Evaluation of Climate Change Impact on Stream Flow: A case study of Awash Bello Sub-basin, Upper Awash River Basin, Ethiopia.

Abstract

Climate change impact and its variability on stream flows are now becoming one of the significant challenges. It occurs because of internal variability within the climate system and external factors. The main objective of this study was to evaluate the impacts of climate change on the streamflow of the Awash Bello sub-basin, Ethiopia, using the soil and water assessment tool. This study contributes to putting direction to plan and manage the streamflow sustainability by the water resource sector of the country. For this study Hydrological data, spatial data, and meteorological data were collected from the ministry of water, irrigation, and electricity, the National meteorological service agency respectively, and the regional climate model from the Coordinated Regional Climate Downscaling Experiment Africa was used. Downscaled future climate projections of precipitation and temperature were extracted from Hadley Global Environment Model 2-Earth System) under two radiative forcing scenarios (RCP4.5 and RCP8.5). Before using the input data especially precipitation and temperature to the SWAT, bias correction was made by using power transformation and variance scaling equation for both in line with the observed data, and areal rainfall by Theissen polygon over the sub-basin was determined. The study area was delineated into 35 sub-basins with 218 hydrological response units. The future projection period was divided into two-time horizons the 2050s (2021-2050) as middle-future and 2080s (2051-2080) and as far-future compared with base period (1990–2019). The result obtained shows that projected mean annual precipitation expected to increase by 4.85% in the 2050s and decreased by 9.87% in 2080s under RCP4.5 while under RCP8.5 precipitation increased by 7.56% in 2050s and decreased by 15.21% in 2080s. The projected minimum temperature increased by 0.78o c and 1.12o c for RCP4.5 and 0.93o c and 1.38o c for RCP8.5 in the 2050s and 2080s respectively. Similarly, the maximum temperature increased by 1.06o c and 1.28o c for RCP4.5 and 1.13 o c and 1.43o c for RCP8.5 in the 2050s and 2080s respectively. The calibration and validation of the model were done using SWAT-CUP and SUFI-2 algorithms and the model indicated good results. The model results showed a good performance with a statistical performance evaluation of R 2 = 0.89, NSE = 0.87 and PBIAS = - 11.9 during calibration and R 2 = 0.88, NSE = 0.86 and PBIAS = -17.1 during validation. The mean annual streamflow percentage of changes increased by 4.22% and 5.71% for a period of 2050s under RCP4.5 and RCP8.5 scenarios respectively. While in the 2080’s it may decrease by 10.25% and 12.15% under RCP4.5 and RCP8.5 scenarios. Generally, there will be the tendency of variation in streamflow for all future time series. Therefore, it is important to consider this variation of flows to structure appropriate guidelines for planning and management of future and existing water resource projects in this study area