IMPACT OF CLIMATE CHANGE ON HYDROLOGICAL BEHAVIOR OF ANKARA WATERSHED, NORTHERN AWASH BASIN, ETHIOPIA

Abstract

precipitation and temperature will increase over eastern Africa in the coming century. The main objectives of the study was to investigate the possible impact of climate change on hydrological behaviors of Ankara watershed Northern Awash basin, by downscaling canESM2 (Canadian Earth System Model of second generation) global climate model using Statistical Downscaling Model (SDSM). Based on IPCC recommendation baseline periods (1985–2015) were used for baseline scenario analysis. Future scenario analysis was performed for the 2020s, 2050s, and 2080s. Globally, canESM2 model is widely applied for climate change studies and it consists of Representative Concentration Pathway (RCP) RCP8.5 (medium to high emission) and RCP4.5 (medium low emission) scenarios. Impact assessment on stream flow and sediment yield was done by Soil and Water Assessment Tool (SWAT) hydrological model. To set up the model for simulation a 20×20m DEM (Digital Elevations Model), land use map and soil map were used. The daily recorded weather data from 1985 to 2015 were used as input to the model. Monthly stream flow and sediment yield data were available from 1987 to 2001. The model was calibrated using both automated and manual calibration for monthly stream flow and sediment yield data. SWAT model performance in simulating monthly stream flow and sediment yield for the study area was satisfactory with Nash–Sutcliffe Efficiency (NSE), coefficient of determination (R2) and percent error (D). The statistical analysis of calibration result of Ankara watershed showed satisfactory agreement between observed and simulated values in the monthly flow simulation, with R2 (0.78 and 0.82), NSE (0.72 and 0.76) and D (19.50 and 9.5) for calibration and validation periods respectively; and for monthly sediment yield simulation R2 (0.77 and 0.79), NSE (0.72 and 0.74) and D (18.5 and 12.5) calibration and validation periods respectively. The estimated soil loss rate from different sub watersheds had ranged from.0.96 tons/ha/yr to 29.42 tons/ha/year, whereas the annual weighted average soil loss rate from the watershed was estimated 13.09 tons/ha/year. Mean annual changes of precipitation and temperature (maximum and minimum) were applied to quantify these impacts. The result of downscaled precipitation and temperature reveals a systematic increase in all future time periods for both RCP8.5 and RCP4.5 scenarios. These increases in climate variables are expected to result in increase in mean annual sediment yield of 16.42%, 20.81%, and 26.08 % for RCP8.5 scenario and by 15.02%, 19.30%, and 16.60 % for RCP4.5 scenario for the 2020s, 2050s, and 2080s, respectively. This increase in sediment yield is double the increase in stream flow due to climate change for all time periods. In conclusion, awareness and understanding of the impacts of climate change is critical globally, continentally, nationally as well as at specific catchment scale. Results of climate change on Ankara watershed using canESM2 model via RCP4.5 and RCP8.5 scenarios downscaled by SDSM showed that temperature and rainfall are expected to increase in all time periods. Future work need to consider also different GCM models and impact of land use change on the catchment for future sustainable development plan.