Water management problems in the Ethiopian rift: Challenges for development

Journal of African Earth Sciences 48 (2007) 222–236 www.elsevier.com/locate/jafrearsci

Water management problems in the Ethiopian rift: Challenges for development Tenalem Ayenew

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Addis Ababa University, Department of Earth Sciences, P.O. Box 1176, Addis Ababa, Ethiopia Received 14 April 2005; received in revised form 18 December 2005; accepted 9 May 2006 Available online 30 March 2007

Abstract The Ethiopian rift is characterized by many perennial rivers and lakes occupying volcano-tectonic depressions with highly variable hydrogeological setting. The rift lakes and rivers were the focal points for relatively large-scale water resources development. They are used for irrigation, soda abstraction, commercial fish farming, recreation and support a wide variety of endemic birds and wild animals. Ethiopia’s major mechanized irrigation farms and commercial fishery are confined within the rift. A few of the lakes have shrunk as a result of excessive abstraction of water; others expanded due to increased surface runoff and groundwater flux from percolated overirrigated fields and active tectonism. Excessive land degradation and deforestation have also played a role. Human factors, in combination with the natural conditions of climate and geology have influenced the water quality. The chemistry of some of the lakes has been changed dramatically. This paper tries to present the challenges of surface water resources development with particular reference to environmental problems caused in the last few decades. The methods employed include field hydrological mapping supported by aerial photograph and satellite imagery interpretations, hydrometeorological and hydrochemical data analysis and catchment hydrological modeling. A converging evidence approach was adapted to reconstruct the temporal and spatial variations of lake levels and the hydrochemistry. The result revealed that the major changes in the rift valley are related mainly to recent improper utilization of water and land resources in the rivers draining the rift floor and the lakes’ catchment, and to direct lake water abstraction, aggravated intermittently by natural factors (climate and tectonism). These changes appear to have grave environmental consequences, which demand urgent integrated basin-wide water management practice. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Environmental problems; Ethiopian rift; Lake level changes; Irrigation; Salinity; Water management

1. Introduction Ethiopia has numerous inland water bodies whose scientific interest and development potential are largely unexplored. It is clear that lakes are important resources for Ethiopians as sources of water for various uses. Most of these inland water bodies are confined within the rift, forming a spectacular chain of lakes and large feeder rivers that originate from the adjacent highlands. The permanent rift lakes are mainly found in the central and southern part of the Ethiopian rift. These lakes are

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highly productive, harboring an indigenous population of edible fish and support a wide variety of other aquatic and wild life. They are globally significant freshwater ecosystems containing important areas of both terrestrial and aquatic biological diversity, and most are becoming degraded as a result of human activities (Chernet et al., 2001; Ayenew, 2004; Ayenew and Demellie, in press). Some of the alkaline lakes are highly productive, whose muddy shores support one of the largest bird populations (flamingo, white pelicans, etc.) in Africa. These lakes also form a vital migration route for Arctic birds during the winter season in the northern hemisphere. The fresh water lakes and their corresponding basins serve as invaluable resources for many people residing in the rift and adjacent highlands. Some of these lakes are

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being used for commercial fishing, irrigation, soda ash extraction and recreation. Although there is increasing use of these resources, they are not yet as permanently altered, damaged, and depleted as some other water bodies in the other developing countries (Zinabu, 2002). However, there seems to be little awareness about the risks on over-exploiting of these lakes. Water abstraction is often being done without the basic understanding of the complex hydrological and hydrogeological system and the fragile nature of the rift ecosystem (Kebede et al., 2001; Ayenew, 2004). Despite the fact that these lakes are of great importance to the national economy and are critical to the survival of millions of people, there were no detailed studies on issues of vulnerability to the various natural and anthropogenic factors such as climate and land-use change, neotectonism, and abstraction of water for various purposes. However, analysis of records available in recent decades has assisted in the understanding of the response of inland water bodies to climate changes and recent man-induced factors (Makin et al., 1976; Chernet, 1982; Ayenew, 1998, 2001a; Kebede et al., 2005). These studies related the major environmental problems to anthropogenic factors. The most important large-scale withdrawals of water in the rift are related to irrigation and trona (NaCO3) production. These activities have reduced the level of some of the lakes and affected the hydrochemical setting (Kebede et al., 1994; Zinabu and Elias, 1989; Ayenew, 2002). Aside from the problems related to lakes, over-irrigation induced salinization of wide farmlands (Hailu et al., 1996) and subsurface irrigation-groundwater inflow resulted in the expansion of Lake Beseka (Halcrow, 1989; Tessema, 1998; Ayenew, 2004). Application of agrochemicals and fertilizers have also slightly changed water and soil chemistry (Dechassa, 1999). Apart from the various inflow and outflow components of the water balance of the lakes and anthropogenic factors, volcano-tectonics and sedimentation played a role in affecting lake levels in the past (Street, 1979; OEPO, 2005). Some studies showed that deforestation and land degradation resulted in the siltation of lakes (Ayenew, 2003a,b; WWDSE, 2001; Gebreegziabher, 2005). At present there is no volcanic activity except for the existence of geothermal activities in the vicinity of lakes, which has little or no role in changing the level of the lakes. However, the occurrence of frequent earthquakes and formation of new faults might have influenced hydrogeologic regime of some lakes (Ayenew, 1998; Tessema, 1998). The levels of most lakes in the rift fluctuate according to the precipitation trends in the adjacent highlands (Street, 1979). For the last four decades, there is no substantial declining trend of rainfall in the region (Ayenew, 2004). It is believed that improper utilization of water will certainly lead to negative consequences for the fragile ecosystem of the rift lakes in the foreseeable future. In connection with this, some studies have been carried out for scientific purposes especially for environmental concerns. The major degradation problems that these lakes are facing currently have been studied over the last few years (Zinabu and Elias, 1989;

