Climate change alters regional hydrologic conditions and results in a variety of impacts on water resource systems. Such hydrologic changes will affect almost every aspect of human well-being. The economy of Ethiopia mainly depends on agriculture; and this in turn largely depends on available water resources. The aim of this study is to assess the impacts of climate change on surface water availability of Upper Awash River Basin by using Soil and Water Assessment Tool (SWAT) hydrological model and Regional Climate Model (RCM). Regional climate model (ECHAM5 with A1B emission scenario) and meteorological variables at local scale was applied for three time periods (2020s, 2050s and 2080s). Bias-correction methods have been developed to adjust RCM climate variables. The results showed that average annual maximum temperature changes for the basin were, for 2020s, 0.53°C; for 2050s, 1.18°C and for 2080s, 1.87°C, relatively to the historical climate (1980-2010). Average annual minimum temperature changes were 0.58°C, 0.82°C and 2.14°C in 2020s, 2050s and 2080s, respectively. The percentages of basin average annual rainfall, based on the ECHAM5 downscaling, were 2.40%, -2.14% and -10.11% for future periods of 2020s, 2050s and 2080s, respectively. The annual stream flow of Upper Awash Sub-Basin was reduced by 2.46% and 18.14% in 2050s and 2080s, respectively, while the stream flow increased in 2020s by 4.90% for A1B scenario. The simulated flow at 2050s and 2080s, with A1B scenario from RCM, showed reduction of runoff by 1.52% and 3.50% respectively in the Sub-Basin and it was directly related to the reduction in precipitation, while the annual runoff increases in 2020s by 8%. Model result showed that about 44.36% of annual rainfall contributes to stream flow as surface runoff. Generally, the results revealed that changes in climatic variables, such as reduction in rainfall and change in both minimum and maximum temperature would have a significant impact on the stream flow and surface runoff, causing a possible reduction on the total water availability in the Sub-Basin.

Keywords: A1B Emission Scenario, Climate Change, Regional Climate Model, SWAT Model, Surface Water