Dilnesaw Alamirew Chekol, Helmut Eggers:
Development of Water Balance Model of Lake Ziway Watershed in Ethiopia


University of Bonn, Institute of Water Resources and Land Improvement, Germany

The study area is part of the Ethiopian Rift Valley that is located in central Ethiopia. It is important sites in Ethiopian Rift from a water resources development point of view. Water and land resource development plans were implemented with short-time interests, though it was understood that the recent increased abstraction of water from lakes and rivers caused some changes in the lacustrine environment. Some of the lakes have reduced in size (ZINABU, 1989) and changes in the chemistry of the lakes have been observed, but it is largely unknown whether the ecosystem has been affected (HALCROW, 1989).

Future uncontrolled abstraction of water will inevitably change the hydrologic balance of the lakes and the ecology of the lacustrine environment. Knowledge of the water balance assists the prediction of the consequences of artificial changes in the regime of streams, lakes, and ground-water basins. The objective of the study is to understand the hydrology of lake Ziway. This requires assessment of water balance components like rainfall, evapotranspiration, runoff and etc, and their interactions. Evapotranspiration was estimated using Modified Penman Method and was compared with the data from evaporation pan. Average rainfall for the area was estimated using Isohyetal Method. The model was developed using the values from each water balance component. Besides analysis of the river flow data from gauging station at Meki town was made and the result has shown that the river will actually dry up at some time during the period December to March. Moreover, the flows that do occur during this period can be particularly low and variable. Based on this river discharge analysis of Meki there can be no prospect for year-round, large-scale irrigation in the Meki catchment based on run-of-river flows. The difference between inflow and outflow is found to be -45.24 mcm. This again indicates groundwater inflow. From the analysis it was found that 85% of the water coming to lake Ziway goes as evaporation, for this reason it is recommendable to look for a technology that could be economically used to reduce evaporation and make available the water for irrigation and other uses.

Keywords: Ethiopia, evapotranspiration, infiltration, rainfall, riftvally lake, run-off


Contact Address: Dilnesaw Alamirew Chekol, University of Bonn, Institute of Water Resources and Land ImprovementNussallee 1, 53115 Bonn, Germany, e-mail: dilnesaw@uni-bonn.de
Andreas Deininger, September 2004