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Kebede et al., 1994; Brook, 1994; MWR, 1999; Zinabu, 2002; Legesse et al., 2003; Ayenew, 2004; OEPO, 2005). As a result of the alarming rate of environmental degradation during recent years, the rift lakes have become the focus of attention by the Ethiopian government, the scientific community and the people residing around the lakes. In this paper, the author intends to evaluate the magnitude of the recent challenges of water resources development in the rift with particular attention to environmental problems associated with anthropogenic factors by giving illustrative evidence. It includes recommendations for corrective measures. The focus of the study is the Main Ethiopian rift (MER) lakes and the main perennial River Awash which drains the floor of the rift, supplying water to Ethiopia’s major mechanized irrigation farms and the urban and rural community of the Afar regional state, all the way to Djibouti. To the knowledge of the author there is not much-published work on the relatively small lakes in many parts of the East African rift system. Although water management problems may vary from country to country in the region, the basic facts would probably be similar, given that the countries in the region share much with respect to geology, climate, and socio-economic development. Therefore, it is the author’s hope that the issues raised and recommendations given for the Ethiopian condition may apply to other countries within the East African rift as well. 2. General description of the region The Ethiopian rift is part of the East African rift system, which extends from the Kenyan border in the south up to the Red Sea in the north. It divides the Ethiopian highlands into two, then widens into the Afar plain and continues and splits into two branches to form the basins of the Red Sea and the Gulf of Aden oceanic rifts. This rift can be divided into four sub-systems: Lake Turkana (Rudolf), Chew Bahir, the MER and the Afar. The seismically active MER transects the uplifted Ethiopian plateau for a distance of about 1000 km, extending from the Afar depression southwards across the broad zone of basins and volcanic ranges to the watershed of Lake Chamo (Fig. 1a). The main focus in this study is the closed Ziway–Shala lakes basin located in central Ethiopia (Fig. 1b). The geological and geomorphologic features of the region are the result of Cenozoic volcano-tectonic and sedimentation processes. Except for some patchy Precambrian outcrops at the southern and northern edges, the rift is covered with Cenozoic volcanic and sedimentary rocks. The rift formation is associated with extensive volcanism. Several shield volcanoes were developed over large parts of the adjacent plateaux. The volcanic products in many places were fissural basaltic lava flows, stacked one over the other, alternating with volcano-clastic deposits derived from tuff, ignimbrite and volcanic ash. The basalt extrusions were interspersed with large accumulations of rhyo-

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ture. The rift valley is distinctly separated from the plateaux by a series of normal faults usually trending parallel and sub-parallel to the NNE–SSW trending rift axis. The floor of the rift is marked by a persistent belt of intense and recent faulting. This tectonically active region is characterized by numerous geothermal manifestations and caldera volcanoes. The Ethiopian rift is bounded to the east and west by highlands characterized by high rainfall. The elevation within the rift varies in a wide range from 1680 m.a.s.l at Lake Awassa to around 120 m below sea level in the Dalol Depression in northern Afar. There are also volcanic hills and mountains both within the rift and the highlands. These hills, basins and volcano-tectonic depressions separate the rift-valley lakes. The annual rainfall within the limits of the rift varies from a little below 100 mm in the Afar up to 900 mm around Lake Abaya in the south. The rainfall is much higher in the highlands; in some places it is as high as 1400 mm annually. The climate is sub-humid in the central MER, semi-arid close to the Kenyan border and arid in Afar. The Dalol Depression, one of the hottest places on Earth, has an average annual temperature of around 50 °C. The floor of the rift is occupied by a series of lakes fed by large perennial rivers originating from the highlands. Most of the principal lakes of Ethiopia, except Lake Tana, lie in the rift or close to a line extending from the northern tip of Lake Turkana on the border with Kenya to Lake Abhe close to the Djibouti border. The MER has seven major lakes used for various purposes. One of them (Koka) is an artificial reservoir constructed in 1960 on the Awash River, for irrigation and hydro-electric power development. The rift lakes are highly variable in size, hydrogeological and geomorphologic setting (Table 1). Excluding the small crater lakes, the alkalinity of the lakes increases as one goes from south to north. In fact terminal lakes without surface water outlets such as Abiyata and Shala have very high alkalinity so that they can be used for abstraction of salts. The lakes in Afar have very high salinity and were used for abstraction of salts for centuries. The main source of water to the rift lakes and rivers is the rainfall in the eastern and western highlands. The most important rivers are the Meki–Katar, Bilate and Awash which feed Lakes Ziway, Abaya and Abhe, respectively.

Fig. 1. Location map (a) Rift valley and adjacent highlands with lakes; (b) the Ziway–Shala basin (main study area with lakes).

lite and trachyte, breccias, ignimbrite and related shallow intrusions (Kazmin, 1979). Most of the rift valley flat plains around lakes are covered with thick lacustrine deposits and volcano-clastic Quaternary sediments. Block faulting has disrupted the rocks and formed a horst and graben strucTable 1 Basic morphometeric and hydrologic data of the lakes Lake

Altitude (m)

Surface area (km2)

Max. depth (m)

Mean depth (m)

Volume (km3)

Salinity (g/1)

Conductivity (lS/cm)

Chamo Abaya Awassa Shala Abiyata Langano Ziway Beseka

1233 1285 1680 1550 1580 1585 1636 1200

551 1162 129 329 176 241 442 3.2

13 13.1 21.6 266 14.2 47.9 8.95 –

– 7.1 10.7 87 7.6 17 2.5 –

– 8.2 1.34 36.7 1.1 5.3 1.6 –

1.099 0.77 1.063 21.5 16.2 1.88 0.349 5.3

1320 925 830 21940 28130 1770 410 7155

Source: Wood and Talling (1988), Halcrow (1989) and Ayenew (1998).

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In addition, many seasonal streams feed the lakes. Abaya and Chamo are seasonally connected by an overflow channel, Ziway and Abiyata by the Bulbula River, Langano and Abiyata by the Horakelo River. Lakes Awassa, Abiyata, Shala and Beskea have no surface water outlet. The largest commercial farms in the country are located along the downstream of the Koka dam, and are irrigated by the regulated flow of the Awash River which drains through the rift, starting from the central highlands through the northern part of the MER and finally ending in Lake Abhe at an altitude of 250 m.a.s.l on the border with Djibouti. Lakes Beseka, Gamari, Afambo and Abhe cover the rift floor within the Awash River basin. The expansion of irrigation is also evident in the MER along the Meki and Katar Rivers and around Lake Ziway. The vast plains covered with colluvio-alluvial and lacustrine deposits along the course of the Awash River are very convenient for irrigation. The Awash River master plan study was conducted in the early 1960s, which later led to the expansion of wide irrigation fields after the construction of the Koka dam, with catchment and reservoir areas of 9909 km2and 177 km2, respectively. At present the largest commercial mechanized farms of Ethiopia (Wonji, Nura-Era, Matahara, Amibara, Melka-Sedi, Melka-Worer and Gewane) are located along the downstream of the Koka dam; growing different fruits, cotton, bananas and sugarcane. 3. Approach The hydrology and hydrogeology of the rift-valley lakes and rivers, particularly in the MER (Ayenew, 1998) and the salinization problems of some of the irrigation fields of the Awash valley (Hailu et al., 1996) have been studied in relative detail. River basin master plan studies outlined some of the problems related to water and land resources development in the region (UNDP, 1973; Wenner, 1973; Makin et al., 1976; Halcrow, 1989). The expansion of some of the lakes (Tessema, 1998; Geremew, 2000; Ayenew, 2004) and the relation of lake levels and climatic factors of some of these lakes were addressed in part, including the water balances (Nidaw, 1990; Tessema, 1998; Ayenew, 2002). These studies were used here as a basis. In this paper, more risk assessment was made, based on time-series of recent hydrological records, development of a systematic relevant database from previous works, and on detection of the spatial variation of lake levels from satellite images and aerial photographs. The lake stage records (since the late 1960s) were used to reconstruct the lake level changes. Information on abstraction of water for irrigation and soda ash production was gathered from relevant institutions. To reconstruct the positions of the different shore lines, multi-temporal satellite images – Multispectral Scanner, MSS (1979), Thematic Mapper, TM (1987, 1989, 2003) 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

