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International Institute for Geo-Information Science and Earth Observation (ITC) 2005

Assessment of the water balance of lake Awassa catchment, Ethiopia.

Gebreegziabher, Y. (Yemane)

Titre : Assessment of the water balance of lake Awassa catchment, Ethiopia.

Auteur : Gebreegziabher, Y. (Yemane)

Etablissement de soutenance : University of Twente International Institute for Geo-Information Science and Earth Observation (ITC)

Grade : Master of Science in Geo-Information Science and Earth Observation 2005

Résumé
Lake Awassa is located in a closed catchment in the Central Main Ethiopian Rift Valley. Its level is rising progressively during the past 2 decades. Despite its economic and social importance to the community within the catchment, its rise poses a frequent flooding problem to the town of Awassa that is established adjacent to the lake. It is a fresh water lake while other closed lakes in the region are alkaline. For better management of the lake water, the hydrologic components which play a significant role in the lake level fluctuations need to be understood. The possible causes for the rise of water level need to be identified and the behaviour of the catchment at large must be understood : For that a water balance study is crucial. Thornthwaite and Mather soil water balance procedure and spreadsheet model have been used in the catchment and the lake water balance studies for the period of 1981-1998 respectively. The spreadsheet model uses monthly evaporation, surface runoff, and precipitation as an input and groundwater component is treated as net groundwater flow. Based on the simulation results it is found that evaporation, rainfall, surface runoff and constant groundwater outflow from the lake constitutes 131, 106, 83 and 43 Mm 3 of the annual average lake water balance respectively. The freshness of the lake water is attributed to the constant groundwater outflow from the lake. Computer program WTRBLN developed on basis of Thornthwaite and Mather soil water balance procedure is used for the catchment water balance calculation. Long-term average monthly climatic variables (rainfall, reference potential evapotranspiration), crop coefficient and water capacity of the root zone are inputs to the model. Using ILWIS/GIS environment, catchment factors like soil type, land use and rooting depth, the water holding capacity of the root zone is defined. Incorporation of land use and climatic variables in the soil water balance program serves to assess the impact of different land use and climatic changes on the catchment runoff. Results of the catchment water balance shows that long-term mean annual values of rainfall, actual evapotranspiration, and catchment runoff (surface and sub-surface runoff) constitutes 1398, 916 and 482 Mm 3 respectively. The results of the lake and catchment water balance analysis shows that the combined effect of climatic and land use changes during the past 25 years most likely resulted in an increase of the catchment runoff and so the lake level.

Version intégrale (ITC)

Page publiée le 28 janvier 2018