摘要：

The Main Ethiopian Rift (MER) is a major structural depression where a number of lakes are found; Awassa is one of these lakes. Pervious studies carried out on lake sediments, in the MER, have suggested that lake levels have dropped by about 210 meters from the level they were 10,000 years ago, at the end of Pleistocene. In contrast, recent trends suggest that lake levels in the MER have been increasing, on average, at a rate of 6 cm / year. Similarly, the gauging data of Awassa Lake have indicated that the lake has been rising at a rate of 7.6 cm / year. Flooding caused by the lake has posed a serious development problem on Awassa town, which is located along the eastern shore of the lake. Environmental isotopes and major ions chemistry were used to define the groundwater flow system and recharge conditions in the study area. Isotope data from four rainfall-sampling stations, gave a Local Meteoric Water Line (LMWL) with slope 7.5 and deuterium excess of 13.3.Three Local Meteoric Water Lines can be proposed for Ethiopia based on sampling carried out at four stations under this project and the GNIP station at Addis Ababa. They are: δH=8 δO+14 for the western high land, δH=7.5 δO+13 for the rift valley and escarpment areas, and δH=8 δO+16 for the eastern highland. The Deuterium versus Oxygen-18 plot of water samples form springs, boreholes and some dug wells show rather closer distribution with mean values of 1.3 and -1.5, respectively. Such a clustered distribution can suggest a homogenous source of groundwater in the study area. The samples from dug wells and lake Awassa fall along evaporation line with slope 5.5 and deuterium excess of 7.6. The evaporation line established by this study is similar to that of evaporation line established for east African lakes. Mean recharge elevation of 2000m was estimated from the dependency of O on elevation (δO = -0.0012*h+0.8718, R= 0.94), which was established from isotopic data of four stations, having different elevation that has continuously been sampled for two years. Correlation of rainfall and evaporation with elevation has given a recharge elevation of 2100m, which is almost similar with isotopically determined recharge elevation. Groundwater in the MER, in general, and in the study area, in particular, is dominated by one water type: Na-HCO, which is also, supported by closer distribution of isotope data. Awassa, being a closed lake, is one of the freshest lakes in the MER with mean EC of 800 μs/cm and pH 8.5. The freshness of the lake is related to shorter turnover time (about 6 years). The lake has groundwater inflow both from the eastern and southern shore and a subsurface out flow towards north. Thermal springs, discharging into Lake Shalla north of Awassa Lake, are believed to have been recharged by Awassa Lake. Unlike thermal springs in the study area, Chitu-Shalla hot springs, north of Awassa Lake, show positive values of O, which can be the result of mixing of enriched Lake Awassa water with depleted paleogroundwater. Contour maps of EC and pH have depicted similar groundwater flow direction as inferred from that of the piezometric map. The ten-years moving average rainfall, in the study area, has shown a decreasing trend whereas discharge data of Tikurwoha River that flows into Awassa Lake show an increasing trend. Similar trends have also been observed in adjacent areas. The decreasing trend of rainfall is related to the increasing trend of aridity of the region (climatic change) whereas the increasing trend of river discharge is related to change in land use (deforestation). The recent lake level increase of Awassa Lake is, therefore, related to increasing trend of surface water input. Many remedial measures have been proposed, to mitigate the problem of lake level rise, among which watershed management is believed to be the best solution according to this study

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