ....................................................................................26
3.3.4.3 Design of irrigation main and secondary canals .............................................................................26
CHAPTER- 4 RESULTS AND DISCUSSION..........................................................................................29
4.1 Crop water requirements .......................................................................................................................29
4.1.1 Monthly climatic data for Base I rice swamp ....................................................................................29
4.1.2 Monthly evapotranspiration ...............................................................................................................30
4.1.3 Crop water requirement and irrigation requirement...........................................................................31
4.2. Hydraulic conductivity.........................................................................................................................32
4.3 Computation of Irrigation Efficiency....................................................................................................34
4.3.1 Measurement of Discharge ................................................................................................................34
4.3.2 Comparison of Project design dimensions and actually found at field for secondary channel ..........35
4.3.2.1 Channel bed width ..........................................................................................................................35
4.3.3 Measurement of flow velocity ...........................................................................................................36
viii
4.3.4 Discharge ...........................................................................................................................................36
4.3.5 Water conveyance efficiency .............................................................................................................37
4.4 Water losses calculation in Base I swamp at plots................................................................................38
4.4.1 Calculation of percolation, infiltration and evapotranspiration .........................................................38
4.4.1.1 Results of water losses calculation of nine plots in Base I swamp. ................................................38
4.4.2 Water use efficiency ..........................................................................................................................39
4.4.3 Determination of discharge for main and secondary channels ..........................................................40
4.5 Design of primary and secondary channel ............................................................................................40
4.5.1 Design of primary channel .................................................................................................................40
4.5.2 Design of secondary channel .............................................................................................................42
CHAPTER-5 CONCLUSION AND RECOMMANDATIONS .................................................................45
REFERENCE..............................................................................................................................................47
APPENDICES ............................................................................................................................................49
ix
LIST OF FIGURES
Figure 1: Levelled and non levelled land ...................................................................................................... 5
Figure 2: Average monthly climatic data for Byimana meteorological station (1979-2009) .....................29
Figure 3: Average monthly ETo for Base I swamp ....................................................................................30
Figure 4: Average monthly rainfall and effective rainfall ..........................................................................31
Figure 5: Crop water requirement and irrigation requirement from June – November ..............................32
Figure 6: Secondary canal bed width variation...........................................................................................35
Figure 7: Economic trapezoidal channel.....................................................................................................41
Figure 8: Primary canal...............................................................................................................................42
Figure 9: Secondary canal...........................................................................................................................44
x
LIST OF TABLES
Table 1: Permeability classes based on hydraulic conductivity of soil. ......................................................16
Table 2: Approximate Available Moisture Holding Capacity of Soils .......................................................17
Table 3: Data collected during the field study ............................................................................................33
Table 4: Secondary channel cross section and wetted area.........................................................................35
Table 5: Average values of flow velocities at three locations of secondary channel ..................................36
Table 6: Discharge in secondary channel ...................................................................................................36
Table 7: Conveyance efficiency..................................................................................................................37
xi
LIST OF APPENDICES
Appendix 1: Crop water requirement from June to November...................................................................49
Appendix 2: Byimana Mean climatic data used as input into Cropwat 8 ...................................................49
Appendix 3: ETo from Byimana mean climatic data using Cropwat 8 ......................................................50
Appendix 4: Rainfall climatic data used in Cropwat 8. ..............................................................................50
Appendix 5: Crop characteristic (rice) ........................................................................................................51
Appendix 6: Crop irrigation scheduling by cropwat ...................................................................................51
Appendix 7: Answer from the farmers of Base I swamp about production (2011) ....................................52
Appendix 8: Ruhango district map .............................................................................................................52
Appendix 9: Climatological data from Byimana meteorological station (1978-2009)...............................53
xii
LIST OF ABREVIATIONS
AAA: Agro Action Allemande
FAO: Food and Agricultural Organization
FC: Field Capacity
GIR: Gross Irrigation Requirement
MINAGRI: Ministère de l’Agriculture et des Resource animal.
NGOs: Non Governmental Organizations
NIR: Net irrigation Requirement
ORSTROM: Office de Recherche Scientifique et Technologique d’outre Mer
CORIBARU: Cooperative des Riziculteurs du Base/ Ruhango
RSSP: Rural Sector Support Project
ZIGAMA CSS: ZIGAMA Credit and Savings Service
1
CHAPTER-1
INTRODUCTION
The agriculture necessitates the optimum and economic use of water in crop production
process. Though water is plentiful on the earth surface, its exploitation and use for
development of agriculture has not been taken into consideration. The area under irrigation in
the world was estimated to only 241.5 Mha and was only 15.98% in the year 1996 (FAO,
1998).
The agriculture production system in Rwanda is characterized by small arable lands, with a
familiar exploitation of 0.8 hectares (TRANSTEC, 2001). In Rwanda, the arable lands are
estimated to 1,380,000 ha (52% of the total lands) for an estimate of about 8.5 millions
population in 2000 year and for a physical density of 325 population/square km
(TRANSTEC,2001).
The swamps occupy the extent of 165,000 ha with only a half cultivable (MINAGRI, 2000).
The irrigation and drainage are applied only for 5992 hectares of development swamp, with
2584 hectares involved in rice cropping (MINAGRI, 2004). Better swamp management will
increase the higher productivity in addition to the hilly cultivation. The government of
Rwanda has been involved in swamp development to enhance the production by providing
the more easily achievable explorable areas in swamps; this is mainly due to irregularity in
rainfall and increase in population growth and to give a guarantee though optimal
productivity of swamps, by better management of irrigation network. Once irrigation not
properly maintained will result in poor production.
The better distribution and management of water in swamp will help in satisfying future food
demands, for that our work in Base I swamp, case study will conduct into: « The study of
water distribution efficiency in Base I rice swamp, Ruhango District ».
I. 1 Problem statement
There is the problem of spatial and temporal distribution of rainfall that has resulted in
soil erosion and moisture stress on agricultural crops; the major cause of low
productivity.
2
Low interest of the farmers to use appropriate agricultural techniques in order to
increase crop production.
In Base I rice swamp; there is poor distribution of water which occurs over the
secondary and primary canals in irrigation system. The northern part of the swamp
gain excess in water while the southern part has a serious deficiency in water and the
problem of water logging.
I. 2 Principal objective
The main objective of this study is the evaluation of current system of irrigation water
distribution and its efficiency in Base I rice swamp and recommendations for proper
management.
I. 3 Specific objectives
The specific objectives of this study are:
To determine the hydraulic conductivity in Base I rice swamp.
To determine irrigation water requirement through cropwat
To determine the irrigation water flow discharge for main and secondary channels
and comparison of cross section (secondary channel) in proposed and actually
implemented.
To determine water losses of the system and irrigation efficiencies.
I. 4 Hypotheses
The following hypotheses are formulated:
Poor water management yields low water use efficiency.
I.5 Justification of the study
To ensure food security in Rwanda it is vital to use its natural resources like land and
water appropriately. Therefore, swamps which are potential source of agricultural
production have to be exploited efficiently. In this regard, Base I swamp was taken to
study irrigation water efficiency. This study would help to know the causes of low
production and present status of water use. Recommendations would help improving land
and water management for sustainable production.
3
CHAPTER-2
LITERATURE REVIEW
2.1 Importance of irrigation
Irrigation is the artificial application of water, with good economic return and no damage to
land and soil, to supplement the natural sources of water to meet the water requirement of
crops (Majumdar, 2004). A crop requires a certain amount of water at some fixed interval
throughout its period of growth. If the water requirement of the crop is met by natural rainfall
during the period of growth, there is no need of irrigation. Crops receive water from natural
sources of water in forms of precipitation, other atmospheric water, ground water and flood
water. Since the amount, frequency and distribution of precipitation which is the principal
source of water for crops are unpredictable, may be insufficient, unevenly distributed,
untimely, and the ground water may be too deep in the soil profile, irrigation becomes
necessary for successful crop growing. Irrigation should, however, be profitable and applied
in times of crop need and in proper amount. Adequate and timely irrigation leads to high
yields. The excess or under irrigation may damage lands and crops. Irrigation applied earlier
to the actual time of crop need results in ineffective irrigation and waste of water, while
delayed irrigation may cause water stress to crops and reduce the yield (Majumdar, 2004).
Irrigation is the key input in crop production. Full benefit of crop production technologies
such as high yielding varieties, fertilizer use, multiple cropping, crop culture and plant
protection measures can be derived only when adequate supply of water is assured on one
hand. High yielding varieties usually have a higher water requirement than ordinary varieties.
The yield potential of these varieties can be fully exploited if an adequate amount of water is
made available, besides other inputs (Gautam and Dastane, 1970). On the other hand,
optimum benefit from irrigation is obtained only when other crop production inputs are
provided and technologies applied. With adequate supply of water and other inputs, crop
production technologies such as multiple cropping can be profitably applied to boost up
growth and yield of crops (Majumdar and Mandal, 1984).
2.2 Harmful effects of over-irrigation
Irrigation is beneficial only when it is properly managed and controlled. Faulty and careless
irrigation does harm to crops and damages lands, besides causing waste of valuable water.
Rice is the exception and it is grown under soil submergence.
4
When plenty of water is available, most of inexperienced irrigation farmers are tempted to
over-irrigate their lands because they assume that with more water, they will get higher yields
without being conscious of the harmful effects. If the water is used judiciously and
scientifically, there would be practically no ill-effects. Therefore, wide knowledge and
experience are required for efficient water management. These following ill-effects can be
effectively reduced and sometimes altogether eliminated by exercising economical and
scientific use of water.
2.3 Classification of irrigation methods
Water management pertains to optimum and efficient use of water for best possible crop
production keeping water losses to the minimum. Serious water losses occur unless it is
properly monitored while irrigating fields. Various methods are adopted to irrigate crops and
the main aim is to store water in the effective root zone uniformly and in maximum quantity
possible ensuring water losses to the minimum. Each method of irrigation has certain
advantages and disadvantages based on certain principles. Factors such as the water supply;
the type of soil; the topography; the land and the crop to be irrigated; socio-economic, health
and environmental aspects, determine the correct method of irrigation to be used. Whatever
the method of irrigation, it is necessary to design the system for the most efficient use of
water by the crop.
According to Majumdar (2004), the common methods of irrigation are broadly grouped
under:
(1) Surface irrigation methods;
(2) Subsurface or sub-irrigation methods;
(3) Overhead or sprinkler irrigation methods;
(4) Drip or trickle irrigation method.
As it is stated above, irrigation is an artificial application of water for creating favorable
conditions for plant growth. The right quantity of water at appropriate time plays an
important role in crop production. The control of water losses during irrigation is an
important aspect to save this priceless commodity. Irrigation water may be applied to crop by
flooding it on the field surface, by applying it beneath the soil surface, by spraying it under
pressure or by applying it in drops.
The surface irrigation method refers to irrigating lands by allowing water to flow over the
soil surface from a supply channel at upper reach of the field (Majumdar, 2004).
5
The subsurface irrigation, also designated as sub-irrigation, involves irrigation to crops by
applying water from beneath the soil surface either by constructing trenches or installing
underground perforated pipe lines or tile lines. The sprinkler irrigation refers to application
of under pressure water to crops in form of spray from above the crops like rain and that is
the reason why it is also called the overhead irrigation. The drip irrigation, also called trickle
irrigation, refers to the application of water at a slow rate drop by drop through perforations
in pipes or nozzles attached to tubes spread over the soil to irrigate a limited area around the
irrigation (Majumdar, 2004).
2.4 Irrigation requirement of rice
2.4.1 Good water management practices for rice cultivation
Worldwide, water for agriculture is getting increasingly scarce. By 2025, 15-20 million
hectares of irrigated rice may suffer water scarcity. Therefore, care must be taken to use
water wisely and reduce water losses from rice fields. A few principles exist to “get the
basics right” for good water management in paddy rice (H. C. Zhang, 2009).
2.4.2 Field channels
In many paddy fields, water flows from one field to another through breaches in the bunds.
Under such conditions, water in an individual field cannot be controlled and field-specific
water management is not possible, construction of channels to convey water to and from each
field, or group of fields, greatly improves the irrigation and drainage of water (H. C. Zhang,
2009).
2.4.3 Land levelling
A well-levelled field is a prerequisite for good water management.
When a field is notlevel, water maystagnate in thedepressions whereashigher parts mayfall dry.
source: http://www.knowledgebank.irri.org.
Figure 1: Levelled and non levelled land
Land levelingEffect of uneven fields.
Remedy: land leveling!
6
Rice is a semi-aquatic plant and hence its water requirements are many times more than most
other food crops with the water requirement of 500 to 2500 mm (T. P. Tuong, 2000). It is,
therefore, a major consumer of the water resources of the country and thus needs careful
water management in order to increase its efficiency of water use. Rice is grown under varied
soil and climatic conditions and its effective root zone depth is 60 cm. Though rice could be
grown on a variety of soils, it grows best on clay loams to clays since these are retentive of
moisture and have low percolation rates of 1-5mm/day (Michael, 1981).
The cultural practices of rice vary widely, depending upon the variety and the local soil and
climatic conditions. However, the condition under which rice is grown could broadly be
grouped into two, namely, low land rice and upland rice. Under low land conditions, rice is
generally transplanted on puddled soils and land is kept under submerged conditions by rain
or irrigation water. The practice of puddling and land submergence, in general, has been
found to reduce the percolation losses, check weed growth, increase the availability of plant
nutrients, regulate soil and water temperature, favour the fixation of atmospheric Nitrogen in
soil through algal growth and improve photosynthesis in the lower leaves due to reflected
light from the water surface (Michael, 1981). The practice of shallow submergence directly
save considerable amount of water as compared to deep submergence.
According to (H. Ikeda and al 2008) the practice of continuous shallow submergence,
however, is possible only when the water supplies are adequate and assured. Land also needs
to be scrupulously leveled to facilitate uniform spreading of water. Weeds, especially the
grassy types, also need to be controlled.
Experimental results are available to show that it is not always necessary to follow the
practice of continuous submergence, especially in the rainy season when the humidity is high
and evaporative demands are low. He also added that under these conditions, the practice of
intermittent submergence, i.e. submergence during the critical stage of initial tillering and/or
flowering and maintenance of saturation to field capacity during the rest of the stages give
yields comparable to those obtain under continuous shallow submergence. The water
supplies, if limited, could safely be curtailed during the non-critical stages of crop growth.
The shortage of water during initial tillering and flowering reduce the yield considerably
while the stages of tillering, grain formation and maturity tolerate water stress to a great
extent (H. Ikeda and al 2008).
7
2.5 Swamps development in Rwanda
The word “swamp” denotes the whole soils of low landscape including humid parts and
marshy soils. Swamps are ecosystems, which have recently caught attention, though the need
for their sustainable development deserves further concern. The importance of swamps is
related to their potential to retain large volumes of water, which can be used for system
maintenance and for dry season agriculture. Agricultural production in Rwanda is extremely
dependent on the rainfall pattern and the climatic uncertainty contributes to wide variations in
crop production. Wherever there are swamps there are people, mainly small farmers and
fishermen. This close association between people and swamps draws attention to the
remarkable strategic importance of these ecosystems in the rural economy and the need for an
effective planning, management and conservation strategy. The arable area is about 825,000
ha, hillside slopes (about 660,000 ha) are not exploited in the dry season and marshlands
(about 165,000 ha) are partially exploited in the rainy seasons depending on their degree of
flooding. About 94,000 ha of marshlands are currently exploited (HYDROPLAN, 2002),
mostly the ones called mineral; whereas the remaining large marshlands made up of peaty or
organic soils covered by Papyrus are not cultivated. However, only 4,000 ha of swamps are
fully equipped with irrigation and drainage systems and 1,200 ha are partially equipped.
