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Awassa, East African rift, hydrology, modeling, Naivasha, water balance


The hydrodynamics and the long-term water balance of two topographically closed crater lakes is presented in a comparative manner using a spread-sheet hydrological model. The main objective of the work is to study the role of groundwater and the effect of water abstraction from lakes Awassa and Naivasha. The rationale of selection of the two lakes separated by thousands of kilometers is the striking similarity of their hydrogeological and geomorphological setting and future intended water uses for large-scale abstraction from the lakes and feeder rivers. The net groundwater outflow from Lake Awassa and the effect of water abstraction from Lake Naivasha under different scenarios were calculated based upon the average monthly hydrometeorological data of rainfall, evaporation and river inflows. The net groundwater flux was obtained from the simulation as a residual of other water balance components and was found to be substantial in both lakes. The result revealed that the annual net groundwater outflow from Lake Awassa to adjacent basins is estimated at 58 ×106 m3. The predicted and recorded lake levels fit well for much of the simulation period. For Lake Naivasha groundwater flows into and out of the lake are successfully estimated based on the predicted water level fluctuations when water abstraction was at a minimal. The most accurate predictions of lake level were derived from the data sets of river discharges known to be from the most-reliable time period after the early 1970s. The model estimated an annual abstraction rate and groundwater outflow from Lake Naivasha of 60×106 m3 and 56×106 m3 respectively. The model demonstrated its validity as a good management tool to predict effects of large-scale pumping and extreme climatic events that may affect the lakes in space and time.