Hydrological Response of the Thiokoye River Basin to Climate Change: An Assessment Using the GR4J Hydrological Model and CMIP5 Climate Projections

This study evaluates the impacts of climate change on surface water resources in the Thiokoye River Basin, a tributary of the Gambia River in West Africa, using the GR4J daily hydrological model implemented in R via the airGR package. Model calibration (1974–1984) and validation (1985–1991), following a warm-up period (1972–1973), were conducted using the SCE-UA algorithm and a multi-criteria objective function combining NSE, log-NSE, KGE, and R², all exceeding 75%, indicating satisfactory model performance. Future climate projections, derived from five CORDEX-Africa regional models (GFDL-ESM2M, HadGEM2-ES, IPSL-CM5A-LR, NorESM1-M, and MIROC) under RCP4.5 and RCP8.5 scenarios, were bias-corrected using the CDF-t method and fed into the model to simulate streamflow until 2100. Results show a general decline in mean annual discharge— comprised between − 30% under RCP4.5 and − 19.6% under RCP8.5 compared to the historical average of 8.71 m³/s—despite stronger warming under RCP8.5 (+ 4.15°C vs. +2.5°C), a paradox explained by the higher flows projected by some models, especially MIROC5. This is consistent with uncertain rainfall trends, with Multi-Model Means (MMM) indicating annual decreases of − 12.71% (RCP4.5) and − 16.55% (RCP8.5), while MIROC5 alone forecasts increased precipitation. Flow variability is expected to intensify, with standard deviations rising to 4.22 m³/s (RCP4.5) and 6.5 m³/s (RCP8.5). Seasonally, runoff is projected to decrease during the wet season, with greater extremes—minimum flows dropping to 0.1 m³/s and peaks potentially reaching 33.1 m³/s under RCP8.5—alongside lower dry-season flows and potential shifts in flow timing. These findings underscore the hydrological vulnerability of the Thiokoye Basin to future climate change and the importance of incorporating ensemble-based, uncertainty-aware modeling approaches into water resource planning. The study reinforces the urgency of adaptive, climate-resilient strategies for basin management, infrastructure design, and agricultural planning to safeguard livelihoods and ecosystems in the face of intensifying climate pressures.