Assessing the impact of future climate change on water availability: Insights from CMIP6 projections and SWAT+ modelling in the Upper Great Ruaha River Basin, Tanzania

Study region Upper Great Ruaha River Basin, Tanzania. Study focus Climate change impacts on water availability were assessed using the Soil and Water Assessment Tool Plus (SWAT+) model forced by climate projections from ten bias-adjusted and downscaled global climate models under SSP1–2.6, SSP3–7.0, and SSP5–8.5 scenarios. Simulations were conducted for a historical period (1961–1990) and two future periods (2036–2065 and 2070–2099). Climate impacts were quantified by comparing future and historical simulations, while hydrological extremes were evaluated using flow duration curves. New hydrological insights (1) Annual temperature was projected to increase under all scenarios by 1.8–1.9°C (SSP1–2.6), 2.3–4.0°C (SSP3–7.0), and 2.6–4.9°C (SSP5–8.5). (2) Wet-season rainfall was projected to increase by up to 24% under SSP5–8.5 in the far-future, while reductions of up to 12.5% are projected under SSP1–2.6 in the mid-future during the onset of the rainy season. (3) These changes increased evapotranspiration, surface runoff, and water-yield, while reducing percolation, baseflow, and river discharge by up to 7.8% under SSP1–2.6 in the mid-future. (4) High-flows were projected to increase by more than 75% in the mid-future across all scenarios, while low-flows declined by more than 10% both future periods. The changes pose substantial risks to agriculture, water supply, energy generation, and local livelihoods. The findings provide local evidence to support climate-resilient water resources management and adaptation planning in the basin.