Hydrological Sciences-Journal'des Sciences Hydrologiques, 47(3) June 2002 493

Recent changes in the level of Lake Abiyata, central main Ethiopian Rift

TENALEM AYENEW Department of Geology and Geophysics, Addis Ababa University, PO Box 1176, Addis Ababa, Ethiopia dgg@telecom.net,et

Abstract The spatial and temporal variations in the level of Lake Abiyata and controlling natural and manmade factors are presented. This study has been made by combining evidence from hydrometeorological and lake level records, water budget analyses, aerial photograph and satellite imagery interpretations, and numerical groundwater flow modelling. The most important components of the water balance of the lake are precipitation, river inflow and evaporation. The lake level has been fluctuating considerably over a wide range (by 6 m during the last 60 years) strongly controlled by the precipitation trends in the adjacent highlands. Climatic changes and consequent reduction in the surface water inputs have resulted in the reduction of its size. Recent abstraction of water for irrigation and soda ash production have drastically changed both the lake level and its hydrochemistry. This change appears to have grave environmental consequences on the fragile rift lacustrine ecosystem.

Key words abstraction; lake level change; hydrological setting; water budget; Ethiopian Rift

Modifications récentes du niveau du Lac Abiyata, rift central éthiopien Résumé Cette étude présente la variation spatio-temporelle du niveau du Lac Abiyata et ses causes naturelles et anthropiques. Elle s'appuie sur la combinaison de données enregistrées d'hydrométéorologie et limnimétriques, d'analyses du bilan hydrique, d'interprétations de photographies aériennes et d'images satellitaires, et de modél­isations numériques de l'hydrogéologie. Les principales composantes du bilan hydrologique du lac sont la précipitation, l'évaporation et le débit des rivières. Le niveau du lac fluctue considérablement dans une gamme importante (environ 6m durant les soixante dernières années) essentiellement en liaison avec la tendance d'évolution des précipitations sur les montagnes avoisinantes. Les changements climatiques et la réduction des apports d'eau de surface ont conduit à une diminution de la taille du lac. Les prélèvements récents pour l'irrigation et la production de soude ont radicalement modifié le niveau et l'hydrochimie du lac. Cette modification engendre de graves problèmes environnementaux dans le fragile écosystème lacustre du rift.

Mots clefs prélèvement; changement de niveau lacustre; hydrologie; bilan hydrologique; rift éthiopien

INTRODUCTION

The physical regimes and the levels of lakes are governed by many natural and anthro­pogenic factors. Climatic, hydrological and man-induced factors control lake levels in many ways (Nicholson et al., 2000; Sene, 2000; Yin et al., 2000). Changes in lake levels result from a shift in the water balance or the net steady-state removal of water via various surficial and subsurface processes. In particular, closed terminal lakes fluctuate significantly in response to climatic changes but tend to maintain equilibrium between input and output. Slight shifts in the regional climate and withdrawal of water on a time scale of decades, or even less, can change the steady-state elevation of terminal lakes by several metres.

Open for discussion until 1 December 2002

494 Tenaient Avenew

Some of the Ethiopian Rift lakes, particularly those located in a terminal position, have undergone significant lake level changes since the 1970s. However, very few of them are being used for irrigation, soda ash (NaCCh) extraction and commercial fish farming. In the last few decades, the ever-growing utilization of water resources in the rift and adjacent highlands has induced salinization of irrigation fields and lake level changes (Hailu et al, 1996; Ayenew, 1998). The chemistry and ecological setting of some of the lakes has changed (UN, 1973; Makin et al, 1976; Kebede et al, 1996). The main controlling factors are not well understood, particularly the relative importance of natural and manmade factors.

Reconstruction of climate and environmental changes over the last few decades is essential for understanding the impact of natural processes and anthropogenic factors on the hydrological setting and ecosystems and to forecast their evolution in the near future. This is especially relevant in the semiarid regions of the African tropics, including the Ethiopian Rift, characterized by large interannual changes in precipitation (Vallet-Coulomb et al, 2001) and where increasing population pressure makes areas more sensitive to the fluctuations of water resources (Servat et ai, 1998).

Analysis of observed records available for recent decades has considerably assisted in the understanding of the response of inland water bodies to climate changes and man-induced factors in many East African rift lakes (Makin et al., 1976; Chernet, 1982). This work focuses on these issues, taking the terminal Lake Abiyata as a model.