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also available since the late 1930s (Benvenuti et al., 1995). Hydrochemical analysis is used as an independent check of the recent changes in hydrological setting. Data on the salinization problems in the Awash basin are extracted from previous studies (Halcrow, 1989; Hailu et al., 1996) supported by recent limited field visits. Based on field observations, interviews with the local community and professionals working in the water sector of the rift, an attempt is made here to present the magnitude of the recent environmental changes. Finally, recommendations are outlined on how the recent changes have to be averted and sustainable and environmentally sound water resource utilization practices can be implemented. 4. Results and discussion 4.1. Lake level changes Except for the interannual and seasonal variations of rainfall, there has been no declining trend of precipitation in the region for the last 50 years (Ayenew, 2004). This has kept the level of some lakes constant, with little or no change. The level of many lakes fluctuates in accordance with the climatic conditions (Street, 1979). However, recent pressure from human activities has led to contrasting lake level changes i.e. expansion and reduction. The major changes are observed in terminal lakes influenced by largescale water abstraction and inflow into the lakes from over-irrigated farms. In contrast, there are some lakes showing rising water-level trends without visible human influences, thought to be due to neotectonism (Ayenew, 2004). The recent lake level changes can be exemplified by taking the three most-affected lakes: Abiyata, Beseka and Awassa. Fig. 2 shows the recent unusual lake level changes established based on monthly average lake stage records. The most drastic changes have been observed in Lakes Abiyata and Beseka, the former is shrinking and the later expanding; rise is also evident in Lakes Awassa and Chamo. 4.1.1. Lake Abiyata Lake Abiyata is a relatively shallow small alkaline terminal lake fed by the Horakelo and Bulbula Rivers originating from the near-by Lakes Langano and Ziway, respectively. The relatively shallow depth and its terminal position make it more susceptible to changes in climate and input from precipitation and river discharge. The main inflow is from direct precipitation and discharge from the two feeder rivers. As a closed lake, the only significant water loss is through evaporation. Groundwater flow model simulations indicate negligible groundwater outflow from the lake (Ayenew, 2001a). Generally 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 the lake for soda extraction, and the utilization of water from feeder rivers and Lake Ziway for irrigation has resulted in rapid reduction in lake levels (Ayenew, 2001b, 2004). The economic

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a

Terminal lake Abiyata declinining in level

Level above gauge bottom (m)

7 6 5 4 3 2 1

10 10

0

95 95

90

85

80

75

70

0

Terminal lakes rising in level 5 4 (m)

3 2 1 0

90

85

80

75

0 70

b

Level above gauge bottom

Year (1970-2000)

Year (1970-2000) Beseka

Awass a

Chamo

Fig. 2. Lake level fluctuations in the Main Ethiopian rift.

feasibility of soda extraction from Lakes Abiyata and Shala was investigated in the early 1980s (Halcrow, 1989). Subsequently, a large production process began in 1985 via a trial industrial plant (Fig. 3b). The present extraction is considered to be the first phase of a larger development scheme. In 1998, the annual amount of water pumped for soda ash extraction from Lake Abiyata was estimated to be 13 million cubic meters (Ayenew, 1998). This is equivalent to a depth of 0.07 m, based on the then

average lake area of 180 km2. From the estimated evaporation figure, it appears that the recent dramatic lake level changes cannot be explained in terms of artificial lake water evaporation alone. The lake level reduction is believed to have been amplified by the large amount of water being pumped from Lake Ziway for irrigation, which is the major supplier of water to Lake Abiyata through the Bulbula River. The fluctuation of lake level for Abiyata follows the same trend as Lake Ziway, with an average time lag of about 20 days (Ayenew, 2003a). Any abstraction of water in the Ziway catchment results in a greater reduction in the level of Lake Abiyata than that of Lake Ziway. Over the past three decades, the depth reached a maximum of 13 m in 1970–1972 and only 7 m in 1989. These extreme levels correspond to water volumes of 1575 and 541 million cubic meter (mcm), and lake surface areas of 213 km2 and 132 km2, respectively. Before 1968, lake level variations, reconstructed from different sources (Street, 1979; Benvenuti et al., 1995; Ayenew, 1998) showed interannual 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) comparable to that of 1989, and a level even further reduced in 1967 (aerial photographs), 1994 and 2004 staff gauge monitoring. The current trend is worrisome. The lake size is currently reduced to around 100 km2. A field visit in April 2005 revealed that the shoreline close to the soda ash plant has receded by 1.3 km from the 1980s position. The reduction of the level of Lake Abiyata is clearly visible from old shorelines seen on satellite images (Fig. 3a). The maximum lake level reduction coincides with the time of large-scale water abstraction for soda production and

Fig. 3. (a) Enhanced panchroimatic TM image of Lake Abiyata showing recently formed strands of shorelines; (b) shoreline position of Lake Abiyata at different times. Numbers in bracket indicate elevation above mean sea level. The outer boundary represents the 1940 shore line (1582 m) and then in decreasing order 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). The inner thick shoreline is the present day (2003) average lake level (1575.2 m); broken line shows current shoreline position.