2.5.1 Classification of the swamps in Rwanda
According to the Ramsar Convention (1971), the definition of wetlands considers a very wide
range of inland, coastal and marine ecosystems, including lakes, flood plains, freshwater
marshes, estuaries and mangroves (Dungan, 1991).
In Rwanda, semi-detailed characterization and classification studies of swamps have been
carried out. There are different types of classification as follows:
a. Classification according to Cambrezy;
b. Classification according to the size of the swamp;
c. Classification according to the natural vegetation;
d. Classification based on the utilization and stages of the development;
e. Hierarchical classification according to the hydrology.
The hydrology of swamps in Rwanda is basically defined on hydrological conditions which
exist on catchments areas. According to HYDROPLAN (2002), the swamps of Rwanda are
classified in three categories:
8
a. The swamps of high altitude which are in general narrow in shape and under certain
conditions can develop the organic soils in peat. They can be used for water storage, but
in some area they can be used for cultivation. These kinds of swamps are for example
Kamiranzovu and Rugezi swamps.
b. The intermediate swamps or swamps of middle altitude which are often more large in
size. They are essentially situated in Central Plateau.
c. The big swamps of low altitude or collectors swamps which are found in the central
part of Rwanda or along the primary hydrographical network composed by rivers
Nyabarongo, Akanyaru and Akagera.
2.5.2 Management and use of swamps
Water management is the key issue in the use and management of swamps. While drainage is
not so problematic in hydromorphic sandy soils, it can be dangerous in peat soils. Under
improved drained conditions, main cultivated crops are rice, maize, beans and vegetables.
Rice is considered as the main crop to be grown on these soils due to its rooting system which
is well adapted to waterlogged conditions and to its growing cycle during the rainy season,
where the peat soils are likely to be flooded. These soils are, however, quite fragile due to the
absence of the mineral component. Therefore, mismanagement of peat soils can lead to its
degradation and permanent loss for agriculture. The major constraints in the swamps
development are among others the lack of experience and technical skills of farmers to
manage water.
The fragile ecosystem of the swamp to be developed is the major problem of a drastic
decrease of agricultural production. The consequences of those problems are very harmful if
care is not taken by those water users.
2.5.3 Legal aspects of swamps
The necessity to develop legislation on marshlands was perceived very early by the
Government of Rwanda, due to the acuteness of land shortage which characterizes the
country. That situation makes swamps the only alternative to reduce pressure on the fragile
slopes as well as to increase production in order to ensure food security for the population.
In 1988, the Rwandan Government began to work out legislation for the exploitation of
swamps with assistance from various international partners such as FAO.
9
In those legal texts, are defined the status and definition of marshlands, their delimitation,
classification, rules of exploitation, institutions in charge, modalities of management,
maintenance and production, contracts of the utilization of the marshlands, etc. According to
the Organic Law of 14/07/2005 (MINIJUST, 2005) determining the use and management of
land in Rwanda as it is stipulated in Article 14, swamps belong to the State private domain
and can be given to associations/cooperatives or private people according to defined
modalities. However, the uplands are personal or for the family. The social implication of this
fact is that the farmer will give more priority and much care to the uplands than the swamps
which do not belong to him. That is why many swamps are not efficiently and properly
managed.
2.6 Size of the Basin
If the area of the basin is large, total flood flow will take more time to pass the outlet, there
by the base of hydrology of the flood flow will widen out, and consequently reduce the peak
flow because total volume of water passing is the same.
2.7 Shape of the Basin
The shape of the drainage basin also governs the rate at which water enters the stream. The
shale of drainage basin is generally expressed by “Farm factor” and compactness coefficient
as defined below.
Farm factor
The axial length (L) is the distance from outlet to the most remote point on the basin, and
average width (B) is obtained by dividing the area (A) by the axial length.
Compactness coefficient =
If A is the area of the basin and re is the radius of the equivalent circle, then
Circumference = = =2
Therefore, compactness coefficient =
Where P= perimeter of the basin
A= area of the basin
9
In those legal texts, are defined the status and definition of marshlands, their delimitation,
classification, rules of exploitation, institutions in charge, modalities of management,
maintenance and production, contracts of the utilization of the marshlands, etc. According to
the Organic Law of 14/07/2005 (MINIJUST, 2005) determining the use and management of
land in Rwanda as it is stipulated in Article 14, swamps belong to the State private domain
and can be given to associations/cooperatives or private people according to defined
modalities. However, the uplands are personal or for the family. The social implication of this
fact is that the farmer will give more priority and much care to the uplands than the swamps
which do not belong to him. That is why many swamps are not efficiently and properly
managed.
2.6 Size of the Basin
If the area of the basin is large, total flood flow will take more time to pass the outlet, there
by the base of hydrology of the flood flow will widen out, and consequently reduce the peak
flow because total volume of water passing is the same.
2.7 Shape of the Basin
The shape of the drainage basin also governs the rate at which water enters the stream. The
shale of drainage basin is generally expressed by “Farm factor” and compactness coefficient
as defined below.
Farm factor
The axial length (L) is the distance from outlet to the most remote point on the basin, and
average width (B) is obtained by dividing the area (A) by the axial length.
Compactness coefficient =
If A is the area of the basin and re is the radius of the equivalent circle, then
Circumference = = =2
Therefore, compactness coefficient =
Where P= perimeter of the basin
A= area of the basin
9
In those legal texts, are defined the status and definition of marshlands, their delimitation,
classification, rules of exploitation, institutions in charge, modalities of management,
maintenance and production, contracts of the utilization of the marshlands, etc. According to
the Organic Law of 14/07/2005 (MINIJUST, 2005) determining the use and management of
land in Rwanda as it is stipulated in Article 14, swamps belong to the State private domain
and can be given to associations/cooperatives or private people according to defined
modalities. However, the uplands are personal or for the family. The social implication of this
fact is that the farmer will give more priority and much care to the uplands than the swamps
which do not belong to him. That is why many swamps are not efficiently and properly
managed.
2.6 Size of the Basin
If the area of the basin is large, total flood flow will take more time to pass the outlet, there
by the base of hydrology of the flood flow will widen out, and consequently reduce the peak
flow because total volume of water passing is the same.
2.7 Shape of the Basin
The shape of the drainage basin also governs the rate at which water enters the stream. The
shale of drainage basin is generally expressed by “Farm factor” and compactness coefficient
as defined below.
Farm factor
The axial length (L) is the distance from outlet to the most remote point on the basin, and
average width (B) is obtained by dividing the area (A) by the axial length.
Compactness coefficient =
If A is the area of the basin and re is the radius of the equivalent circle, then
Circumference = = =2
Therefore, compactness coefficient =
Where P= perimeter of the basin
A= area of the basin
10
There are two types of catchments in general:
Fan shaped catchments
Fern shaped catchments
Fan shaped catchments give greater runoff because tributaries are nearly of the size and
therefore time of flow is nearly the same and is smaller, where as in fern leaf catchments, the
time of concentration is more since the discharge is distributed over a long period.
2.8 Elevation of the watershed
The elevation of the drainage basin, its amount, and hence produce enough effect on the
runoff. The elevation of watershed is a variable factor from point to point in order to
determine the average elevation (Z) of the drainage basin a contour map of the basin is taken
and the Z is then calculated;
Z=
Where, A= area of the basin
A1, A2, A3= area of the area between successive contours
2.9 The type of arrangement of stream channels
If the drainage network of catchment is efficient, water will flow rapidly, and will result in a
higher peak, as the concentration time will be less. Hence the more efficient is drainage, the
more flashy the stream flow will be, and vice-versa. The characteristics of drainage net can
be fairly described by four factors.
Order of stream
The length of tributaries
The stream density
Drainage density
2.9.1 Order of stream
All non branching tributaries, regardless of whether they enter the main stream of its
branches, are termed as first order streams. Stream which receive all non branching
tributaries are on the second order.
10
There are two types of catchments in general:
Fan shaped catchments
Fern shaped catchments
Fan shaped catchments give greater runoff because tributaries are nearly of the size and
therefore time of flow is nearly the same and is smaller, where as in fern leaf catchments, the
time of concentration is more since the discharge is distributed over a long period.
2.8 Elevation of the watershed
The elevation of the drainage basin, its amount, and hence produce enough effect on the
runoff. The elevation of watershed is a variable factor from point to point in order to
determine the average elevation (Z) of the drainage basin a contour map of the basin is taken
and the Z is then calculated;
Z=
Where, A= area of the basin
A1, A2, A3= area of the area between successive contours
2.9 The type of arrangement of stream channels
If the drainage network of catchment is efficient, water will flow rapidly, and will result in a
higher peak, as the concentration time will be less. Hence the more efficient is drainage, the
more flashy the stream flow will be, and vice-versa. The characteristics of drainage net can
be fairly described by four factors.
Order of stream
The length of tributaries
The stream density
Drainage density
2.9.1 Order of stream
All non branching tributaries, regardless of whether they enter the main stream of its
branches, are termed as first order streams. Stream which receive all non branching
tributaries are on the second order.
10
There are two types of catchments in general:
Fan shaped catchments
Fern shaped catchments
Fan shaped catchments give greater runoff because tributaries are nearly of the size and
therefore time of flow is nearly the same and is smaller, where as in fern leaf catchments, the
time of concentration is more since the discharge is distributed over a long period.
2.8 Elevation of the watershed
The elevation of the drainage basin, its amount, and hence produce enough effect on the
runoff. The elevation of watershed is a variable factor from point to point in order to
determine the average elevation (Z) of the drainage basin a contour map of the basin is taken
and the Z is then calculated;
Z=
Where, A= area of the basin
A1, A2, A3= area of the area between successive contours
2.9 The type of arrangement of stream channels
If the drainage network of catchment is efficient, water will flow rapidly, and will result in a
higher peak, as the concentration time will be less. Hence the more efficient is drainage, the
more flashy the stream flow will be, and vice-versa. The characteristics of drainage net can
be fairly described by four factors.
Order of stream
The length of tributaries
The stream density
Drainage density
2.9.1 Order of stream
All non branching tributaries, regardless of whether they enter the main stream of its
branches, are termed as first order streams. Stream which receive all non branching
tributaries are on the second order.
11
Streams of the third order are formed by the junction of two streams of the second order and
so on. In accordance with this system the order number of main stream indicates at once, the
extent of bifurcation of its tributaries is a direct indication of the size and extent of drainage
network.
2.9.2 The length of tributaries
The length of tributaries is an indication of steepness of the drainage basin, as well as of the
degree of drainage. Steep-well drained area generally has numerous small tributaries,
whereas, in plains where soil is deep and permeable, only relatively long tributaries will be in
existence. This factor thus gives the idea of the efficiency of the drainage net. It is generally
better to consider and compare the average length of the same type of tributaries and
especially of first order of tributaries, than to compare the average length of all tributaries.
2.9.3 Stream density
The stream density or stream frequency of a drainage basin may be expressed by relating the
number of stream to the area drained. If Ns is the number of streams in the basin and A is
total area, the stream density Ds can then be expressed as:
D=
ie: The number of stream per km2 in determining the total number of stream, only the
perennial and intermitted streams are included. This factor does not provide a true measure of
drainage efficiency, because a basin having two smaller streams draining only a part of the
basin, and other basin will be indicated to be equally efficient by this factor, whereas it will
not be so, the second being certainly more efficient than the first.
2.9.4 Drainage density
The drainage density is expressed as the length of streams per unit of area. Let Dd represent
the drainage density
L: the total length of perennial and intermittent stream in the basin, and A the area, then:
Dd =
Drainage density varies inversely with the length of overland flow, and therefore provides at
least some indication of drainage efficiency of the basin.
11
Streams of the third order are formed by the junction of two streams of the second order and
so on. In accordance with this system the order number of main stream indicates at once, the
extent of bifurcation of its tributaries is a direct indication of the size and extent of drainage
network.
2.9.2 The length of tributaries
The length of tributaries is an indication of steepness of the drainage basin, as well as of the
degree of drainage. Steep-well drained area generally has numerous small tributaries,
whereas, in plains where soil is deep and permeable, only relatively long tributaries will be in
existence. This factor thus gives the idea of the efficiency of the drainage net. It is generally
better to consider and compare the average length of the same type of tributaries and
especially of first order of tributaries, than to compare the average length of all tributaries.
2.9.3 Stream density
The stream density or stream frequency of a drainage basin may be expressed by relating the
number of stream to the area drained. If Ns is the number of streams in the basin and A is
total area, the stream density Ds can then be expressed as:
D=
ie: The number of stream per km2 in determining the total number of stream, only the
perennial and intermitted streams are included. This factor does not provide a true measure of
drainage efficiency, because a basin having two smaller streams draining only a part of the
basin, and other basin will be indicated to be equally efficient by this factor, whereas it will
not be so, the second being certainly more efficient than the first.
2.9.4 Drainage density
The drainage density is expressed as the length of streams per unit of area. Let Dd represent
the drainage density
L: the total length of perennial and intermittent stream in the basin, and A the area, then:
Dd =
Drainage density varies inversely with the length of overland flow, and therefore provides at
least some indication of drainage efficiency of the basin.
11
Streams of the third order are formed by the junction of two streams of the second order and
so on. In accordance with this system the order number of main stream indicates at once, the
extent of bifurcation of its tributaries is a direct indication of the size and extent of drainage
network.
2.9.2 The length of tributaries
The length of tributaries is an indication of steepness of the drainage basin, as well as of the
degree of drainage. Steep-well drained area generally has numerous small tributaries,
whereas, in plains where soil is deep and permeable, only relatively long tributaries will be in
existence. This factor thus gives the idea of the efficiency of the drainage net. It is generally
better to consider and compare the average length of the same type of tributaries and
especially of first order of tributaries, than to compare the average length of all tributaries.
2.9.3 Stream density
The stream density or stream frequency of a drainage basin may be expressed by relating the
number of stream to the area drained. If Ns is the number of streams in the basin and A is
total area, the stream density Ds can then be expressed as:
D=
ie: The number of stream per km2 in determining the total number of stream, only the
perennial and intermitted streams are included. This factor does not provide a true measure of
drainage efficiency, because a basin having two smaller streams draining only a part of the
basin, and other basin will be indicated to be equally efficient by this factor, whereas it will
not be so, the second being certainly more efficient than the first.
2.9.4 Drainage density
The drainage density is expressed as the length of streams per unit of area. Let Dd represent
the drainage density
L: the total length of perennial and intermittent stream in the basin, and A the area, then:
Dd =
Drainage density varies inversely with the length of overland flow, and therefore provides at
least some indication of drainage efficiency of the basin.
12
2.9.5 Other factors
Beside these four important characteristics of drainage basin other factors such as, the soil in
the catchment, land uses at the various factors influence the runoff.
2.10 Various formulas to compute the discharge
The peak flow can be determined either by rainfall method or statistical method by using a
given frequency rainfall on the basin and determination of peak flow of desired period of
recurrence by means of distribution and analyses the discharges measured on the stream
(Nomand,1978)
Generally, the discharge is shown as the product of channel cross sectional area by the flow
velocity.
