IMPACTS OF CLIMATE CHANGE DYNAMICS ON AVAILABILITY OF WATER IN THE DIDESA RIVER CATCHMENT, WESTERN ETHIOPIA

Abstract

Recently, the Coordinated Regional Climate Downscaling Experiment (CORDEX) initiative has made multiple Regional Climate Models’ (RCMs') outputs available for end users across the African continent. But their accuracy has to be evaluated before running climate simulations in impact studies. Therefore, inthis study the impact of climate change projections on stream flow over the Didesa catchment of Abay basin were evaluated using three independent regional climate model. The performance of all models was assessed in differentstepwise approaches: First, the performance of the RCM simulation results were evaluated in Didesa catchment of Abay basin using bias, Root Mean Squared Error (RMSE), and Coefficient of Variation (CV). The second evaluation was based on the combined influence of corrected RCM simulated temperature and rainfall onhydrological simulations of mean stream flow under current (1989 2018) climatic data or condition. Then, the hydrological simulations of projected midi-term (2021–2050) and long-term (2051-2080) climate conditions were compared with the baseline with two emission scenariosof RepresentativeConcentration Pathways(RCP4.5 and RCP8.5). Findings of this study indicated that the annual rainfall bias of the models varies between 21.9% and 15%, suggesting high variation. Based on the finding, climate change studies may benefit from the use of multi model simulation. Biases of most of the models proved that correction for the systematic error of RCM outputs must be made before the model outputs could be utilized by users. Compared to the base line period, all the models indicated that the total annualrainfall increases in midterm period (2021-2050) and long term (2051-2080) as projected under the RCP4.5. In long-term periods, climate models predicted that a decrease in rainfall in all seasons of the year except for summer season under RCP8.5 scenario, where as rainfall is predicted to increase whencompared to the base line period. The stream flow projections are made for both periods in the future with two emission scenarios of RCP4.5 and RCP8.5. On the other hand, RCMs outputs indicated that the maximum and minimum temperatures were projected to increase over the catchment for both future periods ofboth scenarios with largest increase under RCP8.5.Under all models of RCP8.5 in the midterm and long term periods, the change of mean annual temperature ranges from 1.56oC to +2.27oC and 0.43oC to +3.62oC,respectively. From the stream flow projection relative to the baseline period, the change in mean annual stream flow from both future period are mostly positive,indicating increase in available discharge in the river under RCP4.5 and RCP8.5 scenarios. Flow duration curves showed that there is the probability of occurrence of high flows in both future periods under RCP4.5 and RCP8.5 scenarios relative to the baseline period. The overall conclusion of the study is that Didesa catchment of Abay basin is likely to experiencemore flowin the future than baseline period .