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for irrigation from Lake Ziway after the mid 1980s (Fig. 2). There was a considerable reduction in the volume of Abiyata in 1985 and 1990, amounting to about 425 mcm. According to the then site managers at the Abiyata Soda Ash Factory, inflow from Lake Ziway has diminished from the long-term annual average value of 210–60 mcm in 1994 and 1995, due to both abstraction and the low rainfall of these two years. In wet years, for 50% of the time between November and June, Ziway shows a net loss of storage due to the outflow of water to Lake Abiyata. During August and September, a net gain to storage occurs because of large inflows from the Katar and Meki Rivers. 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 Maki and Katar. A threephase irrigation development project was proposed covering a total area of 5500 ha. Since 1970, major irrigation activities were introduced around the lake and its catchment. The annual abstraction for irrigation in 1998 was estimated to be only 28 mcm (Ayenew, 1998). If all the proposed irrigated areas are developed, the estimated annual water requirement will be 150 mcm (Makin et al., 1976). This would result in a 3 m reduction in the level of Lake

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Ziway, ultimately leading to a drastic reduction in the level of Lake Abiyata and drying up of the feeder Bulbula River. Currently, due to the expansion of flower farms and other vegetable fields, the abstraction is expected to be higher than the 1998 figure. 4.1.2. Lake Beseka In contrast to many East African terminal lakes; Beseka has recently been growing in size as a result of increase in the net groundwater flux into the lake (Tessema, 1998; MWR, 1999; Ayenew, 2004). This lake is located north of the MER at 190 km east of Addis Ababa. Aerial photographs taken at different times have shown that the area covered by the lake was around 3 km2 in the late 1950s; currently the total area is close to 42 km2 (Ayenew, 2004). Fig. 4 and Table 2 illustrate the change of size with time reconstructed from aerial photographs, topographic maps and satellite images. The change in volume was established from bathymetric survey (MWR, 1999). The level of the lake has risen by 4 m over two decades (1976–1997). The time of onset of the expansion is not known exactly. Previous studies tend to agree that the problem was initiated in 1964 when irrigation of the Matahara mechanized

Fig. 4. (a) TM image showing the current size of Lake Beseka (image taken in 2003), (b) shoreline position of Lake Beseka at different times.

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Table 2 Temporal changes of the size of Lake Beseka (modified from MWR, 1999) Period of recording

Elevation (m.a.s.l)

Area (km2)

Width (km)

Length (km)

Depth (m)

1957/1964 January 1972 April 1978 December 13, 1998

940.82 942.77 946.96 950.701

3 11 29.5 39.97

1.09 1.86 2.84 3.5

8 21.5 36.4 44.4

0.58 1.38 3.45 5.8

farm around the lake was started, for cultivation of cotton and citrus fruits. Currently the surrounding estates grow sugarcane. The main changes in the water balance of Lake Beseka come from groundwater inputs, which are related to the recent increment of recharge from the nearby irrigation fields and due to the rise of the Awash River level after the construction of the Koka dam. Some authors relate the expansion of the lake partly to neotectonism (Ayenew, 1998; Tessema, 1998). Prior to the construction of the Koka dam, the Awash River sometimes ran dry between December and March. However, after the construction of the dam there has been fairly steady flow of the river throughout the year. The sustained flow of the Awash throughout the year has become a source of continuous indirect recharge to the groundwater of the area, ultimately feeding Lake Beseka which is located at a relatively lower topographic position. Estimation of the water balance of Lake Beseka shows that groundwater contributes 50% (53.8 mcm/year) of the input to the lake. Sixty four percent of the groundwater input to the lake comes from outside the catchment area, i.e. the Awash River transmission or channel loss and irrigation loss accounting 23.5 and 10.5 mcm/year, respectively (Tessema, 1998). Excess water from irrigation discharged into the lake was estimated to be in the order of 20 mcm (Halcrow, 1989). The reason for this has been poor irrigation efficiency in the surrounding farms. In 1977 the irrigation efficiency was 30%. In 1990 it was reported to have improved to 70% (Halcrow, 1989). The transmission loss from the Awash River and direct recharge are facilitated by the presence of active tensional faults. The favorable geological factors combined with the availability of infiltrated irrigation water have enhanced the groundwater recharge. Isotopic, hydrochemical and geological evidence have shown the occurrence of younger and relatively older cold and thermal waters. Groundwater flows into the lake from the western side (Tessema, 1998; MWR, 1999). The lake level has risen by 4 m during 1976–1977 as evidenced from lake stage records (Fig. 2). The hydrograph of Lake Beseka (starting in 1964) shows that the early part of this increase was gentler followed by steeper rise in recent years. The average lake level rise is 15 cm/year. By the end of 1997 the elevation of the lake was 952.4 m.a.s.l (MWR, 1999). Inspection of the 1:50,000 scale topographic

map shows that the lowest point along its northern water divide is 954 m.a.s.l. The lake level is therefore 1.6 m below the lowest point; if the inputs to the lake continue at the same rate, it will overpass the divide by the year 2008. If inputs increase the overflow could occur even sooner. Recently the government has started pumping out and releasing the lake water into the Awash River, although the ecological effect downstream is unknown. 4.1.3. Lake Awassa The endohric Lake Awassa is one of the few fresh water lakes within the Ethiopian rift. The lake has no surface outflow. The freshness of the lake is maintained by groundwater outflow (Darling et al., 1996; Ayenew, 2001a). The lake is situated in a volcano-tectonic depression called the Awassa caldera with a catchment and lake area of 1160 and 129 km2, respectively. Lake Awassa and the nearby Cheleleka (Shalo) swamp are located in a 25–30 km wide volcano-tectonic depression (Fig. 5). The bordering scarps and volcanic complexes have an elevation difference varying from 200 to 900 m. Awassa town is located right at the eastern shore of the lake at an elevation of 1700 m.a.s.l. Often the town is flooded by the lake during extreme wet years. Unlike the other two lakes, there is no substantial abstraction of water, except limited diversions for local irrigation from the Tikur Wuha River connecting Cheleleka swamp and Lake Awassa. The inhabitants of Awassa town depend on the lake for fishing and recreation. At the national level the lake is a major source of income through tourism. It is one of the biggest bird sanctuaries in Africa. The Corbetti geothermal field, located to the northwest, is recharged by the lake (Darling et al., 1996). The plains around the lake are largely agricultural leaving little natural vegetation. Herds of cattle and sheep are brought to the lake to drink. Along the western shore people drink the lake water. The size of Lake Awassa has changed substantially in the last hundred years. European explorers, who visited the area at the end of the 19th century, provide the first known written descriptions of Lake Awassa. They found Lakes Awassa and Cheleleka united, at least in the wet season (Neumann, 1901). This would require a water level rise of only a few meters. Du Bourg (1903), traveling in 1902, drew the lakes as being separate, with the Cheleleka swamp smaller than drawn by Neumann (1901). Makin et al. (1976) include lake level data obtained from a gauge on Lake Awassa; this shows annual variations in water level of about one meter superimposed on long-term trends. The maximum water depth recorded has varied from 21.6 m in 1937 to 17.8 m in 1964 (Baxter et al., 1965). Unlike the situations in the early nineteenth century, recently the level of Awassa is rising. Systematic lake level recording started since the early 1970s illustrates clearly the rising trend of Lake Awassa (Fig. 2). It also shows strong seasonal and interannual variations. High water levels in the early 1970s flooded parts of Awassa town, established

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Fig. 5. (a) Location of Awassa area (including swamps and the lake), (b) digital elevation model of Awassa caldera, (c) recent ground cracks southwest of Awassa in Muleti area.

some 20 years earlier (Makin et al., 1976).Water levels were high again in late 1996 following a prolonged wet season (Ayenew, 2004).