Q=V x A
Where, Q = Discharge in m3/s
V =Velocity in m/s
A = Cross sectional area in m2
For the estimation of discharge of the stream or channel, the usual formulas are given below:
a) Manning’s-Stricker formula
It is applicable for uniform flow or with variation, for a permanent regime.
Q = AR2/3S1/2
Where, Q =Discharge expressed in cumecs
A = Cross sectional area in m2
R = Hydraulic radius in m
S =Longitudinal slope (channel bed slope in m/m)
n = Manning roughness coefficient
The R is obtained by the relation P = wetted perimeter in m
12
2.9.5 Other factors
Beside these four important characteristics of drainage basin other factors such as, the soil in
the catchment, land uses at the various factors influence the runoff.
2.10 Various formulas to compute the discharge
The peak flow can be determined either by rainfall method or statistical method by using a
given frequency rainfall on the basin and determination of peak flow of desired period of
recurrence by means of distribution and analyses the discharges measured on the stream
(Nomand,1978)
Generally, the discharge is shown as the product of channel cross sectional area by the flow
velocity.
Q=V x A
Where, Q = Discharge in m3/s
V =Velocity in m/s
A = Cross sectional area in m2
For the estimation of discharge of the stream or channel, the usual formulas are given below:
a) Manning’s-Stricker formula
It is applicable for uniform flow or with variation, for a permanent regime.
Q = AR2/3S1/2
Where, Q =Discharge expressed in cumecs
A = Cross sectional area in m2
R = Hydraulic radius in m
S =Longitudinal slope (channel bed slope in m/m)
n = Manning roughness coefficient
The R is obtained by the relation P = wetted perimeter in m
12
2.9.5 Other factors
Beside these four important characteristics of drainage basin other factors such as, the soil in
the catchment, land uses at the various factors influence the runoff.
2.10 Various formulas to compute the discharge
The peak flow can be determined either by rainfall method or statistical method by using a
given frequency rainfall on the basin and determination of peak flow of desired period of
recurrence by means of distribution and analyses the discharges measured on the stream
(Nomand,1978)
Generally, the discharge is shown as the product of channel cross sectional area by the flow
velocity.
Q=V x A
Where, Q = Discharge in m3/s
V =Velocity in m/s
A = Cross sectional area in m2
For the estimation of discharge of the stream or channel, the usual formulas are given below:
a) Manning’s-Stricker formula
It is applicable for uniform flow or with variation, for a permanent regime.
Q = AR2/3S1/2
Where, Q =Discharge expressed in cumecs
A = Cross sectional area in m2
R = Hydraulic radius in m
S =Longitudinal slope (channel bed slope in m/m)
n = Manning roughness coefficient
The R is obtained by the relation P = wetted perimeter in m
13
In practice, it is desirable to use at least three transverse sections. Assume S1, S2, S3, Sn-1, Sn
different section taken and with small variations, the area S of the wetted surface can be
obtained as follow:
S =
Where, 1, 2 ….n are different sections
b) Chezy’s formula
When the flow is turbulent both Chazy and manning Stricker formula are preferable. TheChezy formula is expressed by:
V = C when Q =CS
And V = Average velocity
R =Hydraulic radius
I = Hydraulic gradient, m/m
C=
Source: Ministere francaise de la coopération(1974)
c) Rational formula
It is the commonly used formula in irrigation and drainage. In this formula
Q =
Where, Q= peak flow in m3/sec
C =runoff coefficient
I =rainfall intensity in mm/hr
A =catchment area in hectares
13
In practice, it is desirable to use at least three transverse sections. Assume S1, S2, S3, Sn-1, Sn
different section taken and with small variations, the area S of the wetted surface can be
obtained as follow:
S =
Where, 1, 2 ….n are different sections
b) Chezy’s formula
When the flow is turbulent both Chazy and manning Stricker formula are preferable. TheChezy formula is expressed by:
V = C when Q =CS
And V = Average velocity
R =Hydraulic radius
I = Hydraulic gradient, m/m
C=
Source: Ministere francaise de la coopération(1974)
c) Rational formula
It is the commonly used formula in irrigation and drainage. In this formula
Q =
Where, Q= peak flow in m3/sec
C =runoff coefficient
I =rainfall intensity in mm/hr
A =catchment area in hectares
13
In practice, it is desirable to use at least three transverse sections. Assume S1, S2, S3, Sn-1, Sn
different section taken and with small variations, the area S of the wetted surface can be
obtained as follow:
S =
Where, 1, 2 ….n are different sections
b) Chezy’s formula
When the flow is turbulent both Chazy and manning Stricker formula are preferable. TheChezy formula is expressed by:
V = C when Q =CS
And V = Average velocity
R =Hydraulic radius
I = Hydraulic gradient, m/m
C=
Source: Ministere francaise de la coopération(1974)
c) Rational formula
It is the commonly used formula in irrigation and drainage. In this formula
Q =
Where, Q= peak flow in m3/sec
C =runoff coefficient
I =rainfall intensity in mm/hr
A =catchment area in hectares
14
Orstom formula
Qm = Where, Qm = maximum discharge in cumecs
P = maximum rainfall in meters during 24 hours
K = despondency coefficient
A = area of catchment in m2
Tb = base time in seconds
R = runoff coefficient
Qmax= Qα with α, security coefficient ranged from 1to 2.5(Kalisoni, 2004)
d) Rurangwa formula
Qu = 0.116U0.36 A0.82U-0.03
With, Qu = maximum discharge in m3
U = recurrence period in years
A = catchment area in km2
This formula is recommended for Rwanda. It has been proposed by RURANGWA Eugene
and it is obtained with a correlation of 90% on several catchment areas of Rwanda (Kalisoni,
2004).
2.11 Movement of water into the soil
2.11.1 InfiltrationThe movement of water from the surface into the soil is called infiltration. The infiltration
characteristic of the soil is one of the dominant variables influencing irrigation. Infiltration
rate is a soil characteristic determining the maximum rate at which water can enter the soil
under specific condition including the presence of excess water. It has the dimension of
velocity. The actual rate at which water is entering the soil at any given time is called
infiltration rate. The infiltration rate decreases during irrigation. The rate of decrease is rapid
initially and tends to approach a constant value. Cumulative infiltration is the total quantity
of water entering the soil in a given time (Israelson, 1962).
14
Orstom formula
Qm = Where, Qm = maximum discharge in cumecs
P = maximum rainfall in meters during 24 hours
K = despondency coefficient
A = area of catchment in m2
Tb = base time in seconds
R = runoff coefficient
Qmax= Qα with α, security coefficient ranged from 1to 2.5(Kalisoni, 2004)
d) Rurangwa formula
Qu = 0.116U0.36 A0.82U-0.03
With, Qu = maximum discharge in m3
U = recurrence period in years
A = catchment area in km2
This formula is recommended for Rwanda. It has been proposed by RURANGWA Eugene
and it is obtained with a correlation of 90% on several catchment areas of Rwanda (Kalisoni,
2004).
2.11 Movement of water into the soil
2.11.1 InfiltrationThe movement of water from the surface into the soil is called infiltration. The infiltration
characteristic of the soil is one of the dominant variables influencing irrigation. Infiltration
rate is a soil characteristic determining the maximum rate at which water can enter the soil
under specific condition including the presence of excess water. It has the dimension of
velocity. The actual rate at which water is entering the soil at any given time is called
infiltration rate. The infiltration rate decreases during irrigation. The rate of decrease is rapid
initially and tends to approach a constant value. Cumulative infiltration is the total quantity
of water entering the soil in a given time (Israelson, 1962).
14
Orstom formula
Qm = Where, Qm = maximum discharge in cumecs
P = maximum rainfall in meters during 24 hours
K = despondency coefficient
A = area of catchment in m2
Tb = base time in seconds
R = runoff coefficient
Qmax= Qα with α, security coefficient ranged from 1to 2.5(Kalisoni, 2004)
d) Rurangwa formula
Qu = 0.116U0.36 A0.82U-0.03
With, Qu = maximum discharge in m3
U = recurrence period in years
A = catchment area in km2
This formula is recommended for Rwanda. It has been proposed by RURANGWA Eugene
and it is obtained with a correlation of 90% on several catchment areas of Rwanda (Kalisoni,
2004).
2.11 Movement of water into the soil
2.11.1 InfiltrationThe movement of water from the surface into the soil is called infiltration. The infiltration
characteristic of the soil is one of the dominant variables influencing irrigation. Infiltration
rate is a soil characteristic determining the maximum rate at which water can enter the soil
under specific condition including the presence of excess water. It has the dimension of
velocity. The actual rate at which water is entering the soil at any given time is called
infiltration rate. The infiltration rate decreases during irrigation. The rate of decrease is rapid
initially and tends to approach a constant value. Cumulative infiltration is the total quantity
of water entering the soil in a given time (Israelson, 1962).
15
2.11.2 Factors affecting infiltration rate
The major factors affecting infiltration rate are:
Initial moisture content
Condition of soil surface
Hydraulic conductivity of the soil profile
Texture
Porosity
Degree of swelling of soil colloid and organic matter
Vegetative cover
Duration of irrigation or rainfall
Viscosity of water (Michel,1978)
2.11.3 Measurement of infiltration
Three methods are used for estimating infiltration characteristics of soil are used. They are
the use of cylinder infiltrometers, measurement of subsidence of free water in large basin and
estimation of accumulated infiltration from the water in advance data. The use of cylinder
“infiltrometer” is the most common method.
2.11.4 Permeability
Permeability may be defined as the characteristic of porous medium of its readiness to
transmit a liquid. The equation expressing the flow considers the fluidity of liquid and the
permeability factor called intrinsic permeability.
Only the size and shape of soil particles and pore influence it. Intrinsic permeability is the
same as the hydraulic conductivity expect that it is independent of fluid properties such as
specific weight and viscosity, while the hydraulic conductivity is dependent on the fluid
properties and the change with quality of water.
16
Table 1: Permeability classes based on hydraulic conductivity of soil.
Permeability classes Hydraulic conductivity of soil (cm/hour)
1. Extremely slow <0.0025
2. Very slow 0.0025-0.025
3. Slow 0.025-0.25
4. Moderate 0.25-2.5
5. Rapid 2.5-25.0
6. Very rapid >25.0
Source: Smith and Browning (1975)
2.12 The technical aspects in irrigation network2.12.1 Irrigation efficiency
This is used to evaluate how effectively the available water supply is used to crop production;
water is conveyed through canal system, water courses and channels to crop field.
Irrigation is applied to store water in the effective root zone of soil for use of crops.
A considerable loss occurs after its diversion from sources to its actual use by crops. The
extent of water loss in the process decides the irrigation efficiency. Irrigation efficiency
declines as the water loss increase. A high efficiency of an irrigated project is always
desirable. The efficiency may be estimated for various operations beginning from diversion
of water to its actual use, by crops, uniformity in its distribution in the root zone, its use for
crop productivity economics and so on (H. C. Zhang,2009). The methods of estimating,
factors influencing efficiencies and measure to attain a high level of efficiency are discussed
in this study.
2.12.2 Water requirement of crops
Water requirement of a crop refers to the amount of water required to raise a successful crop
in a given period (Majumdar, 2004).
17
It comprises the water lost as evaporation from crop field, water transpired and metabolically
used by crop plants, water lost during application which is economically unavoidable and the
water used for special operations such as land preparation, puddling of soil, salt leaching and
so on. The water requirement is usually expressed as the surface depth of water in
millimeters or centimeters. Crop water requirement may be mathematically formulated as:
CWR = ET + Wm + Wu + Ws or
CWR = CU + Wu + Ws
Where, CWR = Crop water requirement, cm;
ET = Evapo-transpiration from crop field, cm;
Wm = Water metabolically used by crop plants to make their body weight, cm;
Wu = Economically unavoidable water losses during application, cm;
Ws = Water applied for special operations, cm and
CU = (ET + Wm) is the consumptive use of water by the crop, cm.
2.12.3 Available Water (AW)
The available water is defined as the difference between the moisture content of a soil at the
field capacity (FC), and its moisture content at the permanent wilting point (WP) and it is
usually expressed in millimetres.
These quantities are often described as constants, but this is misleading, because they are only
constant for a given soil, and vary with the texture and composition of the soil. The table 2
gives typical values for the soil moisture.
Table 2: Approximate Available Moisture Holding Capacity of Soils
Soil texture Available water(cm/m depth)
Coarse texture-oarse sands, fine sands, loamy sands. 6 – 10Moderately coarse texture - sandy loams and fine sandy loams. 10 – 14Medium texture - very fine sandy loams, loams, and silt loams. 12 – 19Moderately fine texture - clay loams, silty loams, and sandy clayloams.
14 – 20
Fine texture - sandy clays, silty clays, and clays. 13 – 20
Source: Michael and Ojha (1966)
18
2.12.4 Project irrigation efficiency
Irrigation efficiency is usually expressed as the percentage ratio of the amount of water stored
in crop root zone for crop use in the project command area to the amount of water diverted
from the project source (Majumdar, 2004). It is expressed as,
Ep = 100 , Where, Ep = Project irrigation efficiency in percent;
Ws = Amount of water stored in crop root zone soil;
Wd = Amount of water diverted or pumped from the source.
It evaluates the efficiency of an irrigation project and combines the various component
efficiencies. Improvement of irrigation efficiency is achieved by reducing the water loss that
occurs in various ways during water conveyance and irrigation practices. Principal factors
influencing loss and irrigation efficiencies are design and nature of construction of the water
conveyance system, types of soil, and extent of land preparation and grading, design of the
field, choice of irrigation, choice of irrigation methods and skills of irrigations.
Water is loss through evaporation from water surface in conveyance and distribution system
and crop fields during irrigation, through seepage from conveyance and distribution systems
and through deep percolation in crop field. Also water loss through evaporation occurs during
sprinkler irrigation. Sometimes water is loss by runoff from the field due to negligence of an
irrigator. Irrigation efficiency usually varies from 40 to 70%.
2.12 .4.1 Water Conveyance EfficiencyAs already stated, water is conveyed through canal network, water courses and channels from
sources such as reservoirs, rivers and dams to fields or farms for crop use. Conveyance
efficiency is used to evaluate the efficiency of channels conveying water. It is also used to
measure the efficiency of channels conveying water from wells and ponds to field.
Water conveyance efficiency may be defined as the percentage ratio of the amount of water
delivered to fields or farms to the amount of water diverted from sources.
It is expressed as,
Ec = 100 ,
Where, Ec = Water conveyance efficiency in percent;
Wf = Amount of water delivered to fields or farms (at the head of field supply
channel or farm distribution system);
Wd = Amount of water diverted from sources.
18
2.12.4 Project irrigation efficiency
Irrigation efficiency is usually expressed as the percentage ratio of the amount of water stored
in crop root zone for crop use in the project command area to the amount of water diverted
from the project source (Majumdar, 2004). It is expressed as,
Ep = 100 , Where, Ep = Project irrigation efficiency in percent;
Ws = Amount of water stored in crop root zone soil;
Wd = Amount of water diverted or pumped from the source.