GENERAL OVERVIEW OF THE AREA

Lake Abiyata is a relatively shallow, small alkaline closed lake, located in the Ziway-Shala basin in central Ethiopia (Fig. 1). It lies in a saucer-shaped hollow within a deep faulted trough at an elevation of 1580 m a.m.s.l. The basin is part of the Ethiopian Rift system bordered by high altitude plateaux to the east and west. Four major lakes of different depths and sizes occupy the floor of the rift: Ziway, Langano, Shala and Abiyata, which are fed by perennial rivers originating from the highlands (Table 1). The first three lakes are connected by rivers, while Lake Shala has a closed catchment without surface water connections to the other lakes. A rise in the level of Lake Abiyata to about 1592 m a.m.s.l. would unite the system with Lake Shala. These lakes are the home for many endemic birds and a wide variety of wild animals.

The basin has three physiographic regions: the rift, escarpment and highland. The altitude ranges from 1600 m a.m.s.l. in the rift to over 4000 m a.m.s.l. in the large volcanic peaks of the Eastern Highlands. The climate is humid to subhumid in the highlands and semiarid in the rift valley. The mean annual temperature is around 15°C in the highlands and 20°C in the rift valley. The average annual rainfall ranges from 1150 mm in the highlands to 650 mm in the rift floor (Ayenew, 1998). The main rainy season is between June and September and the dry season lasts from October to February.

The lakes of the Ethiopian Rift experience a wide range of climate, accentuated by the annual north-south movements of inter- and sub-tropical frontal zones across the country. The relatively shallow depth and its terminal position, make Lake Abiyata more susceptible to changes in climate and input from precipitation and discharge. The main inflow is from direct precipitation and discharge from the Bulbula and Horakelo

Recent changes in the level of Lake Ahiyata 495

Eritrea

Djibouti

Study area

Kenya

38 00' 39 30' 8 30

At%%§K Topographic contours Rivers River gauging stations Meteorological stations Major mountains

Major Irrigated fields

8 30'

/ 0 30 km

7 00' 38 00' 39 30'

7 00'

Fig. 1 Location map of the studied basin.

496 Tenaient Ayenew

Table 1 Basic hydrological data of the lakes (source: Wood & Tailing, 1988; Chernet, 1982; Ayenew, 1998).

Lake

Abiyata Langano Shala Ziway

Altitude (m a.m.s

1580 1585 1550 1636

.1.) Lake area (km2)

180 230 370 440

Catchment area (km2)

10740 2000 2300 7380

Maximum (m)

14.2 47.9

266 89

depth Mean (m)

7.6 17 8.6

25

depth Volume (106m3)

957 3800

37000 1466

Salinity (gï1) 16.2 1.88

21.5 0.349

rivers, which are the outflows of lakes Ziway and Langano, respectively. As a closed lake, the only significant water loss from Lake Abiyata is through evaporation (Ayenew, 1998). Groundwater flow model simulations indicate negligible groundwater outflow from the lake (Ayenew, in press). The variability of the annual evaporation rate is far lower than that of the other hydrological budget terms: changes in lake level and volume reflect and amplify the changes in inputs from rainfall and rivers. However, recent development schemes, such as pumping of water from Lake Abiyata for soda ash extraction, and the utilization of water from feeder rivers and Lake Ziway for irrigation has resulted in rapid reduction in lake level.

The economic feasibility of soda ash extraction from lakes Abiyata and Shala was investigated in 1984. Subsequently, a large production process began in 1985 via a trial industrial plant. The present plant at Lake Abiyata is considered to be the first phase of a larger development plan of water abstraction from both lakes Shala and Abiyata. At present, annual artificial water evaporation for soda ash extraction is estimated at 13 x 106mJ (Ayenew, 1998). This is equivalent to a depth of 0.07 m, based on the present average lake area of 180 km2.

Large-scale irrigation was started in the 1970s in the Lake Ziway catchment, taking water directly from the lake and its two main feeder rivers. A three-phase irrigation development project was proposed for selected areas covering a total area of 5500 ha. Since 1970, major irrigation activities were introduced around Lake Ziway and in the Meki and Katar river catchments. The present annual abstraction for irriga­tion is estimated at only 28 x 106 m3. If all the proposed irrigated areas are developed, the estimated annual water requirement will be 150 x 106 m3 (Makin et ai, 1976). This would result in a 3 m reduction in the level of Lake Ziway and would ultimately lead to a drastic reduction in the level of Lake Abiyata.

