Evaluation of Climate Change Impact on Upper Blue Nile Basin Reservoirs

Abstract

Nowadays the sign of climate change and its impact is revealing on different natural and man made systems, in one or other ways. Accordingly, this impact is significant on the water resource system. This study mainly deals with evaluation of the climate change impact on the Gilgel Abay reservoir which is found in Upper Blue Nile Basin, using the reliability, resilience and vulnerability indices (RRV-criteria). Projection of the future climate variables is done by using General Circulation Model (GCM) which is considered as the most advanced tool for estimating the future climatic condition. Statistical Down Scaling Method (SDSM) is applied in order to downscale the climate variables at catchment level. A hydrological model, HBV was utilized to simulate the water balance. The performance of the model was assessed through calibration and validation process and resulted R 2 =0.82 during calibration and R 2 =0.8 during validation. The projected future climate variable shows an increasing trend for both maximum and minimum temperature however, for the case precipitation it doesn’t manifest a systematic increase or decreasing trend in the next century. The evaporation from the open water surface of reservoir reveals an average annual increase by 2.1 % when the projected average annual temperature and precipitation increases from the baseline period by an amount of 0.53 o C and 0.82 % respectively in 2020s under the A2a emission scenario, when the average annual temperature is rise by 1.15 o C and the precipitation increase by 0.85 % in 2050s with A2a emission scenario, the reservoir open water evaporation will expected to increase by 6 %, while in the time horizon of 2080s, the precipitation shows an increase amount by 1.6 % and the temperature raise 1.97 o C consequently the open water evaporation is expected to rise by 22 % for the same A2a emission scenario. On average for both A2a and B2a emission scenarios the time based reliability (the probability of the reservoir to meet the target demand) of Gilgel Abay reservoir shows a value of above 80 %, i.e. 80% of the time the target demand is fully supplied and the resilience (the speed of recovery of the reservoir, form failure) shows value above 60%, a value of 100% resilience shows the reservoir needs very short time to recover itself from failing to meet the demand and the dimensionless vulnerability (the average volumetric severity of failure during failure period divides by the target demand) of the Gilgel Abay reservoir falls in range (25%-30%).The sensitivity analysis of the reservoir with a hypothetical climate change scenario indicates that the reliability and resilience of the reservoir is sensitive to precipitation change than change in temperature on contrary dimensionless vulnerability of the reservoir doesn’t show remarkable difference for both the change in precipitation and temperature