It evaluates the efficiency of an irrigation project and combines the various component
efficiencies. Improvement of irrigation efficiency is achieved by reducing the water loss that
occurs in various ways during water conveyance and irrigation practices. Principal factors
influencing loss and irrigation efficiencies are design and nature of construction of the water
conveyance system, types of soil, and extent of land preparation and grading, design of the
field, choice of irrigation, choice of irrigation methods and skills of irrigations.
Water is loss through evaporation from water surface in conveyance and distribution system
and crop fields during irrigation, through seepage from conveyance and distribution systems
and through deep percolation in crop field. Also water loss through evaporation occurs during
sprinkler irrigation. Sometimes water is loss by runoff from the field due to negligence of an
irrigator. Irrigation efficiency usually varies from 40 to 70%.
2.12 .4.1 Water Conveyance EfficiencyAs already stated, water is conveyed through canal network, water courses and channels from
sources such as reservoirs, rivers and dams to fields or farms for crop use. Conveyance
efficiency is used to evaluate the efficiency of channels conveying water. It is also used to
measure the efficiency of channels conveying water from wells and ponds to field.
Water conveyance efficiency may be defined as the percentage ratio of the amount of water
delivered to fields or farms to the amount of water diverted from sources.
It is expressed as,
Ec = 100 ,
Where, Ec = Water conveyance efficiency in percent;
Wf = Amount of water delivered to fields or farms (at the head of field supply
channel or farm distribution system);
Wd = Amount of water diverted from sources.
18
2.12.4 Project irrigation efficiency
Irrigation efficiency is usually expressed as the percentage ratio of the amount of water stored
in crop root zone for crop use in the project command area to the amount of water diverted
from the project source (Majumdar, 2004). It is expressed as,
Ep = 100 , Where, Ep = Project irrigation efficiency in percent;
Ws = Amount of water stored in crop root zone soil;
Wd = Amount of water diverted or pumped from the source.
It evaluates the efficiency of an irrigation project and combines the various component
efficiencies. Improvement of irrigation efficiency is achieved by reducing the water loss that
occurs in various ways during water conveyance and irrigation practices. Principal factors
influencing loss and irrigation efficiencies are design and nature of construction of the water
conveyance system, types of soil, and extent of land preparation and grading, design of the
field, choice of irrigation, choice of irrigation methods and skills of irrigations.
Water is loss through evaporation from water surface in conveyance and distribution system
and crop fields during irrigation, through seepage from conveyance and distribution systems
and through deep percolation in crop field. Also water loss through evaporation occurs during
sprinkler irrigation. Sometimes water is loss by runoff from the field due to negligence of an
irrigator. Irrigation efficiency usually varies from 40 to 70%.
2.12 .4.1 Water Conveyance EfficiencyAs already stated, water is conveyed through canal network, water courses and channels from
sources such as reservoirs, rivers and dams to fields or farms for crop use. Conveyance
efficiency is used to evaluate the efficiency of channels conveying water. It is also used to
measure the efficiency of channels conveying water from wells and ponds to field.
Water conveyance efficiency may be defined as the percentage ratio of the amount of water
delivered to fields or farms to the amount of water diverted from sources.
It is expressed as,
Ec = 100 ,
Where, Ec = Water conveyance efficiency in percent;
Wf = Amount of water delivered to fields or farms (at the head of field supply
channel or farm distribution system);
Wd = Amount of water diverted from sources.
19
2.12.4.2 Water Application EfficiencyWater application system refers to the efficiency of water application to field, water is
applied to fields by various methods, and those may be:
a) Surface
b) Subsurface
c) Sprinkler or drip methods
The efficiency of those methods individually or for single irrigation of farm fields may be
estimated. The water application efficiency may be defined as the percentage ratio of the
amount of water stored in the crop root zone to the amount of water delivered to fields. It is
expressed as,
Ea = 100 ( )
Where, Ea = Water application efficiency in percent;
Ws = Amount of water stored in the crop root zone soil;
Wf = Amount of water delivered to fields (Bartsnellen. W, 1997).
2.12.5 Efficiency of irrigation practices, water use and operation of irrigation system
Irrigation practices differ from place to place because of physiographic conditions, soil types,
crop grown, and amount of water available and considerations of farmers. An evaluation of
various irrigation practices and water use by crop is essential to have an insight into the
effective use of available water. The following efficiencies are frequently studied:
a) water storage efficiency
b) water distribution efficiency
c) water use efficiency
2.12.5.1 Water Storage Efficiency
Water storage efficiency refers to the percentage ratio of the amount of water stored in
effective root zone soil to the amount of water needed to make up the soil water depleted in
crop root zone prior to irrigation (Majumdar, 2004). It may be expressed as,
Es = 100 ( ) where
Es = Water storage efficiency in percent;
Ws = Amount of water actually stored in root zone soil from the water applied;
We = Amount of water needed to meet the soil water depleted in the crop root zone soil prior
to irrigation.
19
2.12.4.2 Water Application EfficiencyWater application system refers to the efficiency of water application to field, water is
applied to fields by various methods, and those may be:
a) Surface
b) Subsurface
c) Sprinkler or drip methods
The efficiency of those methods individually or for single irrigation of farm fields may be
estimated. The water application efficiency may be defined as the percentage ratio of the
amount of water stored in the crop root zone to the amount of water delivered to fields. It is
expressed as,
Ea = 100 ( )
Where, Ea = Water application efficiency in percent;
Ws = Amount of water stored in the crop root zone soil;
Wf = Amount of water delivered to fields (Bartsnellen. W, 1997).
2.12.5 Efficiency of irrigation practices, water use and operation of irrigation system
Irrigation practices differ from place to place because of physiographic conditions, soil types,
crop grown, and amount of water available and considerations of farmers. An evaluation of
various irrigation practices and water use by crop is essential to have an insight into the
effective use of available water. The following efficiencies are frequently studied:
a) water storage efficiency
b) water distribution efficiency
c) water use efficiency
2.12.5.1 Water Storage Efficiency
Water storage efficiency refers to the percentage ratio of the amount of water stored in
effective root zone soil to the amount of water needed to make up the soil water depleted in
crop root zone prior to irrigation (Majumdar, 2004). It may be expressed as,
Es = 100 ( ) where
Es = Water storage efficiency in percent;
Ws = Amount of water actually stored in root zone soil from the water applied;
We = Amount of water needed to meet the soil water depleted in the crop root zone soil prior
to irrigation.
19
2.12.4.2 Water Application EfficiencyWater application system refers to the efficiency of water application to field, water is
applied to fields by various methods, and those may be:
a) Surface
b) Subsurface
c) Sprinkler or drip methods
The efficiency of those methods individually or for single irrigation of farm fields may be
estimated. The water application efficiency may be defined as the percentage ratio of the
amount of water stored in the crop root zone to the amount of water delivered to fields. It is
expressed as,
Ea = 100 ( )
Where, Ea = Water application efficiency in percent;
Ws = Amount of water stored in the crop root zone soil;
Wf = Amount of water delivered to fields (Bartsnellen. W, 1997).
2.12.5 Efficiency of irrigation practices, water use and operation of irrigation system
Irrigation practices differ from place to place because of physiographic conditions, soil types,
crop grown, and amount of water available and considerations of farmers. An evaluation of
various irrigation practices and water use by crop is essential to have an insight into the
effective use of available water. The following efficiencies are frequently studied:
a) water storage efficiency
b) water distribution efficiency
c) water use efficiency
2.12.5.1 Water Storage Efficiency
Water storage efficiency refers to the percentage ratio of the amount of water stored in
effective root zone soil to the amount of water needed to make up the soil water depleted in
crop root zone prior to irrigation (Majumdar, 2004). It may be expressed as,
Es = 100 ( ) where
Es = Water storage efficiency in percent;
Ws = Amount of water actually stored in root zone soil from the water applied;
We = Amount of water needed to meet the soil water depleted in the crop root zone soil prior
to irrigation.
20
The amount of water needed to be applied through irrigation is equal to the amount of soil
water depleted due to evapo-transpiration, which is described as the net irrigation
requirement.
2.12.5.2 Water Distribution Efficiency
Water distribution efficiency measures the extent to which water is uniformly distributed in
the effective root zone soil along the irrigation run (Majumdar, 2004).
It is described as,
Ed = 100 (1 - )
Where, Ed = Water distribution efficiency in percent;
ȳ = Average numerical deviation in depth of water stored in root zone soil along the
irrigation run from the average depth of water stored during irrigation;
đ = Average depth of water stored during irrigation along the water run.
Water distribution efficiency dictates the permissible lengths of irrigation run. It provides a
measure of efficiency of an irrigation system or method over the other.
2.12.5.3 Water Use Efficiency
Water use efficiency is determined to evaluate the benefits of applied water through
economic crop production. It is very important in crop production and irrigation management.
It is described in the following two ways:
i) Field water use efficiency
This may be defined as the ratio of the amount of economic crop yield to the amount of water
required for crop growing (Majumdar, 2004; Hillel, 1998). It is obtained as follows,
Eu =
Where, Eu = Field water use efficiency expressed in kilogram of economic yield per
Hectare-cm or hectare-mm of water;
Y = Economic crop yield in kilogram per hectare;
WR = Water requirement of the crop in hectare-cm or hectare-mm
20
The amount of water needed to be applied through irrigation is equal to the amount of soil
water depleted due to evapo-transpiration, which is described as the net irrigation
requirement.
2.12.5.2 Water Distribution Efficiency
Water distribution efficiency measures the extent to which water is uniformly distributed in
the effective root zone soil along the irrigation run (Majumdar, 2004).
It is described as,
Ed = 100 (1 - )
Where, Ed = Water distribution efficiency in percent;
ȳ = Average numerical deviation in depth of water stored in root zone soil along the
irrigation run from the average depth of water stored during irrigation;
đ = Average depth of water stored during irrigation along the water run.
Water distribution efficiency dictates the permissible lengths of irrigation run. It provides a
measure of efficiency of an irrigation system or method over the other.
2.12.5.3 Water Use Efficiency
Water use efficiency is determined to evaluate the benefits of applied water through
economic crop production. It is very important in crop production and irrigation management.
It is described in the following two ways:
i) Field water use efficiency
This may be defined as the ratio of the amount of economic crop yield to the amount of water
required for crop growing (Majumdar, 2004; Hillel, 1998). It is obtained as follows,
Eu =
Where, Eu = Field water use efficiency expressed in kilogram of economic yield per
Hectare-cm or hectare-mm of water;
Y = Economic crop yield in kilogram per hectare;
WR = Water requirement of the crop in hectare-cm or hectare-mm
20
The amount of water needed to be applied through irrigation is equal to the amount of soil
water depleted due to evapo-transpiration, which is described as the net irrigation
requirement.
2.12.5.2 Water Distribution Efficiency
Water distribution efficiency measures the extent to which water is uniformly distributed in
the effective root zone soil along the irrigation run (Majumdar, 2004).
It is described as,
Ed = 100 (1 - )
Where, Ed = Water distribution efficiency in percent;
ȳ = Average numerical deviation in depth of water stored in root zone soil along the
irrigation run from the average depth of water stored during irrigation;
đ = Average depth of water stored during irrigation along the water run.
Water distribution efficiency dictates the permissible lengths of irrigation run. It provides a
measure of efficiency of an irrigation system or method over the other.
2.12.5.3 Water Use Efficiency
Water use efficiency is determined to evaluate the benefits of applied water through
economic crop production. It is very important in crop production and irrigation management.
It is described in the following two ways:
i) Field water use efficiency
This may be defined as the ratio of the amount of economic crop yield to the amount of water
required for crop growing (Majumdar, 2004; Hillel, 1998). It is obtained as follows,
Eu =
Where, Eu = Field water use efficiency expressed in kilogram of economic yield per
Hectare-cm or hectare-mm of water;
Y = Economic crop yield in kilogram per hectare;
WR = Water requirement of the crop in hectare-cm or hectare-mm
21
ii) Crop water use efficiency
This may be defined as the ratio of the amount of economic yield of a crop to the
amount of water consumptively used by the crop. It is found out as follows,
ECU (or WUE) =
Where, ECU or WUE = Crop water use efficiency in kilogram of economic yield per hectare-
cm or hectare-mm of water;
Y = Economic yield of crop in kilogram per hectare;
CU = Consumptive use of water in hectare-cm or hectare-mm;
ET = Evapo-transpiration in hectare-cm or hectare-mm.
2.12.6 Economic (irrigation) efficiency irrigation system
Irrigation efficiency is the ratio of actual income (net or gross) attained with the operating
irrigation system, compared with the income expected under ideal conditions. This parameter
is a measure of overall efficiency, because it relates the final return to input cost.
i) Operational efficiency
Operation efficiency is the ratio of actual project efficiency compared with the operational
efficiency of an ideally designed and managed system using the same irrigation method and
facilities. Low operation efficiency shows management or system design problems or both.
ii) Water utilization methods
The water utilisation method should provide a high level of water application efficiency and
also ensure its economic viability, sustained soil productivity and wide adaptability to
prevalent features on the farm. Efficiency water application requires careful attention to all
the factors like:
Quality and quantity of water available
Soil characteristics
Topography of the land
The nature and availability of labour and energy
Economic status of the farmer
21
ii) Crop water use efficiency
This may be defined as the ratio of the amount of economic yield of a crop to the
amount of water consumptively used by the crop. It is found out as follows,
ECU (or WUE) =
Where, ECU or WUE = Crop water use efficiency in kilogram of economic yield per hectare-
cm or hectare-mm of water;
Y = Economic yield of crop in kilogram per hectare;
CU = Consumptive use of water in hectare-cm or hectare-mm;
ET = Evapo-transpiration in hectare-cm or hectare-mm.
2.12.6 Economic (irrigation) efficiency irrigation system
Irrigation efficiency is the ratio of actual income (net or gross) attained with the operating
irrigation system, compared with the income expected under ideal conditions. This parameter
is a measure of overall efficiency, because it relates the final return to input cost.
i) Operational efficiency
Operation efficiency is the ratio of actual project efficiency compared with the operational
efficiency of an ideally designed and managed system using the same irrigation method and
facilities. Low operation efficiency shows management or system design problems or both.
ii) Water utilization methods
The water utilisation method should provide a high level of water application efficiency and
also ensure its economic viability, sustained soil productivity and wide adaptability to
prevalent features on the farm. Efficiency water application requires careful attention to all
the factors like:
Quality and quantity of water available
Soil characteristics
Topography of the land
The nature and availability of labour and energy
Economic status of the farmer
21
ii) Crop water use efficiency
This may be defined as the ratio of the amount of economic yield of a crop to the
amount of water consumptively used by the crop. It is found out as follows,
ECU (or WUE) =
Where, ECU or WUE = Crop water use efficiency in kilogram of economic yield per hectare-
cm or hectare-mm of water;
Y = Economic yield of crop in kilogram per hectare;
CU = Consumptive use of water in hectare-cm or hectare-mm;
ET = Evapo-transpiration in hectare-cm or hectare-mm.
