Future hydrological regime shifts and water security risks in Ethiopia's Awash River Basin: Ensemble CMIP6-driven SWAT+ simulations under SSP2-4.5 and SSP5-8.5

Climate change threatens water security in Ethiopia's Awash River Basin, yet the magnitude of future hydrological shifts remains poorly constrained. Here we present basin-wide projections using three bias-corrected CMIP6 models driving an advanced SWAT+ model. Under the high-emissions SSP5–8.5 pathway by end-century (2073–2100), 100-year flood magnitudes increase by 28–35%, wet-season peak flows increase by 40–100% along critical reaches, and sediment yields escalate by 35–40%, while dry-season baseflows decline by 15–20%. Despite projected precipitation increases (+5–15%), total water yield remains stable (65–85 mm yr⁻¹), as surplus water partitions into enhanced evapotranspiration (+12–18%) and percolation rather than runoff a recharge-dominated regime masking growing water stress. Lowland subbasins exhibit the most pronounced changes (wet-season runoff +30%, low flows -25%), while upland zones show relative stability. Optimized floodplains (11.4% of basin area) currently attenuate peaks by 22% but will be overwhelmed under SSP5–8.5. For a basin sustaining >20 million people and 80% of Ethiopia's irrigated agriculture, mean annual water availability is a misleading risk indicator. The Awash is transitioning toward hydro-intensification, wetter wet seasons, drier dry seasons, and escalating compound flood-drought stress demanding urgent, scenario-informed adaptation.