Streamflow Forecasts and Assessment of the Performance of Dabus Cascade Reservoirs Operation under the impact of climate change. (Case study of Upper and Lower Dabus Reservoirs, Ethiopia)

Abstract

Currently the impact of climate change affects many water resources projects, thus it is important to assess its impact on streamflow and reservoir performance. This study mainly forecast streamflow and assesses the performance of Upper and Lower Dabus reservoirs under the impact of climate change using Reliability, Resilience and Vulnerability indices (RRV). The future climate variables were projected by General Circulation Model (GCM) and downscaled at the basin level for the A1B emission scenario using the Regional Climate Model (RCM) obtained from IWMI. For streamflow generation and reservoirs inflow estimation HEC_HMS model was used using the bias corrected precipitation and Evapotranspiration which was estimated by FAO Penman-Monteith using bias corrected climatic variables. After the flow was forecasted, the performance of the model was assessed via calibration at DabusNearAsosa, SechiNearMendi and Aleltu@nedjo using coefficient of determination (R 2 ) and Nash-Sutcliffe Efficiency (NSE) performance coefficients. Then the model was validated using the parameters optimized during model calibration. The trend of streamflow forecasted at outlet (merging to main Abbay River) was assessed and the inflow generated to reservoirs was used to determine reservoir performance indices (RRV). Finally the inflow to the reservoirs with monthly evapotranspiration from the reservoirs was used as input to HEC_RsSim to simulate and optimize reservoir operation and Power production. The projected mean annual maximum temperature increases from the baseline period by 0.43 0 C, 1.3 0 C and 2.5 0 C for short-term, midterm and long-term respectively whereas minimum temperature increases by 0.47 0 C, 1.53 0 C and 2.83 0 C. Generally the projected future maximum and minimum temperature shows an increasing trend whereas precipitation shows variation (does not reveals clearly increasing or decreasing) for earlier century and decreasing trend in mid and late century. The evapotranspiration shows an increasing trend. The HEC_HMS model shows a good performance at DabusNearAsosa which resulted R 2 =0.90 and NSE=0.89 during calibration and R 2 =0.84 and NSE=0.82 during validation. The streamflow at Dabus outlet shows an average annual increase of 2.83% for earlier century and decrease of 2.83% and 4.56% for mid and late century respectively. The average annual inflow to the upper Dabus reservoirs shows an increasing of 3.17% for early century and decreasing of 2.08% and 4.46% for mid and late century respectively. The average time base reliability of the reservoirs was less than 50% for no reservoir condition and greater than 90% for the other condition considered but volumetric reliability and resilience shows 100% for all conditions. According to the vulnerability result the reservoirs will face shortage of flow which ranges from 8.85% to 88.51%. The result of reservoir simulation shows that the power plant parameters does not shows much significant in all scenarios considered in this study. As a result of these the Dabus sub-basin reservoirs have sufficient potential to produce required power for the country according to reconnaissance level study of the basin demand requirement and even more power can be produced.