2.12.6 Economic (irrigation) efficiency irrigation system
Irrigation efficiency is the ratio of actual income (net or gross) attained with the operating
irrigation system, compared with the income expected under ideal conditions. This parameter
is a measure of overall efficiency, because it relates the final return to input cost.
i) Operational efficiency
Operation efficiency is the ratio of actual project efficiency compared with the operational
efficiency of an ideally designed and managed system using the same irrigation method and
facilities. Low operation efficiency shows management or system design problems or both.
ii) Water utilization methods
The water utilisation method should provide a high level of water application efficiency and
also ensure its economic viability, sustained soil productivity and wide adaptability to
prevalent features on the farm. Efficiency water application requires careful attention to all
the factors like:
Quality and quantity of water available
Soil characteristics
Topography of the land
The nature and availability of labour and energy
Economic status of the farmer
22
CHAPTER-3
MATERIALS AND METHODS
To conduct the studies for accomplishing the objectives, certain materials and methods were
used. The details of area, materials used and procedures followed given in the following
paragraphs.
3.1 Study zone description
The Base I swamp is located about 13 km from Muhanga (Gitarama) town falling under
Ruhango District of southern province of Rwanda. The District of Ruhango has sixteen (16)
swamps on a total surface area of 860 ha. However only 417 ha of swamps are well developed for
agricultural activities (Annual report of Ruhango, District, 2010). Since September 2000, the
German Agro-Action has been managing this swamp. The water used to irrigate this swamp
is coming from Base stream and Ruhondo stream. The total area managed is 105 ha. The
Base I swamp is being exploited by the cooperative CORIBARU under control RSSP,
ZIGAMA CSS and Ruhango district. It is a fertile swamp which produces good yield when
well managed.
3.1.1 Climate
Climatological data were collected from Byimana meteorological station (Altitude 1750m,
coordinates 29o44E, 2o25S). Series of long and continue observation of meteorological
parameters rainfall, temperature, Relative humidity, wind and hour of sunshine per day were
gathered from the above station. The appendix 9 presents the climatological data.
3.1.2 Soil
The pedology of the area constitutes the granite and gneiss. The soils of valleys, marshlands
and riversides are alluvial and colluvial. According to (A.A.A, 2005 and Uwitonze T, 2009)
the soil texture of Base I swamp is sandy loam. The soil is fertile and moderately permeable.
3.1.3 Crop
The main crops grown in Base I swamp are rice; however, maize are also grown in some area
of swamp because of various reasons. Before the management of the swamp the crops were
sweet potatoes, beans, soja, irish potatoes and vegetables.
23
3.2 Materials
To determine better ways of irrigation water use management through highly efficient
irrigation, various materials have been used regarding to the tests and measurements to be
conducted
Computer: this device was used for storing and manipulating data;
Measuring tape Stopwatch, A float wooden piece A bucket A wooden post Ruler Calculator Nails
3.3 Methodology
In this work different measurements and tests have been carried out from September to
November, 2011 in order to have a satisfactorily results to determine better ways of water
management in Base I swamp perimeter. The water of Base stream has been diverted through
main irrigation channel which is supplying water to the experimented plots through
secondary channel. The size of each irrigated plot (upper, middle and down sides at both
sides of swamp) is 25m x 40 m.
Briefly the realized measurements/tests are as follows:
1. Discharge measurement received in secondary channel
2. Estimated irrigation water conveyance efficiency
3. Hydraulic conductivity in the field
4. Determination of water losses in the plots.
5. Data processing and analysis with cropwat
3.3.1 Discharge measurements procedure
The velocity of a canal or stream and hence its discharge, may sometimes be determined
approximately by the use of surface floats and its multiplication to cross sectional area of
stream or channel. To use floats a relatively straight reach of a channel 20 metres long with a
fairly uniform cross section along its length is selected (Stern, 1994; Arora, 2004).
24
To reduce the effects of wind on the float, a long-necked bottle partly filled with water was
used as a float. The velocity of the stream is determined by running the float and noting the
time the float takes to cross the channel section. The float is placed in the centre of the
channel 1 or 2 metres upstream of the start of the measured length L, and the time t taken to
cover the measured distance is noted. Three readings are recorded and the mean was taken.
Care is taken to see that the float does not touch the channel sides (USDI, 1953). The distance
divided by the time gives the velocity of the float, which corresponds to the velocity of the
water at its surface.
Length of channel(L) in meter
Water height into channel(D) in meter
Top and Bottom widths of the channel (T) and (B) in meter
Elapsed time (s)
Velocity(V) in m/sec
It has been found that for regular channels flowing in a straight course under favourable
conditions, the mean velocity of a strip in the channel is approximately 0.85 times its surface
velocity for small streams (Stern, 1979; Majumdar, 2004; USDI; 1953). Since the velocity of
water is the highest on the surface, a constant factor equal to 0.85 is used to multiply the
arrived value of velocity to come to the correct value. The formula for estimating the stream
discharge may be written as, Q = 0.85 ( ) D.V, where Q = Discharge, cm3/s; T = Top
width of the channel at flow surface level, cm; B = Bottom width of the channel, cm; D =
Flow depth in the channel, cm and V = Flow velocity, cm/s.
The found discharge were multiplied by 0.85 factor because of weeds availability in channel
with poor roughness
3.3.2 Estimated irrigation water conveyance efficiency in the perimeter
From the water quantity received at the farm gate at the entrance of main channel and the
estimated water delivered to fields, the water conveyance efficiency has been calculated as;
EC = x100
As the cross sectional of the channel was trapezoidal, thus the cross sectional area calculation
was done as:
The area (A) =
Where, T =Top widt B =Bottom width and D = Depth
24
To reduce the effects of wind on the float, a long-necked bottle partly filled with water was
used as a float. The velocity of the stream is determined by running the float and noting the
time the float takes to cross the channel section. The float is placed in the centre of the
channel 1 or 2 metres upstream of the start of the measured length L, and the time t taken to
cover the measured distance is noted. Three readings are recorded and the mean was taken.
Care is taken to see that the float does not touch the channel sides (USDI, 1953). The distance
divided by the time gives the velocity of the float, which corresponds to the velocity of the
water at its surface.
Length of channel(L) in meter
Water height into channel(D) in meter
Top and Bottom widths of the channel (T) and (B) in meter
Elapsed time (s)
Velocity(V) in m/sec
It has been found that for regular channels flowing in a straight course under favourable
conditions, the mean velocity of a strip in the channel is approximately 0.85 times its surface
velocity for small streams (Stern, 1979; Majumdar, 2004; USDI; 1953). Since the velocity of
water is the highest on the surface, a constant factor equal to 0.85 is used to multiply the
arrived value of velocity to come to the correct value. The formula for estimating the stream
discharge may be written as, Q = 0.85 ( ) D.V, where Q = Discharge, cm3/s; T = Top
width of the channel at flow surface level, cm; B = Bottom width of the channel, cm; D =
Flow depth in the channel, cm and V = Flow velocity, cm/s.
The found discharge were multiplied by 0.85 factor because of weeds availability in channel
with poor roughness
3.3.2 Estimated irrigation water conveyance efficiency in the perimeter
From the water quantity received at the farm gate at the entrance of main channel and the
estimated water delivered to fields, the water conveyance efficiency has been calculated as;
EC = x100
As the cross sectional of the channel was trapezoidal, thus the cross sectional area calculation
was done as:
The area (A) =
Where, T =Top widt B =Bottom width and D = Depth
24
To reduce the effects of wind on the float, a long-necked bottle partly filled with water was
used as a float. The velocity of the stream is determined by running the float and noting the
time the float takes to cross the channel section. The float is placed in the centre of the
channel 1 or 2 metres upstream of the start of the measured length L, and the time t taken to
cover the measured distance is noted. Three readings are recorded and the mean was taken.
Care is taken to see that the float does not touch the channel sides (USDI, 1953). The distance
divided by the time gives the velocity of the float, which corresponds to the velocity of the
water at its surface.
Length of channel(L) in meter
Water height into channel(D) in meter
Top and Bottom widths of the channel (T) and (B) in meter
Elapsed time (s)
Velocity(V) in m/sec
It has been found that for regular channels flowing in a straight course under favourable
conditions, the mean velocity of a strip in the channel is approximately 0.85 times its surface
velocity for small streams (Stern, 1979; Majumdar, 2004; USDI; 1953). Since the velocity of
water is the highest on the surface, a constant factor equal to 0.85 is used to multiply the
arrived value of velocity to come to the correct value. The formula for estimating the stream
discharge may be written as, Q = 0.85 ( ) D.V, where Q = Discharge, cm3/s; T = Top
width of the channel at flow surface level, cm; B = Bottom width of the channel, cm; D =
Flow depth in the channel, cm and V = Flow velocity, cm/s.
The found discharge were multiplied by 0.85 factor because of weeds availability in channel
with poor roughness
3.3.2 Estimated irrigation water conveyance efficiency in the perimeter
From the water quantity received at the farm gate at the entrance of main channel and the
estimated water delivered to fields, the water conveyance efficiency has been calculated as;
EC = x100
As the cross sectional of the channel was trapezoidal, thus the cross sectional area calculation
was done as:
The area (A) =
Where, T =Top widt B =Bottom width and D = Depth
25
3.3.3 Hydraulic conductivity tests
In fact, hydraulic conductivity calculation helped us to know the capacity of saturated soil to
transmit water through it. Reverse auger hole method was used to determine hydraulic
conductivity at field. For this the following process were adopted
Select and level the site;
Dig a hole of 30cm diameter width and 40cm depth;
Collect water to fill the hole using a bucket until is saturated;
Determine initial depth after filling with water;
Using a stopwatch, record a final depth after a certain time internal (5 minutes);
Calculate K using Lewis following formula:
K = 1.15 x r [ ]
Where, K = hydraulic conductivity; cm/sec;
ho = initial depth of water from bottom of hole to water surface, cm;
ht = final depth of water from bottom of hole to water surface, cm;
r = radius, cm;
∆t =time interval; min
3.3.4 Calculation of evapo-transpiration, infiltration, percolation
3.3.4.1 Water losses at plots level
Usually, water consumption is expressed in water depth. Water consumption in depth is
obtained by measuring the change of water level in the paddy field when both water supply
and outflow from the paddy field are controlled. Daily water consumption is defined as water
consumed in the paddy field. For this purpose, the “stake and nail” method has been used for
its easiness and applicability in saturated conditions. Plots for this measurement were
sampled depending on location in the swamp, reliability to inflow and outflow measurement
of water and farmers’ willingness to allow experiments to be conducted in their rice fields.
The sample farm plots were monitored for water losses due to paddy transpiration,
evaporation from standing water in the fields, and lateral and deep percolation.
The procedure is as follows:
25
3.3.3 Hydraulic conductivity tests
In fact, hydraulic conductivity calculation helped us to know the capacity of saturated soil to
transmit water through it. Reverse auger hole method was used to determine hydraulic
conductivity at field. For this the following process were adopted
Select and level the site;
Dig a hole of 30cm diameter width and 40cm depth;
Collect water to fill the hole using a bucket until is saturated;
Determine initial depth after filling with water;
Using a stopwatch, record a final depth after a certain time internal (5 minutes);
Calculate K using Lewis following formula:
K = 1.15 x r [ ]
Where, K = hydraulic conductivity; cm/sec;
ho = initial depth of water from bottom of hole to water surface, cm;
ht = final depth of water from bottom of hole to water surface, cm;
r = radius, cm;
∆t =time interval; min
3.3.4 Calculation of evapo-transpiration, infiltration, percolation
3.3.4.1 Water losses at plots level
Usually, water consumption is expressed in water depth. Water consumption in depth is
obtained by measuring the change of water level in the paddy field when both water supply
and outflow from the paddy field are controlled. Daily water consumption is defined as water
consumed in the paddy field. For this purpose, the “stake and nail” method has been used for
its easiness and applicability in saturated conditions. Plots for this measurement were
sampled depending on location in the swamp, reliability to inflow and outflow measurement
of water and farmers’ willingness to allow experiments to be conducted in their rice fields.
The sample farm plots were monitored for water losses due to paddy transpiration,
evaporation from standing water in the fields, and lateral and deep percolation.
The procedure is as follows:
25
3.3.3 Hydraulic conductivity tests
In fact, hydraulic conductivity calculation helped us to know the capacity of saturated soil to
transmit water through it. Reverse auger hole method was used to determine hydraulic
conductivity at field. For this the following process were adopted
Select and level the site;
Dig a hole of 30cm diameter width and 40cm depth;
Collect water to fill the hole using a bucket until is saturated;
Determine initial depth after filling with water;
Using a stopwatch, record a final depth after a certain time internal (5 minutes);
Calculate K using Lewis following formula:
K = 1.15 x r [ ]
Where, K = hydraulic conductivity; cm/sec;
ho = initial depth of water from bottom of hole to water surface, cm;
ht = final depth of water from bottom of hole to water surface, cm;
r = radius, cm;
∆t =time interval; min
3.3.4 Calculation of evapo-transpiration, infiltration, percolation
3.3.4.1 Water losses at plots level
Usually, water consumption is expressed in water depth. Water consumption in depth is
obtained by measuring the change of water level in the paddy field when both water supply
and outflow from the paddy field are controlled. Daily water consumption is defined as water
consumed in the paddy field. For this purpose, the “stake and nail” method has been used for
its easiness and applicability in saturated conditions. Plots for this measurement were
sampled depending on location in the swamp, reliability to inflow and outflow measurement
of water and farmers’ willingness to allow experiments to be conducted in their rice fields.
The sample farm plots were monitored for water losses due to paddy transpiration,
evaporation from standing water in the fields, and lateral and deep percolation.
The procedure is as follows:
26
(i) Install a stake on which a nail is attached at exactly the level of the water;
(ii) Close inlet and outlet of water in the plot;
(iii) Measure and record water level and rainfall at a regular time basis;
(iv) Calculate daily change of water level.
The losses through evaporation, evapo-transpiration, percolation and infiltration in the six
plots of rice at a vegetative a stage
H1= initial height of water
H2 = height of water after 24 hours
H3 =height of water after 48 hours
D1T1 = the corresponding time to H1
D2T2 = the corresponding time to H2
D3T3 = the corresponding time to H3
∆H1 = Difference in water level 1
∆H2= Difference in water level 2
Thus, the total losses =
3.3.4.2 Irrigation water requirement using cropwat
All relevant data have been analyzed mostly using computer software such as Excel to
calculate some parameters needed for discharge calculations and FAO Cropwat 8.0 to
estimate crop water requirements. The following steps were followed for calculation of
irrigation water requirements:
Calculate reference evapotransptraion ETo using FAO CROPWAT Program;
Determine crop coefficient Kc value using FAO-Crop parameters;
Calculate effective rainfall (Pe);
Calculate Net irrigation Requirement (NIR);
3.3.4.3 Design of irrigation main and secondary canals
For this design, Lacey’s theory has been used as method in order to arrive on the design of
irrigation networks.
26
(i) Install a stake on which a nail is attached at exactly the level of the water;
(ii) Close inlet and outlet of water in the plot;
(iii) Measure and record water level and rainfall at a regular time basis;
(iv) Calculate daily change of water level.
The losses through evaporation, evapo-transpiration, percolation and infiltration in the six
plots of rice at a vegetative a stage
H1= initial height of water
H2 = height of water after 24 hours
H3 =height of water after 48 hours
D1T1 = the corresponding time to H1
D2T2 = the corresponding time to H2
D3T3 = the corresponding time to H3
∆H1 = Difference in water level 1
∆H2= Difference in water level 2
Thus, the total losses =
3.3.4.2 Irrigation water requirement using cropwat
All relevant data have been analyzed mostly using computer software such as Excel to
calculate some parameters needed for discharge calculations and FAO Cropwat 8.0 to
estimate crop water requirements. The following steps were followed for calculation of
irrigation water requirements:
Calculate reference evapotransptraion ETo using FAO CROPWAT Program;
Determine crop coefficient Kc value using FAO-Crop parameters;
Calculate effective rainfall (Pe);
Calculate Net irrigation Requirement (NIR);
3.3.4.3 Design of irrigation main and secondary canals
For this design, Lacey’s theory has been used as method in order to arrive on the design of
irrigation networks.