Although the lake level rise is obvious, the causes are not clearly understood. Some of the probable causes are land-use changes, climate and neotectonism. The land-use

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of the catchment has been changed in the last few decades; and the rise in the level of the lake has been explained in terms of increase in surface runoff as a result of excessive deforestation (WWDSE, 2001). The study conducted in the Katar River watershed in the adjacent Ziway–Shala basin revealed that land-use change of present day cultivated/grass land to woodland would decrease the discharge at the outlet of the river by about 8 % (Legesse et al., 2003, 2004). In other words deforestation increases surface runoff by the same order of magnitude. Considering the land-use changes of Awassa catchment and the catchment area, the high lake level rise cannot be entirely explained by land-use change. In fact, water diversion from feeder rivers has risen slightly. Correlation of climatic variables and lake level changes also did not clearly illustrate the relative importance of natural and anthropogenic factors (Geremew, 2000; Ayenew, 2004; Gebreegziabher, 2005). Studies made in the last few decades on groundwater resources in the rift faults, evidenced the positive role of open tensional faults and ground cracks in the substantial transfer of groundwater to the lakes (Tessema, 1998; Ayenew, 1998, 2001a; Ayalew et al., 2004). Recently very large ground cracks have been created in the Muleti area southwest of Lake Awassa (Fig. 5c). These cracks have been found to be the major conduits to groundwater transfer into the lake. According to many of the local people residing in the Muleti area southwest of Lake Awassa, a small Derba pond (about 5 km2 size) disappeared in less than two years time after the formation of these cracks (Fig. 5b). The pond at present is a muddy plain (Gebreegziabher, 2005). This has been confirmed by the author during the field visits in October 2004. The geologic and topographic setting indicates that the ultimate destination of the water from the pond is Lake Awassa located at the lowest topographic position in the basin 1680 m.a.s.l.. In the adjacent basin north of Lake Langano, spring discharges have been drastically increased after the formation of new faults (Ayenew, 1998). These evidences indicate the importance of tectonic activity in affecting the hydrogeology of the rift. The land-use changes and neotectonism might have also been important for the rising of Lake Chamo. 4.2. Hydrochemical changes Comparison of long-term water quality changes in several Ethiopian rift lakes with differing exposure to human influences may further illustrate some of the issues raised so far. The changing lake levels are reflected in changes of the hydrochemical and limnological setting. These chemical changes have promoted some changes in the aquatic ecology (Tudorancea and Taylor, 2002). The hydrochemical modification can be exemplified by comparing the changes that have taken place in Lake Beseka (with little human impact in its catchment) and Lake Abiyata, which has been affected by different sets of human activities.

According to Zinabu (2002), a study of Ethiopian riftvalley lakes (Kebede et al., 1994; Alemayehu et al., 2006) has shown that Lake Beseka decreased drastically in its total ionic concentration by about a factor of 10 over 30 years. This change in ionic concentration appears to have been accompanied by a shift in the phytoplankton community: Spirulina platensis, the characteristic bluegreen algal species found in African soda lakes was reportedly dominant in 1961 (Wood and Talling, 1988) but totally absent in 1991. On the other hand Lake Abiyata has shown a considerable increase in ionic concentration since 1961. Changes in phytoplankton composition also seem to have taken place in Lake Abiyata: a dominant population of Spirulina species was reported from Lake Abiyata in the 1960s (Wood and Talling, 1988). In the 1990s the same species was not observed (Kebede and Willen, 1996). Close observation of the shoreline, lake stage records and satellite images suggest that the lake has shrunk significantly in the last four decades (Ayenew, 2003b). Although the soda ash extraction plant on Lake Abiyata may have increased the evaporation rate of the lake water in the last 20 years or so, diversion of the inflows for irrigation purposes and flushing from deforested and heavily grazed catchment might have contributed to the increase in the concentrations of ions (Zinabu and Elias, 1989). Another study on the Ethiopian rift lakes indicated that three different trends in the salinity of the lakes have occurred in the last three decades. Lakes Ziway and Shalla have maintained the same salinity they had in the 1960s. Lakes Langano and Awassa have become more diluted. The salinity of Lakes Abiyata, Abaya and Chamo has increased over the last 30 years. The changes that have taken place in the conductivity of the Ethiopian rift-valley lakes in the last three decades suggest a similar trend (Table 3). Although these changes in salinity can take place due to evapotranspiration and/or solute inputs, the intensity of the human activity in their catchments must have contributed to the contrasting trends in their salinity (Zinabu, 2002). Recent study by Ayenew (2004) demonstrated the temporal changes of the ionic concentration of Lakes Abiyata and Beseka, all attributed to anthropogenic factors. Water input–output relationships are the dominant feature of the status in the salinity series of the rift-valley lakes (Wood and Talling, 1988). If accompanied by a maintained lake level or volume and negligible seepage losses, 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/l), the alkalinity chan-

T. Ayenew / Journal of African Earth Sciences 48 (2007) 222–236

231

Table 3 Temporal changes of the chemistry of Lake Abiyata, ions expressed mg/l Source

Tame of sampling

Omer-Cooper (1930) Loffredo and Maldura (1941) De Filippis (1940) Talling and Talling (1965) Wood and Talling (1988) Von Damm and Edmond (1984)

November 1926 April 1938 1939 May 61 January 76 November 1980 November 1980 October 1981 March 1991

Salinity (g/l) 8.1 8.4

Alkalinity (g/l) 80 81 210 166 138 180 297 326

19.4 16.2 12.9 21 26

Ca

Mg

Na

K

Cl

0.5 0.4 0.2 <0.15 <0.1 0.1 <0.01

0.8 0.5 0.1 <0.6 <0.1

125 130 140 277 222 194 231 378 416

1.9 10.3 8.5 6.5 4.9 6.9 9.9 9.7

42 42 40 91 51 54 82 121 88

<0.01

0.1

SO4 1.4 15 22.5 0.3 4 5.7 24

Total cation 133 150 285 228 199 238 388 425

Source: Kebede et al. (1996) and Ayenew (2004).

