mpact of Climate Change on Reservoir Water Potential; The case of Legedadi - Dire Catchments

Abstract

Climate change variability and its impact on stream flows are now becoming one of the significant challenges. Developing countries like Ethiopia is likely to be affected most. The Legedadi and Dire catchments under consideration are situated in the upper north western Awash sub basin. The evaluation of the impact and the trends of the change is very important at watershed level. Therefore the main objectives of this study is to investigate the reservoir water potential of Legedadi and Dire catchments by water balance approach and project simulate of future climate change effects on the catchments and reservoir. The conceptual semi distributed Hydrologiska Byråns Vattenbalans-Avdelning (HBV) model computes runoff from observed daily rainfall, daily temperature, and estimates of potential evapotranspiration, catchment characteristics and runoff data for calibration. Projection of future climate variables is achieved by the use of General Circulation Model (GCM) outputs which is downscaled for present and future time horizon under HadCM3A2a and HadCM3B2a scenarios. Statistical Down Scaling Method (SDSM) is applied to downscale the climate variables at catchment level. HBV model calibration and validation results indicated that the model simulate historical daily discharges with an acceptable accuracy. The objective function of the model is good for both calibration and validation goodness of fit (Reff) obtained is about 0.82 and 0.84 for Legedadi case and 0.77 and 0.78 for dire case. The projected future climate change shows an increasing trend for rainfall, minimum and maximum temperature in comparing with respect to the observed period. The hydrological impact of future period scenarios indicated that there would be both an increasing runoff trends for Hadley Centre for Climate Prediction HadCM3A2a and HadCM3B2a climate forcing scenarios. For A2a scenario, the mean annual runoff for legedadi increased by 4.2, 9.2 and 14.4%, Dire mean annual runoff is increased by 5.2, 11.9 and 8.6% for 2030s, 2050s and 2080s respectively. The results for B2a scenario legedadi mean annual runoff increased by 5.8, 6.4 and 13.1%, Dire mean annual runoff is increased by 6.6, 7.6 and 13.5% for the time period of 2030s, 2050s and 2080s respectively. This study shows that there is excess surface water potential for Legedadi (92.39 MMC) and Dire (35.67 MMC) which is greater than the current reservoir potential of legedadi (42.8 MMC) and dire (23 MCM). Therefore, this can be a clue to the decision makers to understand the issue and design strategies to use this additional water for the city