Impact of Climate Change on Flood Frequency of Guder Watershed, Upper Abay River Basin, Ethiopia

Abstract

Climate change is a global concern as one of the most challenging and threatening issues of the 21st century to the world devastating natural hazards, which can significantly damage human lives and properties. This study examined the Impact of Climate change on Flood frequency in the Guder watershed in upper Abay river basin of Ethiopia. For the future, CORDEX-Africa data output of three RCMs under RCP 4.5 and RCP 8.5 climate scenarios were predicted for two horizons time (2041-2070) and (2071-2100) as the mid-term and the long-term respectively with a baseline period of (1971-2000). The three RCMs model performances were evaluated in terms of BIAS, RMSE and R2 evaluation criteria, how the RCMs perform in simulating the rainfall. In terms of BIAS, and RMSE the RCA4 model performed best whereas the CCLM4-8 model performed poorest. The biases of climate variables were removed by Delta change and Variance Scaling for precipitation and temperature respectively. Non-parametric Mann-Kendall test was carried out to detect trends of observed and future projected climate variables. The result of MK test shows; the future precipitation shows increase trend. HEC-HMS Model was used to simulate future daily stream flow data in both time horizons to checking model performance. The R2 and Nash- Sutcliffe Efficiency (NSE) values for the Watershed were 0.76 and 0.73 for calibration and 0.79 and 0.76 for the validation respectively shows good performance of the model. Flood frequency analysis considering the GEV distribution, AM data series, and the Maximum likelihood method for parameter estimation was selected based on goodness of fit. The future flood in different time horizons under dominant RCP’s will expected to increase for 2, 5, 10, 25, 50, 100, 200 and 1000 return periods. The quantile estimated from observed and simulated discharge for three RCMs in both terms of Guder watershed indicated that RACMO22T for both terms of RCP4.5 and RCP8.5 revealed high quantile estimates. In the future, the change of flood magnitude for CCLM4-8 and RACMO22T in AM series by GEV show increasing in all return periods under RCP4.5 and RCP8.5 of both terms by 1.2 %and 1.4% respectively. But RCA4_RCP8.5 mid-term of return period shows decrease with magnitude of -0.3% while RCA4_RCP4.5 long-term is increased. The magnitude change of analysis shows increasing future Flood frequency in dominant RCP scenarios for both mid-term and long-term periods