ged from 80 to 326 mg/l, and pH varied between 9.5 and 10.1. The conservative anion chlorine showed a twofold increase over 42 years (Omer-Cooper, 1930). The dominant cation (sodium) increased more than threefold. Between 1984 and 1991 the sodium chloride levels of the lake water increased from 0.25 to 0.7 mg/l, sodium carbonate increased from 0.44 to 1.24 mg/l and sodium fluoride from 0.02 to 0.05 mg/l (Halcrow, 1989; Ayenew, 2002). Lake Beseka presents a completely different hydrochemical picture: it has changed to a nearly fresh lake from an extremely alkaline water body over the last 45 years. The electrical conductivity has gone down from 74,170 lS/cm to 7440 lS/cm between 1961 and 1991, corresponding to a change in size from 3 to 35 km2 (Table 4). Application of fertilizers and over-irrigation also affected soil and water chemistry in some places. One of the obvious influences of application of fertilizers and over-irrigation is the drastic increase of nitrate in irrigated fields and salinization, respectively. Besides, the natural high concentration of fluoride in the rift, and improper agricultural activities caused a severe groundwater management problem (Halcrow, 1989; Lloyd, 1994). The concentration of fluoride reaches as high as 150 mg/l in the MER close to alkaline lakes and in thermal springs (Ayenew, 1998; Chernet et al., 2001). The study carried out in the irrigation fields of the Wonji sugar plantation (7000 ha), just downstream of the Koka dam that shows high nitrate concentration due to excessive application of fertilizers, high population density with lack of septic tanks and animal breeding (Dechassa, 1999). The Wonji plain is an active agro-industry area with high population density and urbanization. Within the plantation alone, around 50,000 people are living. In some wells the nitrate content reaches as high as 50 mg/l. The sugarcane

plantation uses 200–600 kg/ha urea fertilizer, accounting for a total of over two million kilograms annually. This might have also increased the potassium and sulphate content of the water. Different types of herbicides and insecticides are also used, although the effect on water chemistry is not well established. Even though there are no clear indications of euthrophication, algal blooms were observed in some small reservoirs and abandoned ponds in the Awash valley (Dechassa, 1999). The development of algae is due to nutrient supplied from sugar cane plantations and the surrounding areas. Euthrophication is also observed in some of the rift lakes due to high nutrient fluxes from fertilizers in their catchment. The typical example is Lake Abiyata and moderate manifestations occur in Lake Ziway. 4.3. Irrigation and salinization Salinization is a critical problem in the Awash valley irrigation fields. The most affected area is the MelkaSedi–Amibara irrigation project in the Middle Awash valley bordering the right bank of the Awash River, and located in the arid southern Afar region at an elevation of around 750 m.a.s.l (Fig. 6). The high temperature of the region (annual average 26.7 °C) and low annual rainfall (500 mm) and the high evaporation have aggravated the salinization process. The Metahara sugar plantation has also suffered from over-irrigation and subsequent water logging that led to salinization of wide areas of farmland (Tessema, 1998). Until 1997 nearly 30 ha of farmland have been abandoned due to salinization and 150 ha of land have become unsuitable for ploughing by tractor in and around the existing plantation (Halcrow, 1989). Table 5 illustrates the critical problems of salinization in the different irrigation fields within the Awash valley. The

Table 4 Temporal changes of the chemistry of Lake Beseka (ions expressed mg/l) Source

Time of sampling

EC (lS/cm)

Total cations

Total anions

Na

K

Ca

Mg

HCO3 + CO3

Cl

SO4

pH

Talling and Talling (1965) Kebede et al. (1994)

1961 1991

74,170 7440

784 80

831 71

774 79

10 2

<0.15 0.1

<0.6

580 46

154.8 13

98 12

10 9

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T. Ayenew / Journal of African Earth Sciences 48 (2007) 222–236

a

0

80 0

85 0

ma in h

i gh

wa y

7 50

Awa sh

Ri ve r

9 0 30'

To

Mi le

40 15'

a na Riv e r

a sh Aw

R iv

er

ma i

Ke se m

n ca na l

Ri ve

r

K eb

main irrigated areas (with loical salinization) Expansion areas topographic contours

b

Groundwater level rise (m)

750

5 4 3 2 1 0 1981

1983

1985

1987

1989

Year (1980-1988)

Fig. 6. (a) Amibara irrigation project areas. (The inset indicate photo piezometric monitoring in the salinized and abandoned irrigation field.) (b) Groundwater level rise due to over-irrigation.

problem is related to uncontrolled irrigation practise and lack of knowledge on crop-water requirements and water management. This has been illustrated by the studies made on the Amibara, Melka-Sedi and Melka Worer irrigation projects (Halcrow, 1989; Hailu et al., 1996).

Studies of crop-water requirements and irrigation methods have been made in the Awash valley since 1964 and the first feasibility study was completed in 1969 in Amibara (Halcrow, 1989). The main crops produced are cotton and bananas with limited areas of pasture, cereals and veg-

T. Ayenew / Journal of African Earth Sciences 48 (2007) 222–236 Table 5 Irrigated areas and salinized areas in the different Awash valley irrigation projects Scheme name

Altitude (m.a.s.l)

Degaga 1350 Awara Melka 850 (Kesem) Yalo (Kebena) 850 Melka-Sedi 750 Amibara 750 Gewane/Maro Gala 550 Mile (SF) 400 Dubti (SF) 400 Awssa Asaita 350 Karadura 350 Total

Net irrigated area (ha)

Saline area (ha)

171 1140

20 145

410 3047 434 2071 580 5300 2631 163

220 1165 56 100 20 300 20 80

15,947

2126

etables. The two main irrigation methods are basin and furrow irrigation for banana and cotton fields, respectively both require accurate land grading. The gravity irrigation system was designed on the basis of a 24 hours operation, and comprises a network of secondary, tertiary and field canals, which distribute diverted Awash River water. The crop-water requirement for bananas is 1842.9 mm/year. The net requirement is around 2000–2400 mm/year. The available water which includes the net irrigation plus the effective precipitation in the region ranges from 2200 to 2600 mm/year. Based on 75% irrigation efficiency and 8% leaching requirement, the gross irrigation requirement is about 3170 mm/year. Often the water used exceeds the common calculated crop-water requirement. There is in fact some irrigation water flow control in canals. However, there is no real information as to how much water is being released for the various farm plots. It is believed that the amount of water released is far greater than the crop-water requirement. This is clear from the extensive salinization and abandoned fields after the implementation of irrigation in the region. The high soil salinity is related to groundwater level or capillary rise due to over-irrigation. The inset in Fig. 6 shows the average groundwater level rise between 1981 and 1988; the photo shows salinized soils in abandoned irrigation fields in the Amibara area. With time, groundwater progressively rose and subsequently salinization became critical. The problem is more pronounced in the banana fields, which use basin irrigation. Unfortunately no routine monitoring of soil salinity levels has been undertaken so that there is no definite proof of correlation between soil salinity, groundwater level and capillary rise in areas where the water table is less than 1 m below the surface and the extent of water loss by capillary action is uncertain. However, limited monitoring of piezometers shows rapid rise of groundwater during peak irrigation periods. In the shallow piezometric system, over-irrigation brings about capillary rise and contributes significantly to the salinization process. The Amibara irrigation project has 71 piezometers located