DATA AND METHODOLOGY

Hydrometeorological data (since the late 1960s) were used to reconstruct the recent lake level changes and to correlate these changes with catchment hydrometeorological factors, although some of the data are inconsistent and erratic. Information on abstraction of water for irrigation and soda ash production was gathered from relevant institutions. The conventional water balance estimation allowed an understanding of the relative importance of the various components of the hydrological cycle. Evapora­tion was estimated using the Penman (1948) method and pan records close to the lake. Rainfall records since the late 1960s and river discharge records of Bulbula and Horakelo rivers (since 1975) were used for inflow estimations. The net groundwater flux was estimated as a residual of the other surface water balance components. An

Recent changes in the level of Lake Abiyata 497

independent estimate of the groundwater flux to the lake was made based on steady-state groundwater flow modelling using the modular three-dimensional finite difference groundwater flow model, MODFLOW (MacDonald & Harbaugh, 1988).

To assess the spatial variation of lake levels and to reconstruct the positions of the different shore lines, multi-temporal satellite images: Multispectral Scanner, MSS (1979), Thematic Mapper, TM (1987, 1989), the National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer, NOAA-AVHRR (1994, 1995) and SPOT (1993), as well as panchromatic aerial photographs at the scale of 1:50 000 (1965, 1967), were used. Scattered data on lake levels were also available since the late 1930s (Benvenuti et al., 1995) and the author collected additional data since 1994 by installing a staff gauge at Lake Abiyata.

RESULTS AND DISCUSSION

Table 2 summarizes the different fluxes of Lake Abiyata estimated by conventional water balance (on average 30 years of hydrometeorological recording) and by steady-state groundwater flow model simulation. The most important components of the water balance are inflow from rivers, direct precipitation and evaporation. Inflow to the lake is dominated by the Bulbula River, whose long-term average annual discharge is 184 x 106 m3, which is less than that of the main feeder rivers of Lake Ziway, the Katar and Meki rivers, with average annual flows of 392 x 106 and 267 x 106m3, respectively. The annual inflow from Lake Langano through the Bulbula River, and from the ungauged surface runoff, accounts for 45.6 x 106 and 15.4 x 106 m3, respect­ively. Compared to the other water balance components, the utilization of lake water for soda ash extraction and the net groundwater inflow are quite low.

The level of Lake Abiyata is influenced strongly by the input into Lake Ziway, which transfers water through the Bulbula River. However, the monthly gains of Lake Abiyata to storage are meagre and less than 5% in most dry months. For the first half of the year there is a loss in each month of 5% of the total volume of Lake Ziway. Without a large outflow from Lake Ziway through the Bulbula River in any extended period of wet years, the level of Lake Abiyata will fall consistently. Unlike lakes Ziway and Langano, this pattern of uninterrupted net loss over a number of dry years followed by a sharp recovery during extreme wet years is the essential hydrological behaviour of Lake Abiyata (Fig. 2).

The seasonal and interannual variability in evaporation and groundwater fluxes is low (Ayenew, 1998). Therefore, the large amplitude in lake levels is directly related to the inflow from rivers and direct precipitation on the lake. The inflow from rivers is intimately linked to the amount of rainfall in the highlands. Correlation of catchment

Table 2 Estimated annual water balance of Lake Abiyata in 106 nr'.

Conventional water balance: Groundwater model: PI Rj + Sr E A G,-G„ G, G,-G0

113 245 372 13 27 27 26

PI - direct precipitation on the lake, JR, = inflow from rivers, Sr = surface runoff from ungauged catchments, E = evaporation; A = abstraction for soda ash, G, = groundwater inflow; G„ = groundwater outflow.

498 Tenalem Ayenew

e 6 -o x>

§ ID 4

o jo

•3

2 -

1 -

J 0 4

70 73 76 79 82 85 Year (1969-1998)

9) 94 97

- Ziway Abiyata Langano

Fig. 2 Lake levels reconstructed from mean monthly staff gauge records.

rainfall with lake levels indicates that all three lakes fluctuate in accordance with the climatic conditions of the region, with the exception of a few lakes located outside of the basin influenced by irrigation (Ayenew, 1998). The recent lake level fluctuations also reflect changes in the precipitation conditions over the adjacent highlands.

