Attribution and Streamflow Sensitivity Analysis in the Upper Blue Nile River Basin Using a Top-down Modeling Framework

This study applied the Budyko framework and elasticity method to quantify the relative contribution of climate and watershed characteristics to streamflow changes in 95 watersheds in the Upper Blue Nile River Basin (UBNRB). The Water and Energy Processes (WEP) model was successfully verified and used to simulate the streamflow and potential evapotranspiration of the watersheds. The study period was divided into base (1983-1998) and change (1999-2018) periods based on Pettitt's test to assess the changes in streamflow. Precipitation showed moderate fluctuations during the study period (1983-2018) with an increasing trend. However, potential evapotranspiration exhibited very low fluctuation, leading to a lower contribution to streamflow changes than precipitation. The watershed characteristics coefficient (ω) decreased overall during the study period. Nearly 81% of the UBNRB watersheds had aridity index values between 0.6 and 1.1, indicating humid conditions. Watersheds with higher aridity index and lower ω were more sensitive to streamflow change. Streamflow in the UBNRB tended to increase more due to changes in watershed characteristics than due to climate variations. Nearly 68% of the watersheds showed an increase in streamflow during the change period due to the combined effects of climate and watershed characteristic variations. Overall, the Eastern and Southern source regions and the Northwestern lowlands of the basin were highly affected by these changes. These areas need more attention to sustainably manage the basin's water resources.