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randomly, where the groundwater has been measured monthly since 1984. The long-term average depth to groundwater varies between 1 and 15 m. Groundwater modeling was conducted using the Aquifer Simulation Model to delineate the areas most affected by groundwater level rise (Hailu et al., 1996). The result indicates the presence of wide local domes and cones of depression created by over-irrigation and local pumping, respectively. There is still a substantial area with very shallow groundwater levels prone to capillary rise and subsequent salinization. Higher water table areas are those highly irrigated fields without drainage systems. The irrigation activities around Lake Ziway have also had a considerable effect on the water level and soil salinity (Ayenew, 2003b). Recently the problem has become much more aggravated due to uncontrolled pumping of the lake water by private investors, for horticulture. Since the irrigation in this area is a year-round process, its effect on the water level is magnified, especially during times of low precipitation and high evaporation (Zinabu and Elias, 1989). Several rivers that flow into one or more of the rift lakes have been diverted for irrigation. The rivers Meki, Katar and Bulbula which flow into Lake Ziway and Lake Abiyata, respectively, are used for irrigation. This practice has caused water level reduction in both lakes because of the reduced inflows. A similar phenomenon seems to have taken place in the Lake Abaya–Chamo basin as a result of the diversion of the rivers Kulfo and Sile for irrigation (Zinabu, 2002). 4.4. Other factors There are other anthropogenic influences on the lake catchments which directly or indirectly affect the lakes. Clearing of forests, animal grazing, and other reductions in the vegetation of the catchment areas have expanded considerably during recent years (Hillman, 1988). This resulted in the increase of the silt and nutrient load of the water. Given the present level of deforestation in Ethiopia (about 150,000–200,000 ha of forest per year), which results from the usage of wood for fuel and construction and from the population-driven need to increase cultivable land, forest clearing currently poses a serious problem. Most of the factors that encourage soil erosion are very common in Ethiopia, and some of the large lakes have suffered from the consequences of the linked processes of plant cover removal, erosion, and sedimentation. When erosion takes place, the torrents, carrying soil particles, usually rich in nutrients, may end up in lakes. The result of such increased nutrient loading is eutrophication. This phenomenon in turn can lead to fish-kills caused by decreased oxygen concentrations, algal blooms, and other interrelated consequences. Such events have already been reported in some of the Ethiopian rift-valley lakes. Fishkills, algal blooms, and the associated death of wildlife in Lakes Chamo (Amha and Wood, 1982) and Abiyata (Kassahun, 1982) are attributed to the human impact in the catchment areas (Zinabu, 2002).

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Small-holder farming and state farms in the drainage areas of the Ethiopian rift-valley lakes have modified the catchment areas of the lakes and have contributed to enhanced nutrients and particulate run-off, especially when the grasslands are overgrazed and the fields are tilled or fertilized. It is hard to say how much of the soluble fertilizers and pesticides used remain on land, and how much of it passes to the water bodies. There is a growing interest in using fertilizers in the region which will increase the nutrient load into lakes. So far there has been no study made with respect to impacts of pesticides and fertilizers on the water quality. However, it is certain that such practices, aimed at increasing food production, also result in harmful long-term effects on the water bodies and in the accumulation of toxic chemicals in the bodies of fish. Urbanization in close proximity to the rift-valley lakes is also among the greatest potential causes of changes in water quality. Although there is very little indication of acute cultural pollution in the Ethiopian lakes at the moment, the potential problems deserve some attention. Most of the fast-growing cities like Ziway, Meki, Awassa and Arbaminch are in the neighborhood of the rift-valley lakes. The growing population and industrialization of these cities can have potentially serious consequences for the lakes. It is possible that domestic and industrial wastes may find ways into the lakes. The effluents from the textile factories in Awassa and Arbaminch are likely to end up in Lakes Awassa and Abaya, respectively. Septic tank leachate may also reach the lakes and cause water pollution. 5. Recommendations The current and likely future uncontrolled water abstraction will certainly have negative repercussions which are thought to bring grave consequences to the fragile rift environment. It is clear that many environmental problems arise from the process of development itself. Therefore, although all the development programs that the country is planning to carry out appear to be indispensable, their environmental impacts should be considered before any of the development programs are launched; and their negative impacts should be minimized where and when possible. Increased emphasis should be placed on preventive planning based on environmental impact assessment. From this study, it is clear that serious environmental problems are around the corner. This demands an urgent and comprehensive water management and planning strategy, requiring the process of protecting and developing the water resources in a broad, integrated, and foresighted manner. In practice, this is a complicated endeavour, since comprehensive water management involves a number of functions that are closely related but which are carried out by different agencies and organizations. The functions include water law and policy making, regulation, technical assistance and coordination, monitoring and evaluation, administration and financing, public education and involvement.