Precipitation records for 1968-1988e at Butajira, a highland station in the Ziway-Abiyata catchment, reveal the importance of highland rainfall in controlling recent lake levels (Fig. 3). However, the correlation between mean monthly rainfall at Butajira station and the corresponding monthly lake levels of Lake Abiyata is relatively low (r = 0.5), due to the time lag between precipitation events and lake level changes. The correlation between the mean annual lake stage and the corresponding mean annual precipitation, before the commencement of soda ash extraction, is much higher (r = 0.92). Except for the interannual and seasonal variations of rainfall, there has been no declining trend of precipitation in the region in the last forty years. This has kept the level of many lakes with little or no change. However, after the commencement of large-scale abstraction of water in the late 1980s in the Abiyata catchment, substantial regression of the lake has occurred (Table 3).

In exceptionally wet years, the level of the lake rose substantially. From recent historical data (since the early 1980s), the maximum level was recorded in 1991 and in late 1996. In 1996, the region experienced the highest rainfall in 30 years. In the same year the level of Lake Abiyata rose by 1.9 m with respect to the bottom of the staff gauge (there is no specific reference altitude for the gauges) as compared to the record taken in the dry year of 1994. The level was at 4 m in 1977 and rose to a maximum level of 6.14 m in November 1979. Afterwards it progressively declined until it reached 0.24 m in July 1990. There was a 5.6 m drop in lake level between 1979 and 1989.

Referring to Fig. 2, the level of Lake Abiyata has never fallen below that of the other two lakes, with respect to the elevation of the bottom of the staff gauge. Yet, from 1985 to 1990 it dropped to below the level of the other lakes. However, the large

Recent changes in the level of Lake Abiyata 499

500 Butajira Station

6 -•

5 --

,4 -•

a 2

1 --

0 A

76 79 82 85 Year (19694991)

Lake Abiyata

\

j^J

Fig. 3 levels

70 73 76 79 82 85 88 91 Year (1969-1991)

Long-term mean monthly precipitation at Butajira station and the corresponding of Lake Abiyata.

Table 3 Changes in the size and volume of Lake Abiyata ( 1985-1991 ).

Year

1985 1989 1990 1991

Volume (106m3)

826.2 405.0 401.4

1300.0

Area (km2)

162.7 135.0 134.7 183.0

Maximum (m)

10.2 6.8 6.75

12.0

depth Elevation (m a.m.s.l.)

1576.9 1573.5 1573.45 1580.0

increase in level in 1993 seems to be improbable. In 1994 the level was very low. Abstraction of water or climatic factors cannot explain this drastic reduction—the most likely explanation is data error.

In wet years, for 50% of the time between November and June, Lake Ziway shows a net loss of storage. The loss is directly related to evaporation and outflow to Lake Abiyata. However, during August and September a net gain to storage occurs. This gain is transferred directly to Lake Abiyata. At times the gain reaches as much as 17%

500 Tenaient Ayenew

of the lake volume. The fluctuation of Lake Abiyata follows the same trend as Lake Ziway, with an average time lag of about 20 days. Any abstraction of water in the Ziway catchment results in a greater reduction in the level of Lake Abiyata than in that of Lake Ziway.

Lake Abiyata was at its highest stage in the early 1940s, the late 1970s and early 1980s and at its lowest level in 1967 and in the mid 1970s, and the late 1980s and 1990s. The rainfall record of Addis Ababa (1900-1995), located some 200 km north of the studied basin, shows that the late 1930s was the wettest period. This is reflected by the larger size of the lake in the 1940s, as shown in Fig. 4(b), which shows the different levels of the shore of Lake Abiyata reconstructed from lake stage records, multi-temporal satellite images, aerial photographs and field observations.

