CLIMATE CHANGE IMPACT ON STREAM FLOW OF UPPER BLUE NILE BASIN: BASED ON CMIP5 CLIMATE MODEL OUTPUTS

Abstract

The study aims to assess the impact of climate change on runoff of upper Blue Nile basin North West part of Ethiopia, using CMIP5 climate model outputs (RCP4.5 & RCP8.5) scenarios. The study is different from previous studies since a high resolution but downscaled climate data and new climate scenarios were used. Dynamically downscaled daily rainfall and temperature data of CMIP5 simulation were obtained from CORDEX- Africa Program of RCPs future projection for two periods (20202050) and (20512080) .These climate scenarios data were bias corrected for serving as input to the hydrologic model for impact analysis. Evapotranspiration was estimated using daily temperature by Hargreaves method. Mann Kendaltest was used together with Sen’s Slope estimator for determining the trend and Slope magnitude using XLSTAT software. The test indicates observed mean annual rainfall, average temperature and evapotranspiration were increasing trend. And both scenarios suggest that projection of temperature has increasing tendency, but projection of rainfall is inconsistent and unclear trend in the basin. Summer rainfall is expected to be enhanced in future for both scenarios. For RCP4.5 the projected average warming increases to more than 0.21 o c in 2050s while for RCP8.5 the projected average warming increase more than 0.28 o c in 2080s relative to the historical. To estimate the impact of climate change, a hydrological model (HEC-HMS) was forced for stream flow simulation at Kessie, Belles and Didessa catchments in the basin using rainfall and calculated PET scenarios for 2050s and 2080s. Average runoff volume will increase up to +39.3% and +45.9% during summer season for future periods under RCP4.5 and RCP8.5 scenarios. However average annual runoff volume becomes decrease in the basin. The average annual runoff volume declines for 2020-2080 was insignificant and inconsistent in the basin. Climate change impact analysis was held on hydrology of surface water (runoff volume) without considering the influence of Lake Tanna reservoir on the runoff volume in hydrologic modeling.