MULTI RESERVOIRS OPERATION UNDER CHANGING LAND USE LAND COVER: A CASE STUDY OF OMO GIBE CASCADED RESERVOIRS, ETHIOPIA

Abstract

Land use/cover change (LULCC) has been altering the hydrologic system of our watersheds. It is a global challenge to predict their effects on water availability. The main aim of this study was to assess the impacts of land use/cover changes on the operational strategy of Omo Gibe cascaded reservoirs in Ethiopia. To execute this task, geo-spatial data of the basin, hydro-meteorological data (1990-2014) in and nearby the watershed were used. To evaluate the LULCC in the watershed, 2003 and 2014 land use map was generated using the Maximum Likelihood Algorithm under supervised classification using ERDAS Imagine software. The inflow to the reservoir was simulated using Soil and Water Assessment Tool (SWAT) whereas; the operational analysis was done by using MODSIM 8.1 model. As compared to 2003, the 2014 LULC has shown dramatic decrease in bush land and forest land by 8.91 % and 13.62% respectively. However, settlement and agricultural lands were increased by 12.64% and 14.15% respectively. The overall accuracy of the two maps was resulted with an optimum value of 94.71% and 98.41% respectively. SWAT model calibration and validation was done by using 1990-2006 and 2007-2014 hydro-meteorological data respectively. The performance of the model was evaluated by using NSE, R2 and PBIAS. The result shows an overall good shape agreement between the observed and simulated discharge. In between 2003 and 2014, the inflow generated at Gibe-III reservoir has shown increment by 5.96m3/s (15.66MMC/year). In connection to this, the energy produced by using MOD SIM model revealed increase by 2.34%. However, significant change in reservoir guide curve was not observed. The reason for high inflow change might be negative change in bush and forest land observed contributing to runoff increase throughout the period. In general, policy makers, stake holders and others should consider watershed management practices for the efficient utilization of the reservoirs.