26
(i) Install a stake on which a nail is attached at exactly the level of the water;
(ii) Close inlet and outlet of water in the plot;
(iii) Measure and record water level and rainfall at a regular time basis;
(iv) Calculate daily change of water level.
The losses through evaporation, evapo-transpiration, percolation and infiltration in the six
plots of rice at a vegetative a stage
H1= initial height of water
H2 = height of water after 24 hours
H3 =height of water after 48 hours
D1T1 = the corresponding time to H1
D2T2 = the corresponding time to H2
D3T3 = the corresponding time to H3
∆H1 = Difference in water level 1
∆H2= Difference in water level 2
Thus, the total losses =
3.3.4.2 Irrigation water requirement using cropwat
All relevant data have been analyzed mostly using computer software such as Excel to
calculate some parameters needed for discharge calculations and FAO Cropwat 8.0 to
estimate crop water requirements. The following steps were followed for calculation of
irrigation water requirements:
Calculate reference evapotransptraion ETo using FAO CROPWAT Program;
Determine crop coefficient Kc value using FAO-Crop parameters;
Calculate effective rainfall (Pe);
Calculate Net irrigation Requirement (NIR);
3.3.4.3 Design of irrigation main and secondary canals
For this design, Lacey’s theory has been used as method in order to arrive on the design of
irrigation networks.
27
a) Lacey’s theory
Lacey’s theory is based on the concept of regime condition of the channel.
The regime condition will be satisfied if:
The channel flows uniformly in unlimited incoherent alluvium of the same character
which is transported by the channel.
The silt grade and silt charge remains constant
The discharge remains constant
Lacey states that:
“The silt carried by the flowing water is kept in suspension by the vertical component of
eddies”.
The eddied are generated at all the points on the wetted perimeter of the channel section. (N
Basak.2002).
b) Formula used during the design
Lacey’s design equations:
1) drf 76.1 ; where dr = mean dia of silt in mm
2)52 140vAf ; )(&)/( 2mAsmv v velocity ; AreaA
3)6
12
140
Qfv ; smQ /3 Q= discharge
4) QP 75.4 ; )(mP P= wetted perimeter
5) 31
23
4980R
fS &6
1
35
3340Q
fS
6) Regime scour depth:3
1
47.0
fQR ; R=Hydraulic radius or scour depth
7)
63
5
3340 SfQ
S: side slope
28
c) Limitation of Lacey’s theory
Although this theory is used, it has some limitations
1) The concept of true regime is theoretical and cannot be achieve practically;
2) The various equations are delivered by considering the silt factor f which is not
constant at all;
3) The concentration of soil is not taken into account;
4) Silt grade and silt change are not clearly defined;
5) The characteristics of regime channel may not be same for all cases. (K.R.Arora
2004)
29
CHAPTER- 4:
RESULTS AND DISCUSSION
4.1 Crop water requirements
The crop water requirement for the rice was obtained from climatic data used as input into
CROPWAT 8. The study has been conducted from August to October. However, the actual
measurements were also made to estimate the deep percolation losses in actual field
conditions. The 30 years climatic data viz. rainfall, maximum and minimum temperature,
Relative humidity, wind speed, sunshine, solar radiation and pan evaporation, were collected
from Byimana meteorological station (appendix 2).
4.1.1 Monthly climatic data for Base I rice swamp
The figure 2 shows mean climatic data (appendix 2) for Base I swamp collected from
Byimana meteorological station which is nearby the swamp. These data cover the period of
30 years (1979-2009).
Figure 2: Average monthly climatic data for Byimana meteorological station (1979-
2009)
0
20
40
60
80
100
120
140
160
JAN FEB M AR APR MAY JUN JUL AUG SEPT OCT NOV DEC
Min,Temp ◦C
Max,Temp ◦C
Relative Humidity %
Wind Speed km /day
Sunshine Hours
30
The figure 2 shows that for Base I swamp the average maximum temperature observed over
the period of 30 years ranges from 22.90C to 24.50C with highest during July and August.
However the average minimum temperature ranges from 11.40C to 13.50C with lowest in
October.
The above graph also shows that the minimum humidity is 59% in August and the maximum
humidity is 85 % in April. The minimum wind speed is 110 km/ day in April and the
maximum is 147 km/ day observed in August. The maximum sunshine hour are 7.9 in July
while the minimum sunshine hours are 5.2 recorded in April.
4.1.2 Monthly evapotranspiration
Figure 3: Average monthly ETo for Base I swamp
The figure-3 shows that for Base I swamp; the maximum ETo is 4.02 mm/day in August
followed by September 3.87mm) and July (3.78mm), and the minimum ETo is 3.10 mm/day
in May. As it is shown on the above graphs the high value of ETo are observed during dry
season while the low value are observed during rainy season.
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
JAN FEB M AR APR MAY JUN JUL AUG SEPT OCT NOV DEC
Eto
(mm
/day
)
Month
Eto mm/day
31
4.1.3 Monthly rainfall and effective rainfall
Figure 4: Average monthly rainfall and effective rainfall
It is evident from figure-4 that maximum rainfall (201 mm) is received during April followed
by May (147mm) and November (136mm) and the minimum rainfall (12mm) is recorded
during July. The maximum effective rain was 136.4mm in April and the minimum is 11.8
mm in July (appendix 4)
4.1.3 Crop water requirement and irrigation requirement
Rice crop was planted from June to November with a total of 180 days (FAO, 2003). ETc
and Irrigation requirements have been determined on decade (10 days) and daily basis. The
figure 5 shows calculated ETc and Irrigation Requirement on decade basis.
31
4.1.3 Monthly rainfall and effective rainfall
Figure 4: Average monthly rainfall and effective rainfall
It is evident from figure-4 that maximum rainfall (201 mm) is received during April followed
by May (147mm) and November (136mm) and the minimum rainfall (12mm) is recorded
during July. The maximum effective rain was 136.4mm in April and the minimum is 11.8
mm in July (appendix 4)
4.1.3 Crop water requirement and irrigation requirement
Rice crop was planted from June to November with a total of 180 days (FAO, 2003). ETc
and Irrigation requirements have been determined on decade (10 days) and daily basis. The
figure 5 shows calculated ETc and Irrigation Requirement on decade basis.
31
4.1.3 Monthly rainfall and effective rainfall
Figure 4: Average monthly rainfall and effective rainfall
It is evident from figure-4 that maximum rainfall (201 mm) is received during April followed
by May (147mm) and November (136mm) and the minimum rainfall (12mm) is recorded
during July. The maximum effective rain was 136.4mm in April and the minimum is 11.8
mm in July (appendix 4)
4.1.3 Crop water requirement and irrigation requirement
Rice crop was planted from June to November with a total of 180 days (FAO, 2003). ETc
and Irrigation requirements have been determined on decade (10 days) and daily basis. The
figure 5 shows calculated ETc and Irrigation Requirement on decade basis.
32
Figure 5: Crop water requirement and irrigation requirement from June – November
The figure 5 shows that peak irrigation requirement was 42.8 mm in 3rd decade of July
followed by 37.1 mm in the 1st decade of August, 35.3 mm in 3rd decade of August, 35 mm in
the second decade of August respectively. The lowest irrigation requirement was in1st
(0.7mm) while in 2nd and 3rd decade of November there is no need of irrigation water due to
maturity of crop.
The highest ETc was observed in 3rd decade of August (51.5mm) followed by the 3rd decade
of July, however the irrigation requirement in September was less than the irrigation
requirement of July and August because of the crops’ stage approaching towards maturity
and reduction in crop water requirement and irrigation requirement is reduced. While in July
and August the crop water requirement and irrigation requirement was high because the crop
was in development stage.
4.2. Hydraulic conductivity
Hydraulic conductivity is the permeability of saturated soil. Normally it indicates the
transmission of water in different direction under saturated soil conditions. As rice fields are
mostly having water stagnation, high hydraulic conductivity leads to severe water losses.
Therefore, hydraulic conductivity was determined at Base I swamp by using inverse auger
33
Table 3: Data collected during the field study
K=1.15 x r [ ]
K1= =0.012 cm/sec
K2= = 0.00603 cm/sec
K3= = 0.00402 cm/sec
K4= = 0.0023 cm/sec
K5= = 0.00345 cm/sec
K6= =0.000805 cm/sec
K = = =
K = 0.004768 cm/sec, we say
K = 4.768 x10-3cm/sec
The K measured was 4.768 x10-3cm/sec
S. No Height1(Ho in cm) Height2(Ht in cm) Time (min)1 40 30 52 30 22 53 22 17.5 54 17.5 14.9 55 14.9 12 56 12 11.4 5
33
Table 3: Data collected during the field study
K=1.15 x r [ ]
K1= =0.012 cm/sec
K2= = 0.00603 cm/sec
K3= = 0.00402 cm/sec
K4= = 0.0023 cm/sec
K5= = 0.00345 cm/sec
K6= =0.000805 cm/sec
K = = =
K = 0.004768 cm/sec, we say
K = 4.768 x10-3cm/sec
The K measured was 4.768 x10-3cm/sec
S. No Height1(Ho in cm) Height2(Ht in cm) Time (min)1 40 30 52 30 22 53 22 17.5 54 17.5 14.9 55 14.9 12 56 12 11.4 5
33
Table 3: Data collected during the field study
K=1.15 x r [ ]
K1= =0.012 cm/sec
K2= = 0.00603 cm/sec
K3= = 0.00402 cm/sec
K4= = 0.0023 cm/sec
K5= = 0.00345 cm/sec
K6= =0.000805 cm/sec
K = = =
K = 0.004768 cm/sec, we say
K = 4.768 x10-3cm/sec
The K measured was 4.768 x10-3cm/sec
S. No Height1(Ho in cm) Height2(Ht in cm) Time (min)1 40 30 52 30 22 53 22 17.5 54 17.5 14.9 55 14.9 12 56 12 11.4 5
34
According to the table this shows that the soil on which hydraulic conductivity has been
measured was 4.768 x10-3cm/sec (17.1cm/hr), which means that the soil of Base I is in rapid
class (table-1), very pervious, and care should be taken in order to avoid losses and this
improve puddling operation.
4.3 Computation of Irrigation Efficiency
To determine the conveyance and water application efficiencies field measurements were
done during the study period. The conveyance efficiency was determined in the secondary
irrigation channels covering the whole area under developed swamp.
4.3.1 Measurement of Discharge
To measure the discharge, area- velocity method was used. In this method, measurement of
cross sectional area of selected channel and flow velocity is done. To know the design cross
section and velocities, The Agro Action Allemande document was also consulted and
compared with the actual field conditions. As both channels are not used at a same time,
measurements were taken at one channel and calculations were made. The measurements of
channel section with three locations were done on secondary channel. The following table
indicate the detailed data. It was also found with observation ,project document and
interaction with farmers that both channels on both sides of the swamp are used at the same
time for irrigation and sections of both the channels are of the same size and some parts of the
swamp ridges do not get water because the secondary channels are not well levelled and of
less quantity of water that cannot reach the end of the swamp and that part which do not get
much water is occupied by maize crop.
35
Table 4: Secondary channel cross section and wetted area
Items Projectdesign Section 1 Section 2 Section 3
1 2 3 Av. 1 2 3 Av 1 2 3 AvCanal bedwidth (m)
0.4 0.3 0.2 0.29 0.26 0.3 0.35 0.36 0.34 0.4 0.3 0.23 0.31
Surfacewaterwidth(m)
1 0.56 0.6 0.67 0.61 1 0.6 0.8 0.80 0.8 1.2 0.56 0.85
Top width(m)
1.4 1.9 2 1.8 1.90 1.4 1.4 1.4 1.40 1.4 1.4 1.4 1.40
Ht ofembankment (m)
1.5 1.6 1.7 1.6 1.63 1.5 1.2 1.3 1.33 1.5 1.4 1.5 1.47
Waterheight (m)
0.35 0.32 0.39 0.4 0.37 0.23 0.25 0.26 0.25 0.23 0.25 0.24 0.24
Wettedarea(m2)
0.245 0.14 0.16 0.19 0.16 0.15 0.12 0.15 0.14 0.14 0.19 0.09 0.14
Av.wettedarea (m2)
0.16 0.16 0.14 0.13 0.14 0.12
4.3.2 Comparison of Project design dimensions and actually found at field for secondary
channel
4.3.2.1 Channel bed width
The following graph shows the variation of canal bed width for 3 canal sections
Figure 6: Secondary canal bed width variation
00.05
0.10.15
0.20.25
0.30.35
0.40.45
Section 1 Section 2 Section 3
Comparison beetween Canal bed
actual canal bed (m)
canal bed per agro actionallemande(m)
36
The above figure shows that observed canal bed width was below the designed. This means
that the prevailing problem in selected portions is siltation where transported soil from
catchment area and eroded channel might have raised the channel.
4.3.3 Measurement of flow velocity
The velocity of flow was also measured in all three locations three times at selected channel.
The average values of velocity at each section are presented in the table below.
Table 5: Average values of flow velocities at three locations of secondary channel
Item Location-1 Location-2 Location-3 Average
Length(m) 40 40 40 40
Time(s) 59 64 71 64
Velocity * 0.85(m/s)
0.58 0.53 0.48 0.53
4.3.4 Discharge
The discharge was determined by the multiplication of Average cross section and velocity of
flow at each location. The details are as follows:
Table 6: Discharge in secondary channel
Cross Sectional area
Sq. M
Velocity (m/sec) Discharge (m3/sec)
Location-1 0.16 0.58 0.093
Location-2 0.142 0.53 0.075
Location-3 0.139 0.48 0.067
Average discharge 0.078
37
4.3.5 Water conveyance efficiency
The difference of discharge in three locations is different due to losses on its way. The water
conveyance efficiency is the ratio of water diverted from source to water reached at plot.
Conveyance efficiency is:
Ec=100
Where:
Wf= Amount of water delivered to the fields (at head of field supply channel or farmdistribution system)
Wd= Amount of water diverted from sources
Therefore the observed conveyance efficiencies between first and third locations in the
channel can be taken as conveyance efficiency.
Table 7: Conveyance efficiency
Location Discharge
(m3/sec)
Conveyance Efficiency = (Discharge at
location 2/Discharge at location1) x 100
Location-1 0.093
Location-3 0.067 72%
From the above results, it is evident that due to losses, water conveyance efficiencies are
reduced. The measurements show that the loss of 28% is found. According to Majumdar
(2004), the losses may vary from 25 to 60 percent of water diverted for irrigation. He also
added that in the unlined canals, water ways and channels, the water loss is usually heavy and
the same is attributed mainly to seepage. In fact, growth of undesirable vegetation along and
in canals and in channel beds and sides is also responsible for additional losses.
In addition to that, there were many cracks in channel beds and bunds which may lead to
water losses by seepage. In fact, the canals are not well maintained.
37
4.3.5 Water conveyance efficiency
The difference of discharge in three locations is different due to losses on its way. The water
conveyance efficiency is the ratio of water diverted from source to water reached at plot.