Comprehensive, integrated water management is not easy to implement. It is clear that the water use needs to be seen in a much broader context than current and past practices. More emphasis has to be placed on a better basin-wide planning and management of water and land resources. The present and future projects must not be assessed as separate entities. It is impossible to manage a system effectively if some of its parts can be manipulated separately. Integrated water management in the basin requires hard work and a joint effort between professionals, policy-makers and society, but has to be started under the umbrella of environmental protection and sustainable development. In many developing countries for questions related to environmental issues, policy-makers are usually reluctant and respond to what they think the public wants. The ultimate question, however, is which public will be heard. As we have seen, the lakes serve many people and each has a right to use the resource (industry, agriculture, transportation, commerce, tourism and recreation). All need something from the lakes and rivers and place their own demands on the resource. Solutions to the problem must be seen to be ecologically interrelated. Society has to strike a balance between environmental protection and requirements for resources, energy, and the necessities that present life demand. There must be a legal body responsible for protecting the systems. An environmental authority should develop appropriate policies and guidelines that can be adapted nationally and locally to bring about environmentally sound management of the fragile and complex lakes. Water management requires a good systematic database, which is lacking. The data we have is scanty and incomplete in most cases. They do not give us sufficient background knowledge of the lakes and their catchments. Without basic information on the physical, chemical, and biological aspects of the lakes, it is difficult to evaluate any qualitative and/or quantitative changes. This demands a national effort to develop a good database of the rift valley as a whole and the lakes in particular. In the absence of enough information, scientists, policy makers, and the public at large seem to believe that water quality and quantity changes are not amongst the problems of the country. Probably the water scarcity problem in some parts of the country, which is more seriously considered, has caught the attention of the public so much that quality issues are not raised. Yet the evidence of water quality changes in the Ethiopian lakes is already strong enough to call for immediate corrective measures (Zinabu, 2002). The first and foremost corrective measure is to acknowledge the existence of serious environmental problems and to be determined to act vigorously. There is a limited monitoring system in the rift, i.e. river and lake level gauges. If present, they are not automatic and are poorly maintained. Both physical and socio-economic data are required for proper management and optimum use of lake resources. However, for a large number of Ethiopian lakes, regular monitoring of essential water

T. Ayenew / Journal of African Earth Sciences 48 (2007) 222–236

quality parameters (physical and chemical) is not generally carried out or the data are not available to many potential users. The situation with socio-economic factors is even worse. Impacts of lakes on the lifestyles of intended beneficiaries like farmers, fishermen, and domestic water consumers, have not been evaluated; thus monitoring in this regard is almost non-existent. Monitoring should also provide quantification of all the components of the hydrologic cycle in order to establish the long-term water balance of the lakes and their catchments. For a long time Ethiopia did not have sound legislation or water-use policies directed toward conservation of lakes or aquatic organisms. The Ethiopian Parliament has recently approved legislation on environmental protection, and an Environmental Protection Authority (EPA) has been established, but no actions that warrant protection of the lakes have been taken as yet. Recently the Oromia Environmental Protection Office (OEPO) started consultation with relevant institutions to implement integrated water management for the protection of Lakes Abiyata and Ziway. Whoever starts the positive effort of integrated water management and environmental protection strategy, the following have to be given utmost priority. The major problems at the forefront are the continuing decline in the level of Lake Abiyata and expansion of Lake Beseka, including the ever-growing salinization problem in the Awash basin and around Lake Ziway. Future largescale abstraction of water from Lakes Abiyata, Shala and Ziway must be carefully assessed. There is particular urgency for reassessing the proposed full-scale soda ash production and the three-phased irrigation project in the Ziway catchment. As part of this effort, it is important to quantify more precisely the amount of water being pumped from Lakes Ziway, Abiyata and from the two main rivers, Katar and Meki. If a decision is made to implement the planned projects fully, environmental impact assessment should include the Ziway and Langano catchments. In addition, effects on the fish and bird life of the two lakes and water supply problems of the Bulbula River must be included. Upstream use of water must only be undertaken in such a way that it does not affect water quality and quantity significantly to downstream users. Control provisions require a network of river monitoring stations in order to establish short- and long-term fluctuations in water quantity and quality. The salinization problem will remain the greatest threat to the rift valley fertile farmlands, both in the Awash valley and the Ziway area. For the sake of increasing agricultural production and improving investment in the region, the fertile ground should not be converted to unusable bare plains. It is prudent that the farming system should consider proper crop-water requirement studies and irrigation scheduling. Proper irrigation scheduling has also to be made in irrigation fields to protect the rise and expansion of Lake Beseka. This needs studies on the duration of

235

growing period and type of crops, water balance studies and continuous monitoring of piezometers, soil and water salinity. Proper drainage structures and land grading are also required to reduce salinization and flushing of the salts from the topsoil. Land-use and water-use are inseparable. Deforestation, rapid land-use change for farming, and overgrazing are likely to affect the hydrologic regime of the rift lakes. Protection of the rift acacia trees, bushes associated with grasslands and the lake marginal vegetation is essential. A dense sward of well-managed grassland and trees can withstand trampling of land and reduce soil erosion and the rate of lake sedimentation. The growing towns around the lakes should have proper town planning strategy that leads to the protection of the aquatic environment by properly designing solid and liquid waste desposal sites. The pollution sources have to be controlled to reduce the threat of further pollution of the groundwater and surface water bodies and eutrophication of lakes and reservoirs. Physical and chemical properties of soils have to be checked from time to time to regulate fertilizer and pesticide consumption. Water quality monitoring stations are required to detect the spatial and temporal changes of water quality. Acknowledgements The author is grateful to the Department of Earth Sciences, Addis Ababa University for the field logistic support. Thanks go to the Geological Survey of Ethiopia, Ethiopian Meteorological Services Agency, Ministry of Water Resources, Ethiopian Mapping Authority and Abiyata Soda Ash Factory for providing relevant data. References Alemayehu, T., Ayenew, Tenalem, Kebede, Seifu, 2006. Hydrochemical and lake level changes in the Ethiopian rift. Journal of Hydrology 316 (1–4), 290–300. Amha, B., Wood, R.B., 1982. Limnological aspects of an algal bloom on L. Chamo in Gamo Goffa Administrative region of Ethiopia in 1978. SINET: Ethiopian Journal of Science 5 (1), 1–19. Ayalew, L., Yamagishi, H., Reik, H., 2004. Ground cracks in Ethiopian rift valley: facts and uncertainties. Engineering Geology 75, 309–324. Ayenew, T., 1998. The hydrogeological system of the lake district basin, Central Main Ethiopian rift. Published PhD thesis, Free University of Amsterdam, The Netherlands, 259p. Ayenew, T., 2001a. Numerical groundwater flow modeling of the Central Main Ethiopian rift lakes basin. SINET: Ethiopian Journal of Science 24 (2), 167–184. Ayenew, T., 2001b. Recent changes in the level of Lake Abiyata, Central Main Ethiopian rift. Journal of Hydrological Sciences 47 (3), 493–503. Ayenew, T., 2002. Recent changes in the level of Lake Abiyata, Central Main Ethiopian rift. Hydrological Sciences 47 (3), 493–503. Ayenew, T., 2003a. Evapotranspiration estimation using thematic mapper spectral satellite data in the Ethiopian rift and adjacent highlands. Journal of Hydrology 279 (1–4), 83–93. Ayenew, T., 2003b. Environmental isotope-based integrated hydrogeological study of some Ethiopian rift lakes. Journal of Radioanalytical and Nuclear Chemistry 257 (1), 11–16.

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