WATERSHED MODELING AND MANAGEMENT ASPECTS OF THE GILGEL ABBAY SUB-BASIN

Abstract

Water resources are basic renewable natural resources that are the essential requirements for the existence and development of any society. Proper utilization of these resources demands assessment and management of the quantity and quality of the water resources both spatially and temporally. This study focused mainly on hydrological characterization and application of a GIS based semi-distributed watershed model (HEC-HMS) to establish rainfall­ runoff relationships in the Gilgel Abbay Sub-basin, which is found in the upper most part of the Abbay (Blue Nile) River Basin. From the hydrological characterization it could be observed that the Gilgel Abbay sub-basin has got a high annual runoff yield. The mean annual discharge at the outlet of the sub-basin (gauging station) is 1044.2mm and the mean annual rainfall is 2030.9mm; hence the runoff coefficient is 0.513. The semi distributed GIS based model, HEC-HMS was applied for both long term and short term simulations. And in both cases four combinations of basin models were used. According to RA2 and IVF criteria, the model set containing Initial and Constant method of excess runoff volume determination, Snyder Unit Hydrograph and Recession method of base flow estimation was found to be the best model combination for short period flood forecasting. In accordance with the same criteria, model combination containing Deficit and constant loss, Snyder unit hydrograph and Recession as a flood forecasting model has given satisfactory result for long-term simulation for the study area. Six years of hydrological and climatic time series data were used to calibrate the model. Another four years of the record were used for model validation. The Vl main inputs to the model were daily rainfall (mm), potential evaporation (mm) and observed daily flow (m 3 /s). The main output from the model is daily stream flow (m 3 /s). After the model was calibrated and validated the impact of climatic change on the from the catchment was analyzed