Conveyance efficiency is:
Ec=100
Where:
Wf= Amount of water delivered to the fields (at head of field supply channel or farmdistribution system)
Wd= Amount of water diverted from sources
Therefore the observed conveyance efficiencies between first and third locations in the
channel can be taken as conveyance efficiency.
Table 7: Conveyance efficiency
Location Discharge
(m3/sec)
Conveyance Efficiency = (Discharge at
location 2/Discharge at location1) x 100
Location-1 0.093
Location-3 0.067 72%
From the above results, it is evident that due to losses, water conveyance efficiencies are
reduced. The measurements show that the loss of 28% is found. According to Majumdar
(2004), the losses may vary from 25 to 60 percent of water diverted for irrigation. He also
added that in the unlined canals, water ways and channels, the water loss is usually heavy and
the same is attributed mainly to seepage. In fact, growth of undesirable vegetation along and
in canals and in channel beds and sides is also responsible for additional losses.
In addition to that, there were many cracks in channel beds and bunds which may lead to
water losses by seepage. In fact, the canals are not well maintained.
37
4.3.5 Water conveyance efficiency
The difference of discharge in three locations is different due to losses on its way. The water
conveyance efficiency is the ratio of water diverted from source to water reached at plot.
Conveyance efficiency is:
Ec=100
Where:
Wf= Amount of water delivered to the fields (at head of field supply channel or farmdistribution system)
Wd= Amount of water diverted from sources
Therefore the observed conveyance efficiencies between first and third locations in the
channel can be taken as conveyance efficiency.
Table 7: Conveyance efficiency
Location Discharge
(m3/sec)
Conveyance Efficiency = (Discharge at
location 2/Discharge at location1) x 100
Location-1 0.093
Location-3 0.067 72%
From the above results, it is evident that due to losses, water conveyance efficiencies are
reduced. The measurements show that the loss of 28% is found. According to Majumdar
(2004), the losses may vary from 25 to 60 percent of water diverted for irrigation. He also
added that in the unlined canals, water ways and channels, the water loss is usually heavy and
the same is attributed mainly to seepage. In fact, growth of undesirable vegetation along and
in canals and in channel beds and sides is also responsible for additional losses.
In addition to that, there were many cracks in channel beds and bunds which may lead to
water losses by seepage. In fact, the canals are not well maintained.
38
4.4 Water losses calculation in Base I swamp at plots
4.4.1 Calculation of percolation, infiltration and evapotranspiration
4.4.1.1 Results of water losses calculation of nine plots in Base I swamp.
Plot H1 (cm) Day I, H2 (cm) Day II, ∆H1 (cm) Day III, H3 (cm) ∆H2 (cm)
1 8.1 9h30a.m 6.3 9h30a.m 1.8 9h30a.m 5.5 1.72 6.8 9h30a.m 5.2 9h30a.m 1.6 9h30a.m 4.6 1.63 9.0 9h30a.m 7.5 9h30a.m 1.5 9h30a.m 6.6 1.94 7.0 9h30a.m 5.1 9h30a.m 1.9 9h30a.m 4.5 1.65 8.7 9h30a.m 7.2 9h30a.m 1.5 9h30a.m 6.4 1.86 9.2 9h30a.m 7.5 9h30a.m 1.7 9h30a.m 6.8 1.77 9.0 9h30a.m 7.0 9h30a.m 2.0 9h30a.m 6.1 0.98 10 9h30a.m 8,0 9h30a.m 2.4 9h30a.m 6.7 1.39 7.8 9h30a.m 6.4 9h30a.m 1.4 9h30a.m 5.9 0.5
D: Day; T: Time; ΔH: Water level difference
Total water losses for nine plots = (1.8 + 1.6+ 1.5+ 1.9 + 1.5 +1.7+2.0+2.4+1.4+ 1.7 + 1.6 +
1.9 + 1.6 + 1.8+1.7+0.9+1.3+0.5) cm
Total losses for nine plots are equivalent to 28.8 cm.
Average water losses per plot = 28.8/9 cm = 3.2 cm/plot/2 days = 3.2 cm/1000 m2/ 2days.
Daily average water losses = 3.2/2 = 1.6 cm/day.
The average water depth maintained was equal to (8.1+6.8+9.0+7.0+8.7+9.2+9.0+10+7.8)/9
= 8.4cm.
The average water losses in the field through ETO and deep percolation are 1.6 cm/day.
The average losses through ETO from Appendix 3 are 737.1 mm/180 days which equal to
average ETO of 4.09 mm/day. The average water losses including ETO and deep percolation
are 1.6cm/day.
Therefore, considering the depletion of water from root zone as 4.09 mm/day, then the total
water applied to compensate water depletion from root zone and deep percolation would be
16.0 mm. Hence loss due to deep percolation = 16 – 4.09 = 11.91 mm say 12 mm
Hence, water application efficiency Ea = 4.09 x 100/16 = 25.56% say 25.6%
The overall efficiency would be Ec x Ea
= 0.70 x 0.256 x 100 = 17.92% say 18%
39
In fact, the deep percolation loss in wet rice is exceptionally high and ranges from 38.3% to
as much as 80% of the water applied in various soils (Mandal and Majumdar, 1983).
Considering gross irrigation (700.2mm) while net irrigation (490.1mm) determined from
cropwat it is clear that 70% irrigation efficiency has been considered. However actual
measurement of irrigation efficiency shows only 18% and actual irrigation requirement
would be 413.6 mm/0.18 = 2297 mm.
4.4.2 Water use efficiency
The average production level of rice in Base I swamp is 5.6 tons/ha (Apendix 7). It is evident
that to produce 5.6 tons of rice from one hectare, an amount of 22970 m3water will be
required. Therefore the water use efficiency would be 5600 kg/22970 m3= 0.24 kg/ m3This
means, to produce 1kg of rice 4.16 m3of water is required.
In Base I swamp, the water application is very poor and it shows that the area under crops is
less due to huge water losses. There is lack of knowledge of farmers in rice water
requirement and its management. The type of soil with high hydraulic conductivity (17.1
cm/hr) and low level of puddling and levelling is the main cause of lower efficiency.
Therefore, the farmers have to manage well the small amount of water received in their rice
fields by uniformly applying it and preventing the deep percolation and run-off losses. The
efficiency may be very low in a badly managed farm and higher in a well managed farm.
Majumdar (2004) stated that the application efficiency can be increased to approach 80% if
crops are under-irrigated by applying lower amount of water than needed because of water
scarcity or high-priced water.
Under-irrigation may completely prevent deep percolation and run-off of water, but it is
undesirable as crops suffer from water stress and give lower yields. Proper land levelling and
grading is a good manner (way) for efficient water application. This is needed to avoid
accumulation of excess water in lower spots leading to deep percolation loss and under-
irrigation of higher spots, and to achieve uniform run and distribution of water in the field.
40
4.4.3 Determination of discharge for main and secondary channels
The peak irrigation requirement found through cropwat was 4.3 mm. considering 18%
efficiency the gross amount of discharge for irrigation can be determined as follows:
Gross irrigation requirement =Net irrigation requirement in m x Area in sq.m/ overall
efficiency;
= 4.3 x10 -3 x 105 (ha) x 10000/0.82 = 5506 cum.
Assuming that irrigation will be supplied for 10 hours;
Hence discharge of main channel would be
5506/(10 x 3600) = 0.152 cumecs
As there are two secondary channels one channel will handle 0.152 cumecs/2 = 0.076 cumecs
4.5 Design of primary and secondary channel
4.5.1 Design of primary channelPrimary canal which take water from intake and divert it in secondary channel, these canalsare located around the edge of the swamp.
Q= 0.152 cumecs
Side slope=1.5:1
As the soil is sandy loam then df; mean particle size df =0.32 because the coarse silt is taken
as standard silt (K.R.Arora,2004).
Silt factor f=1.76df= 1.760.32=0.99 say 1.0
Velocity= 6(0.152x121/40)= 0.32 m/sec
Cross section area = Q/V= 0.152/0.32= 0.475m2
Wetted perimeter= P=4.75Q= 4.750.152=1.85 m
Hydraulic radius=0.47 3Q/f= 0.47 30.152/1= 0.25 m
R = A/P=0.475/1.85= 0.25m
Longitudinal slope:2440
1
152.03340
1
3340 61
35
61
35
Q
fS
During the design of channel, consider the most economical trapezoidal channel section.
41
Figure 7: Economic trapezoidal channel
1) Half top width is equal to side length
T/2 = (b+2my)/2= y1+m2 ( This is the condition for the best side slopes)
m = 1.5
2) tanӨ = 1/m or arctan1/m= Ө and then Ө=340 made to the horizontal. The geometric
elements for the most economical trapezoidal channel section are:
A=(b+my)y
P=2(b+my)
R=y/2
T/2=(b+2my)/2= y1+m2
tanӨ=1/m
A=0.475=(b+my)y=0.475=(b+1.5y)y
{b+(2x1.5y) }/2=y1+1.52
or (b+3y)=3.6y
b=0.6y
P=2(b+my)
1.85=2(0.6y+1.5y)
1.85=4.2y
y=1.85/4.2=0.44m
If b=0.6y
b=0.6x0.44=0.264m
Then our channel will have the following dimensions:
Depth=0.44m
Bed width: b=0.264m
42
Top width; T=b+2my=0.264+2(1.5x0.44)=1.584m
Figure 8: Primary canal
4.5.2 Design of secondary channel
Secondary and third level canals are excavated with respect of their constant cross section
area. They often have the trapezoidal shape and the side slope vary with the type of soil. The
role of secondary and tertiary canals is to transport water with maximum efficiency to the
place of utilization (fields) (HYDROPLAN, 2002). Secondary canal is small than primary,
the discharge which pass in secondary canal is half to that pass in the primary.
Q=0.152 cumecs /2 = 0.076 cumecs
Side slope=1.5:1
As the soil is sandy loam then df; mean particle size df = 0.32 because the coarse silt is taken
as standard silt (K.R.Arora,2004).
Silt factor f=1.76df= 1.760.32=0.99 say 1.0
Velocity= 6(0.076x12/140)= 0.28 m/sec
Cross section area = Q/V= 0.076/0.28= 0.27m2
Wetted perimeter= P=4.75Q= 4.750.076=1.3 m
Hydaulic radius=0.47 3Q/f= 0.47 30.076/1= 0.199 m say 0.2m
R = A/P=0.475/1.85= 0.2m
Longitudinal slope:2174
1
076.03340
1
3340 61
35
61
35
Q
fS
Our channel will be the most economical one with the following characteristics:
1) Half top width is equal to side length
T/2=(b+2my)/2= y1+m2 ( This is the condition for the best side slopes) and m = 1.5
43
2) tanӨ=1/m or arctan1/m= Ө and then Ө=340 made to the horizontal. Ther geometric
elements for a most economical trapezoidal channel section are:
A= (b+my)y
P= 2(b+my)
R= y/2
T/2= (b+2my)/2= y1+m2
TanӨ= 1/m
A=0.27= (b+my)y = 0.27=(b+1.5y)y
{b+(2x1.5y) }/2=y1+1.52
Or ( b+3y) = 3.6y
b= 0.6y
P=2(b+my)
1.3 = 2(0.6y+1.5y)
1.3= 4.2y
y = 1.3/4.2= 0.30m
If, b=0.6y
b = 0.6x0.3= 0.2m
44
Then the channel will have the following dimensions:
Depth=0.3m
Bed width: b=0.2m
Top width; T=b+2my=0.264+2(1.5x0.44)=1.1m
Figure 9: Secondary canal
The most economical secondary channel should contain the above values which have the
small dimension compared to the existing dimensions of the channel from the actually design.
45
CHAPTER-5
CONCLUSION AND RECOMMANDATIONS
The methodology as described in chapter 3 was adopted for the present study. The data
collected, analyzed and discussed. It was found that the soils are highly permeable with 17.1
cm/hr hydraulic conductivity (rapid category) leading to enormous water losses. Results also
indicated that very low overall irrigation efficiency with 18% poses high water losses to the
tune of 82 %.
The water use efficiency was 0.24 kg rice /m3 he required discharge of the main and
secondary channel found was 0.152 cumecs and 0.076 cumecs respectively. The desired cross
section of main and secondary channel found was 0.47 sq.m and 0.27 sq.m respectively; this
is more due to low velocity of design. The secondary channel bed width as proposed by the
project was more (0.1m) than actually found due to siltation and weed infestation. However
due to problems such as lack of water during dry season, low use of fertilizers etc, farmers
are not able to get more production in whole areas. Therefore, other crops like beans, maize
and vegetables are also grown in about 41 hectares while the 7 hectares are subjected to water
logging due to low elevation with respect to water level of main stream during rainy season
and water stagnation during rest of the year due to seepage water from adjacent area. This
leads to the losses of cultivated crops. The negative consequence of variable discharge in the
irrigation canal is that water will be available at upstream fields but not available at the tail
end fields caused by poor land levelling. This can provide the conflicts among the farmers of
downstream and upstream side.
With the above conclusions drawn during this study, the following recommendations are
proposed:
Farmers should be organized and sensitized for effective and efficient irrigation water
management;
Capacity building programmes on efficient water and other inputs management must
be organized by the MINAGRI in coordination with the Ministry of Natural Resources,
agricultural research and educational institutions;
Demonstrations for appropriate input management be done regularly.
MINAGRI should promote rice production, processing and marketing should be
established to safeguard the interest of rice growers;
46
Water users’ associations should be responsible for collecting the prescribed water
charges, timely maintenance of infrastructures and management of the irrigation
scheme;
Lining of peripheral canals and regular maintenance of irrigation and drainage
channels and other infrastructures should be done for long time benefits.
Such study should be conducted in Base I in both agricultural season (A&B).
47
REFERENCE
1. Agro-Action Allemande, 2005. Etude d’aménagements hydro-agricoles et de
protection des basins versants pour les marais de Rugeramigozi Amont, Biringanya
Base et Kiryango.
2. Arora, K.R., 2004. Irrigation, Water Power and Water resources engineering,
Standard Publishers distributors, Fourth Ed., New Delhi.
3. Bartstsnellen W, 1997: Exploitation et entretien des réseaux d’irrigation FAO, Rome.
4. Dungan, P.J. 1991. Wetlands Management: a critical issue for conservation in Africa.
In: Proceeding of the SADC Wetlands Conservation Conference for Southern Africa.
Eds. T.Matiza and H.N. Chabwela.
5. FAO, 1994, Irrigation water delivery models.
6. Gautam, O.P. and Dastane, N.G., 1970. Agronomic practices and water-use patterns
for higher crop yields. Agriculture year book-New Vistas in Crop Yields. ICAR, New
Delhi.
7. H. C. Zhang, 2009: Study on cultival science problem of direct-seeded rice. Beijing:
Chinese agricultural science press, (In Chinese).
8. H. Ikeda, A. Kamoshita, J. Yamagishi, M. Ouk, and B. Lor, (2008) Assessment of
management of direct seeded rice production under different water conditions in
Cambodia. Paddy and Water Environment, Vol. 6.
9. HYDROPLAN, 2002. Schéma directeur de l’aménagement des marais, de protection
des basins versants et de la conservation des sols, Kigali.
