Assessment Of Climate Change Impact On Hydrology Of Lower Awash Sub Basin, Ethiopia

Abstract

This thesis research focusing on assessing the impact of climate change on hydrology over Lower Awash Sub Basin using downscaled climate projection from GCM product outputs of HadCM3. The Statistical Down Scaling Model (SDSM) was used to downscale the GCM data from A2 and B2 emissions scenarios for future climate predictions. The output generated from the statistical downscale model (precipitation and temperature) were used to derive a hydrological model (Soil and Water Assessment Tool, SWAT). SWAT was calibrated and was used to assess the hydrological response over Lower Awash Sub Basin due to climate change. SWAT hydrological model simulation was run for the baseline and three future scenarios (2020s, 2050s, and 2080s) to understand climate change impact on hydrological climate variable by keeping constant calibrated non-climate variables of soil and land use and some climate variables such as windspeed, sunshine hour and relative humidity. The average water balance components of A2 and B2 scenario for three-time slices of 2020s, 2050s and 2080s were compared with the baseline hydrological variables. The performance of SWAT model in simulating the stream flow was shown to be good with a coefficient of determination (R2) 0.92 and 0.77 and the Nash and Sutcliffe efficiency (NSE) of 0.88 and 0.75 for calibration and validation periods, respectively. A simulation study of climate change impact on the basin demonstrates that the hydrology of the basin is very sensitive to climate change with decreased surface runoff 6.85, 7.86 and 7.73% by 2020s, 2050s and 2080s respectively for A2 scenario and about 6.66, 8.25 and 5.29% for 2020s, 2050s and 2080s respectively for B2 scenario. Temperature, rainfall and lateral flow are likely to be increased towards end of century over the study area. Increasing temperature decreased the surface runoff, water yield and increased evapotranspiration for future scenarios. The basin is less sensitive to precipitation as compared to temperature changes. Generally, the projected minimum and maximum temperatures in two scenarios is within the range projected by IPCC which reported average temperature will increase in future.