Unravelling the hydrologic impact of climate change in the Great Ruaha River Basin, Tanzania

Abstract In recent decades, the Upper Great Ruaha River Basin (UGRRB) has experienced severe water scarcity due to alterations in its river discharge. However, the extent to which these alterations may be attributed to climate change versus land use/cover changes, water withdrawals, etc., remain uncertain. This study aims to assess the historical impact of climate change in the water balance and river discharge using a modelling approach. The Soil and Water Assessment Tool Plus (SWAT+) model was developed, calibrated and validated for the basin. By forcing the model with both factual (including anthropogenic climate change) and counterfactual (excluding anthropogenic climate change) climate data for 1901–2019, the impact of climate change was quantified as the difference between the two model simulations. Furthermore, long–term trends and magnitudes from simulations were assessed by Mann–Kendall and Sen’s slope statistical approaches, respectively at 5% significance level. The SWAT+ model yielded satisfactory performance with a Nash-Sutcliffe Efficiency of 0.76 and 0.67 during monthly calibration and validation periods, respectively. Attribution results from factual and counterfactual model simulations revealed increases in annual precipitation, evapotranspiration, surface runoff, lateral flow and percolation, as well as simulated annual river discharge attributable to climate change. Conversely, analysis of observed annual river discharge data revealed a consistent and significant decline during 1963–2017 (–1.029 m 3 s −1 yr −1 , p < 0.05). This discrepancy highlights the influence of non–climatic factors such as land use changes and increased water use. These findings underline the urgent need of integrating both climatic and non–climatic factors into basin’s water management frameworks to mitigate emerging water challenges.