10. Islaerson, Orson W. And Hensen, VaughE, 1962.Irrigation principales and practices,
John Wiley and Sons. Inc; USA, and topan company Ltd.Japan.
11. KALISONI, J, 2004: Etude de dimensionnement des ouvrages linéaires et de mise en
valeur du marais Runukangoma, ISAE Busogo.
12. Machibya, M. And Makarius, M., 2005. Comparison Assessment of Water Use and
Damage between Modern and Traditional Rice Irrigation Schemes: Case of Usangu
Basin, Tanzania. In: International Journal of Environmental Research and Public
Health, http://www.ijerph.org, MDPI.
13. Majumdar, D.K., 2004. Irrigation Water Management. Principles and Practices,
Prentice-Hall of India Private Limited, New Delhi.
48
14. Mandal, A.K. and Majumdar, D.K., 1983. Effect of phasic soil submergence and
saturation on growth and water use in Rice in semi-arid lateritic tract of West Bengal.
15. Michael, A.M. and Ojha, T.P., 1966. Principles of Agricultural Engineering, Vol. II,
Jain Brothers, First Ed., Jaipur, India.
16. Michael, A.M. and Ojha, T.P., 1981. Principles of Agricultural Engineering, Vol. II,
Jain Brothers, First Ed., Jaipur, India.
17. MINAGRI, 2001: Rapport du projet d’amenagement du marais de l’AKAGERA,
Kigali.
18. MINAGRI, 2003: Formation sur organisation, gestion et entretien des périmètres
d’irrigation, Kigali.
19. MINIJUST, 2005. Official Gazette of the republic of Rwanda. Organic Law
determining the use and management of land in Rwanda, Kigali. Rapport provisoire,
Gitarama.
20. Ruhango district, 2010. Rapport annuel sur le developpement.
21. Smith, 1975: Swamplands soil properties in tropical humid regions, Washington D.C.
22. TRANSTEC, 2004, Rapport sur le recensement de la population au niveau national,
MINALOC, Kigali.
23. T. P. Tuong, A. K. Singh, J. D. L. C. Siopongco, and L.J. Wade, Constraints to highyield of dry-seeded rice in the rainy season of a humid tropic environment. PlantProduction Science, Vol. 3, pp. September 164-172, 2000.http://books.irri.org/9789712202193_content.pdf
49
APPENDICES
Appendix 1: Crop water requirement from June to November
Month Decade stage Kccoeff
Etcmm/day
Etcmm/decade
Eff rainmm/decade
Irr.Req.mm/decade
June 1 Init 1.05 3.51 35.1 16.7 18.4June 2 init 1.05 3.64 3.64 6.4 30.0June 3 init 1.05 3.75 37.5 5.6 31.9July 1 Dev 1.07 3.94 39.4 4.5 35July 2 Dev 1.12 4.22 42.2 1.9 40.2July 3 Dev 1.16 4.48 44.8 5.4 43.8August 1 Mid 1.18 4.64 46.4 9.4 37.1Aug 2 Mid 1.18 4.74 47.4 12.1 35.3Aug 3 Mid 1.18 4.68 46.8 16.1 35.3Sep 1 Mid 1.18 4.62 46.2 21 25.2Sep 2 Mid 1.18 4.56 45.6 25.3 20.3Sep 3 Mid 1.18 4.46 44.6 26.3 18.3Oct 1 Mid 1.18 4.37 43.7 26.8 16.8Oct 2 Mid 1.18 4.26 42.6 28.1 14.5Oct 3 Late 1.12 3.91 39.1 30.6 12.4Nov 1 Late 1.04 3.50 35.0 34.3 0.7Nov 2 Late 0.96 3.12 31.2 37.3 0Nov 3 Late 0.90 2.87 28.7 24.3 0Total 737.1 332.1 415.3
Decade = Period of 10 days.
1. Decade means first period of 10 day of the month.
2. Decade means second period of 10 days of the month.
3. Decade means third period of 10 days of the month.
Appendix 2: Byimana Mean climatic data used as input into Cropwat 8
PARAMETERS UNIT JAN FEB MAR APR MAY JUN JUL AUG SEPT OCT NOV DEC
Min,Temp ◦C 13 13.2 13.2 13.5 13.5 12 13.2 12.2 12.9 11.4 12.9 12.8
Max,Temp ◦C 24 23.9 23.7 23.9 22.9 24.1 24.5 24.5 24.4 24.3 24.4 23.4RelativeHumidity % 78 76 79 85 82 71 61 59 69 76 82 81
Wind Speedkm/day 140 140 138 110 112 114 123 147 144 142 146 142
Sunshine Hours 5.7 5.5 5.6 5.2 5.3 7.2 7.9 7.6 6.2 5.8 5.5 5.4
50
Appendix 3: ETo from Byimana mean climatic data using Cropwat 8
Appendix 4: Rainfall climatic data used in Cropwat 8.
52
Appendix 7: Answer from the farmers of Base I swamp about production (2011)
S. NAME YIELD/Ha (Tone)A 7.4B 6.5
NORTHERN PART C 7D 6.5E 7.5S.NAME YIELD/HaF 4.8G 4
SOUTHERN PART H 3.5I 5.5J 3.5
Average Production (Tone) 5.62
Appendix 8: Ruhango district map
53
Appendix 9: Climatological data from Byimana meteorological station (1978-2009)
1) Maximum temperature; station of Byimana
Date Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
1979 24.75 24.858 23.835 23.113 24.132 24.313 24.283 26.112 25.423 24.874 24.84 25.45
1980 25.75 23.025 24.022 23.806 24.512 24.556 23.619 25.954 24.94 23.674 21.96 23.51
1981 23.36 25.346 24.038 23.79 22.929 23.733 24.493 24.1 24.573 23.896 24.186 24.19
1982 23.61 24.532 23.858 23.42 22.683 23.476 24.609 25.506 25.713 26.016 24.07 21.91
1983 24.65 24.644 24.683 21.563 23.396 24.226 23.595 22.94
1984 24.74 23.907 23.396 22.67 22.354 23.31 24 24.538 24.263 23.516 22.363 23.5
1985 24.4 23.514 23.383 22.886 23.264 22.46 24.445 25.248 23.39 24.245 23.1 23.75
1986 24.35 24.075 24.451 23.463 22.412 22.21 22.525 24.009 23.77 23.2 22.69 23.29
1987 23.47 22.772 22.383 22.31 21.974 22.19 21.951 23.974 25.213 24.387 21.976 22.94
1988 23.77 23.178 23.558 23.39 22.406 22.963 23.683 24.848 25.176 24.629 23.056 24.32
1989 23.39 24.357 23.316 22.53 22.038 22.76 23.596 23.141 24.45 24.361 24.036 22.89
1990 23.82 23.085 23.638 22.05 21.822 22.42 21.638 24.438 23.546 24.016 23.323 22.84
1991 23.4 23.472 23.793 22.913 21.703 22.206 23.283 24.99 24.2 22.856 23.97
1992 24.55 25.317 23.996 22.723 22.638 22.776 24.058 24.18 24.773 24.093 23.323 23.5
1993 23.71 24.467 23.79 22.69 22.283 22.55 20.919 24.764 24.123 25.141 23.76 22.83
1997 23.83 23.764 23.012 23.21 22.712 22.5 22.693 23.464 22.796 23.49 23.673 23.18
1998 24.5 22.662 24.038 23.176 22.293 22.42 23.271 23.696 24.053 24.6 24.013 23.93
1999 23.63 23.514 23.506 22.78 22.841 22.913 24.003 24.067 24.466 24.825 23.006 24.06
2000 24.15 24.453 23.022 23.106 22.506 22.766 23.88 24.477 25.156 24.496 23.12 22.93
2001 23.75 24.057 24.525 23.04 21.887 22.306 23.377 24.819 26.176 25.677 24.59 23.59
2002 24.3 24.727 24.151 23.57 22.496 23.5 23.371 24.664 24.93 24.641 23.286 23.57
2003 22.443 22.274 22.64 23.258 24.387 24.577 24.81 23.67
2004 24.916 23.09
2005 23.9 24.03 24.245 22.973 22.271 23.566 22.29 24.532 25.49 23.561 23.393 23.55
2006 24.41 24.327 24.358 23.34 22.09 21.993 22.638 24.048 24.08 23.951 22.776 22.72
2007 23.62 23.621 23.296
2008 23.971 22.822 23.87 24.561 23.756 23.75
2009 23.387 23.473
Average 24.07 23.988 23.762 22.956 22.612 22.969 23.338 24.426 24.581 24.382 23.415 23.46
54
2) Minimum temperature; station of Byimana
Date Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec1977 12.75 13.155 12.703 13.506 12.996 11.146 10.941 11.312 12.91 12.8 11.31 12.181978 12.69 13.082 13.093 13.086 13 11.516 11.99 12.203 13.33 12.877 13.85 13.5
1979 13.43 12.467 12.612 12.996 13.254 11.75 10.719 12.648 12.6 13.283 12.87 12.64
1980 12.75 12.825 12.416 13.16 14.048 11.473 10.722 9.6064 12.08 12.948 13.06 12.83
1981 13.11 13.262 13.361 13.41 12.593 11.62 10.725 11.761 12.91 12.677 12.29 12.26
1982 12.14 12.685 13.348 13.276 13.058 10.853 10.603 12.387 12.806 13.532 13.1 12.99
1983 12.9 13.578 13.735 13.793 13.219 12.63 12.096 12.041 13.053 12.971 13.28 12.6
1984 12.8 12.717 13.158 13.69 13.806 11.976 11.158 11.516 12.386 13.025 13.01 13.46
1985 13.24 14.158 13.767 14.14 13.516 12.43 12.309 13.08 12.89 13.264 13.21 12.911986 13.24 14.157 13.977 13.626 13.422 11.74 11.496 11.541 13.453 13.177 13.21 13.091987 13.69 13.792 13.883 14.23 13.696 11.1 11.809 12.996 12.863 13.409 12.103 11.251988 11.82 11.5 11.5 11.63 11.838 9.7066 10.554 10.509 11.1 11.245 11.163 10.81989 11.05 11.786 11.312 11.713 11.732 11.42 8.5258 10.786 11.458 11.026 11.21990 11.6 12.025 11.254 11.47 11.803 9.7633 8.8516 10.551 11.686 11.387 11.133 10.381991 10.17 12.228 14 14.176 13.935 13.62 12.961 12.59 12.973 13.577 13.55 12.791992 13.03 13.16 12.783 13.45 13.629 12.263 11.787 12.616 12.946 13.874 13.036 13.061993 13.01 12.624 13.245 12.956 13.793 12 12.161 12.806 13.76 13.88 14.26 13.191997 13.59 14.307 13.98 14.4 14.329 13.35 12.374 12.835 14.413 14.229 14.163 14.171998 14.33 14.289 14.79 14.396 13.971 13.103 12.119 13.025 13.236 13.583 13.13 14.021999 13.65 14.31 13.458 14.536 14.164 12.433 11.483 13.216 13.286 14.18 13.983 13.612000 14.03 13.9 13.377 14.436 14.48 12.616 11.961 12.303 14.056 13.574 13.41 13.822001 13.54 13.575 14.658 14.336 11.896 12.077 13.177 13.013 13.59 13.24 13.352002 13.15 13.153 13.058 13.976 13.89 12.383 11.019 12.8292003 13.82 13.725 11.646 11.206 12.777 13.287 13.526 13.462004 13.577 13.82005 13.44 13.73 14.125 13.613 14.367 13.183 12.067 12.619 13.503 13.174 13.266 13.922006 13.73 14.162 14.122 14.51 13.722 13.463 11.838 12.051 13.536 14.212 13.453 13.412007 13.5 13.557 12.3412008 10.887 13.677 13.246 13.541 13.333 13.452009 12.94 14.887 13.833Averag
e12.94 13.238 13.233 13.551 13.495 12.034 11.35 12.257 12.913 13.197 12.961 12.89
55
3) Rainfall ; station of Byimana
Date Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec1977 84.6 84.2 145.9 149.2 188.4 6.3 0 30.4 53.3 171.4 233.9 132.91978 112.2 97.6 137.3 273.7 39 1.6 3.5 25.2 52.5 110.2 71.7 45.21979 69.3 121.2 150.4 96.5 99.2 2.1 7 0.8 120.7 144.6 222.2 199.31980 168.5 13.4 130.4 123.1 199.3 15.5 18.4 93.1 104.4 272.4 124.2 131.11981 167.4 99.9 80.2 205.6 402.4 47.5 0 43.8 148.5 33.7 110.8 205.91982 93.3 162.5 106.5 272 68.7 9.9 66 22.1 44.5 134.8 144.6 70.41983 89.7 69 102.4 282.9 141 8 0 42.5 103.7 111.5 146.4 78.11984 30.7 164.5 163 179.8 61.9 22.7 0 100.8 72.2 65.2 136.4 43.71985 40 63 111.3 169.3 293.6 49.4 24 7.8 130 30.9 143.4 193.91986 128.2 171.5 213.8 187.8 91.6 67.8 8.8 0.2 36.7 55.4 170.6 96.11987 88.9 85.7 123.1 121.7 135.4 1.4 0.3 0 65.5 65.3 106.9 44.21988 265.2 152.7 170.9 232 56.3 19.7 17.3 56.2 56.8 103 169.2 64.41989 103.7 132.3 80.8 193.3 197.6 0 16.8 126.6 51.6 42.9 115.2 121.21990 63.3 225.2 68.3 84 103.2 111.5 0 52.2 106.9 223.4 83.31991 85.5 104.2 84 258.9 232.7 4.7 0 36.4 204.8 105.3 189.5 73.11992 76.5 32.6 276.5 173.3 202.4 105.9 87.9 7.8 84.7 34.5 121.7 55.41993 136 82.6 73.9 231.8 142.7 3.4 52.4 14.6 135.2 151.1 92 160.51997 65.1 99.6 113.9 118.5 143.6 31.5 0 82.6 90 94.6 74.3 82.51998 116.3 87.2 105.4 237.4 93.6 7.2 5.5 66.3 119.3 121.7 161.6 109.21999 85.1 125.2 243.1 173 127.8 22 0 39.5 37.1 55.4 106.1 106.92000 210.1 150.3 36.3 185.6 234.7 52.3 0 21.5 5.5 28.8 140.9 129.92001 81.9 96.9 86.2 204.5 160.1 3.8 0 8.9 154.2 110.4 182.9 122.52002 62.2 69.7 150.7 186.9 174.2 0.2 0 148.8 107.5 79.7 62.9 83.42003 68.2 83.3 45.3 247.5 224.6 41.3 0 6.12004 12 200.7 131.4 283.7 70.5 3.3 18.6 36.7 46.4 124.7 177.4 151.42005 78 88 97.5 181.5 26 0.2 65.9 42.2 24.9 171.6 149.1 68.92006 83 126.7 163.7 263.9 65.8 30.6 0 0.2 181.8 116.5 157.9 143.32007 169.6 155.4 128 412.8 183.6 51.2 0 25.1 43.1 145.5 97.5 130.12008 153.1 222.6 111.5 190.5 220.7 87.4 0 32.8 167.7 163.3 344.3 80.82009 90.3 172.3 216.6 218.4 113.1 4.6 0.3 105.4 91.7 97.4 102.9 56.1
Average 110.3 122.72 129.84 201.17 147 30.571 12.456 40.051 84.915 102.95 136.49 108.3