Assessment of Climate Change Impact on Surface Water Availability: Gerhu-Sirnay Catchment, Mereb Sub- Basin,

Abstract

Investigating surface water availability under climate change impact is vital to ensure water resource sustainability. The general objective of this thesis was to assess the impact of climate change over surface water availability of Gerhu-Sirnay catchment using CORDEX-RCA4 with soil and water assessment tool (SWAT). To achieve this, quality of observed data was accepted for outlier, adequacy and consistency tests, and CORDEX-RCA4 datasets were passed the Power transformation and variance scaling bias correction and square root normalization. The baseline monthly stream flow (1990-2003) was modeled using SWAT, calibrated (1992-1999) and validated (2000-2003) in SWAT-CUP under SUFI2 tool. The CORDEX-RCA4 models were compared their performance at the baseline period (1990 to 2001) using volumetric metrics and Taylor diagram to predict future precipitation and stream flow variability by the best fit RCA4 model under RCP4.5 and 8.5 emission scenarios for 2050s and 2080s periods. The results showed that SWAT was very good at modeling baseline stream flow indicated by R 2 , NSE, PBIAS and RSR as 0.93, 0.94, 6.3% and 0.13 for calibration, and 0.82, 0.89, 10.4% and 0.07 for validation respectively. ICHEC-EC-EARTH-RCA4 was best fitted by scoring 0.9838, 0.0000, 0.9838 and 0.0162 for VHI, VFAR, VCSI and VMI respectively for volumetric, and 0.749, little less than 75 and little less than 100 for CC, NRMSE and δN respectively and better at annual scale at Taylor diagram. The baseline variability of seasonal rainfall indicated that an increments on the winter and autumn and decrease on the spring season. In the 2050s and 2080s of both emission scenarios significant increase and decrease was projected than the baseline periods at seasonal and annual scales. The mean annual rainfall was decreased by; 7.58% and 9.82% at 2050s and 2080s, and 4.92% and 9.28% during 2050s and 2080s under RCP4.5 and RCP8.5 respectively. The total change of rainfall was; 9.89% for 2050s and 13.52% for 2080s, and 8.79% at 2050s and 13.31% at 2080s for RCP4.5 and RCP8.5 scenarios respectively. Future annual stream flow will be decreased by; 8.84% in 2050s and 10.59% in 2080s, and 6.32% in 2050s and 9.88% at 2080s under RCP4.5 and RCP8.5 respectively. The total annual stream flow change will be; 9.88% during 2050s and 13.67% at 2080s, and 9.96% at 2050s and 13.86% at 2080s for the RCP4.5 and RCP8.5 scenarios respectively. Findings of this study indicated that climate change has significant impact over surface water availability of Gerhu-Sirnay catchment. To conduct policy oriented climate change impact over surface water availability, future researchers should consider multiple; climate variables, dynamic drivers and uncertainty analysis, and improve CORDEX inputs