WATER BALANCE AND RESERVOIR OPERATION ANALYSIS FOR OPTIMAL POWER PRODUCTION FOR CASCADED HYDROPOWER PLANTS (A CASE STUDY OF UPPER OMO-GIBE RIVER BASIN, ETHIOPIA)

Abstract

Increasing competition for water across sectors (agriculture, industries, domestic, and power generation) increases the importance of the river basin as the appropriate unit of analysis to address the challenges facing water resources management; and the basin requires proper planning and management of water resources available to provide essential information for policy makers in their resource allocation decisions. Hence, this study mainly focuses on water balance and reservoir operation analysis for optimal power production for cascaded hydropower plants in the upper Omo-Gibe river basin. SWAT model was used to simulate the current inflow to the cascade reservoirs. The performance of the model has been evaluated through calibration and validation process. Model sensitivity analysis at Abelti gauging station shows ESCO, Soil_Z and Alpha_Bf as the most sensitive parameters. The overall evaluation indicated that SWAT model satisfactorily simulates river flows in the study area with R 2 of 0.84 and NSE of 0.81 during calibration. During validation, R 2 of 0.79 and NSE of 0.69 were obtained. HEC-ResSim (Hydrologic Engineering Center-Reservoir System Simulation) was used to simulate water allocation and operation of existing, ongoing and planned reservoirs in Omo-Gibe basin. Three water resources development scenarios were applied to analyze reservoir operation of the basin (Scenario T, Scenario A and Scenario B). Scenario-A result shows that an average annual energy output of 786.97, 1806.40 and 6446.00 GWh/year for Gibe I, Gibe II and Gibe III respectively. The mean maximum pool level in Gilgel Gibe-I and Gibe-III was estimated to be 1671 m.a.s.l and 891.9m.a.s.l.in October whereas the minimum mean pool level was estimated to be 1658.5 m.a.s.l. in May and 859.6m.a.s.l.in June respectively. In scenario B, due to regulation of flow at upstream, the annual power production in Gibe III was increased and the simulation result shows an average annual energy production of 6514.00 GWh/year. The mean maximum pool level in Gibe-III was 895m.a.s.l.in September whereas the minimum mean pool level in Gibe-III pool was 860 m.a.s.l. in July.