Combined effect of climate change and catchment characteristics on streamflow response in the Upper Blue Nile River...

Recently, climate change and human activities such as deforestation are putting drastic pressure on water security in the Upper Blue Nile River Basin, Ethiopia. Despite this, the spatial and temporal variability in streamflow responses to these drivers remains insufficiently understood. In this study, we used the modified Mann-Kendall test and Sen's slope estimator to quantify streamflow trends from 1981 to 2018 in 25 catchments, and we applied a Budyko-based elasticity framework, considering the impact of potential evapotranspiration (E₀), precipitation (P), and catchment characteristics (ω) to attribute these trends. Results showed that six catchments- Gumara, Azuari, Koga, Megech, Main Beles, and Rib-exhibited statistically significant rise in annual streamflow, with rates ranging from 3.5 to 10.5 mm year^-1. In contrast, catchments such as Dura and Upper Rib showed decreasing streamflow trend despite rising precipitation, indicating complex hydroclimatic interactions. Elasticity of streamflow to P (ε_p) ranged from 0.60 to 0.83. In contrast, elasticity to Eo(ε_Eo), and ω(ε_ω) ranged from -0.40 to -0.17, and -0.48 to -0.11, respectively. Upstream catchments-such as Gelgel Abay and Gumara-exhibited higher sensitivity to climatic variability, as reflected in their coefficients. Generally, 67% of the observed streamflow variability was attributed to climate change, while catchment characteristics contributed to the remaining 33% of the streamflow variability, with distinct spatial differences. This conclusion shows the need for customized management approaches to address both shifts in climatic variables and changes in vegetation.