Future Changes in River Discharge: Insights from CMIP6 Model Simulations for the Great Ruaha River Basin, Tanzania

Water is the main source of sustenance for millions of people living within the Great Ruaha River Basin (GRRB). However, water scarcity resulting from dwindling river discharges has emerged as a major challenge, affecting livelihoods and threatening the survival of dependent ecosystems. With the ongoing global climate change, it is anticipated that water stress in the basin will intensify as a result of disrupted hydrological cycle. This study assessed the potential changes in discharge resulting from future climate change in the GRRB during the (i) the mid-future (2036–2065) and (ii) the far future (2071–2099) periods. Five Global Circulation Models (GCMs) from the Coupled Model Intercomparison Project Phase 6 (CMIP6), applied under two Shared Socioeconomic Pathways (SSP) scenarios (SSP3-7.0 and SSP5-8.5), were utilized. The calibrated Soil and Water Assessment Tool (SWAT+) was used to evaluate the impact of climate change on discharge patterns. Climate projections indicated that, temperatures are expected to rise by 2–4°C by the end of the century under both scenarios, with evapotranspiration rates increasing by 0–2%. Annual average precipitation is projected to vary by -1% to 3% compared to the historical baseline (1981–2010). Interannual variability showed a projected decrease in precipitation during the mid-future and an increase in the far future. Similarly, long-term annual discharge trends revealed declines in the mid-future under both scenarios, with increases toward the far future. Mean monthly discharge indicated minor changes (-1% to 11%). Low flows are projected to remain relatively stable while high flows will exhibit mixed patterns, ranging from -8% to 7%. These findings highlight increased water stress in the mid-future, with potential recovery in the far future, underscoring the need for sustainable, climate-resilient water management to protect livelihoods and GRRB’s ecosystems in the face of changing climate.