Figure 5 shows a panchromatic Thematic Mapper (TM) image taken on 21 November 1989 showing the lake level reduction. The relative lake level data covering different periods since the late 1960s have been combined with bathymétrie maps, aerial photographs and satellite images. From the record, it may be seen that the level has drastically reduced. Over the past three decades, the depth reached a maximum of 13 m in 1970-1972 and 7 m in 1989. These extreme drops in levels correspond to water volumes of 1575 x 106 and 541 x 106mJ, and lake surface areas of 213 and 132 km2, respectively. Before 1968, lake level variations, reconstructed from different sources (Street-Perrott, 1982; Benvenuti et al, 1995; Ayenew, 1998), showed inter-annual fluctuations of the same order of magnitude, with, for example, a high level in 1940 and 1972, a low level in 1965 (inferred from aerial photographs, Makin et ai, 1976) comparable to that of 1989, and a level even further reduced in 1967 (aerial photographs) and 1994 (field checks).

There was a considerable reduction in the volume of Lake Abiyata in 1985 and 1990, amounting to about 425 x 106 mJ, or 51% of its present volume. According to site managers at the Abiyata Soda Ash Factory, inflow from Lake Ziway has diminished from the long-term annual average value of 210 x 106 to 60 x 106 m3 in 1994 and 1995 due to the low rainfall of these two years. This resulted in a substantial reduction of the level of the terminal Lake Abiyata. For both industrial and environ­mental reasons, this reduction has become a great concern.

The reduction of the level of Lake Abiyata is also reflected in the changes in its ionic concentration. Water input-output relationships are the dominant feature of the status in the salinity series of the rift lakes (Wood & Tailing, 1988). If accompanied by a maintained lake level or volume and negligible seepage-out, evaporation loss can balance inflow plus direct precipitation; thus, with time, the lake becomes more saline. The extent of ionic enrichment depends on the lapse of time since the system became closed and on the changing rate of abstraction and evaporation over time.

Compilation of the sparse chemical data available since 1926 (Kebede et al, 1996) and chemical analysis since 1995 (Ayenew, 1998) has revealed a considerable increase in the total dissolved solids. Between 1926 and 1998, the salinity fluctuated more than 2.6 times (from 8.1 to 26 mg F1), the alkalinity changed from 80 to 326 mg F1, and pH varied between 9.5 and 10.1. The conservative anion chloride showed a two-fold increase over 42 years (Omer-Cooper, 1930). The dominant cation, sodium, increased more than three-fold. Between 1984 and 1991 the sodium chloride levels of the lake water increased from 0.25 to 0.7 mg F1, sodium carbonate increased from 0.44 to 1.24 mg f ' and sodium fluoride from 0.02 to 0.05 mg F1 (Halcrow, 1989; Ayenew, 1998).

Recent changes in the level of Lake Abiyata 501

less than 3 m

3 to 8 m

8 to 10 m

streater than 10 m

6 km

6 km

Fig. 4 (a) Simplified current bathymétrie map and (b) shoreline position of Lake Abiyata at different times. The outer boundary represents the 1940 shoreline (1582 m a.m.s.l.) and the inner thick shoreline is the present day average lake level (1575.8 m a.m.s.l.) The lines between, from the outside, represent the shoreline in 1971 (1580 m), 1983 (1578.8 m), 1984 (1578.5 m), 1976 (1578 m), 1985 (1576.9 m), 1996 (1577 m), 1997 (1576.9 m), 1995 (1576 m) and 1967 (1575 m).

502 Tenaient Avenew

Fig. 5 lnhanced panchromatic TM image taken in 1989 showing the lake shores indicating the different lake levels.

In general the level of Lake Abiyata fluctuates according to the precipitation trends in the highlands. However, the recent drastic decline in its level and the increase in salinity coincides with the time of large-scale water abstraction. The current and future uncontrolled water abstraction will have obvious environmental repercussions, which are thought to bring grave consequences to the lacustrine environment in the foreseeable future. The protection of the environment requires integrated water management at a basinwide scale. Changes in Lake Abiyata should be perceived jointly with the abstraction of water for irrigation around Lake Ziway.

Acknowledgements The author is grateful to the Department of Geology and Geophysics, Addis Ababa University for the field logistic support since 1994. Thanks go to the Ethiopian Meteorological Services Agency, Ministry of Water Resources, Ethiopian Mapping Authority and Abiyata Soda Ash Factory for providing relevant data. Sincere thanks also go to Dr Mohammed Umer and an anonymous referee for their constructive comments.

Recent changes in the level of Lake Abiyata 503

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Hydrobiologia 158, 29-67. Received 12 June 2001; accepted 19 February 2002