EVALUATING THE IMPACT OF CLIMATE CHANGE ON SURFACE WATER RESOURCES FOR ESTIMATING CROP WATER DEMAND: A CASE STUDY OF BIRR WATERSHED, ETHIOPIA

Abstract

Ethiopia is one of the most susceptible countries to the impact of climate change due to dependency on rain-fed agriculture. This study focuses on the evaluating of climate change impact on surface water resources for estimating crop water demand in Birr watershed, Abay Basin, Ethiopia. CORDEX-Africa data output of Hadley Global Environment Model 2- Earth System (HadGEM2-ES) for Birr catchment was selected under representative concentration pathways (RCP2.6, RCP 4.5 and RCP 8.5) scenarios for making future climate projections. Initially, data quality checked and consistency for observed data was performed. In addition, the percent bias (PBIAS), coefficient of variation (CV), and absolute mean square error (AME) was used to evaluate the systematic error of rainfall and the relationship of observed and future model output. The assessment of trends in climate variables was carried out using Mann-Kendall’s tests and Sen’s slope estimators. The result of Mann-Kendall’s test indicated that, observed annual rainfall shows decreasing trend while the average temperature shows an increasing trend. Also, it was found that in the future all RCPs except RCP4.5 at Debecha station show decreasing trend, but, the average temperature will be expected to increase in both time periods. The result of CROPWAT show that crop water and irrigation water requirement for Maize will expect to increase throughout the growing periods especially, crop water demand for maize will strongly increase during development stage. Further, HBV model was calibrated (R2 =0.79) and validated (R2 =0.76) which is a good representation of the catchment. The result of the projected steramflow indicates that, in the future the minimum flow under RCP8.5 and RCP4.5 scenarios statically decrease and a significant down-ward change-point might be occurred in the years of 2055 and 2035, respectively. Similarly, for 1-day and 7-day maximum flow under RCP8.5 and 90-day flow under RCP4.5 will be expected to decrease significantly and a significant change-point will occur at 2060s and 2030s, respectively. Conversely, under RCP2.6, the minimum as well as maximum flow will decrease insignificantly and a significant change-point might not be occurring in the coming six decades.