Influence of existing reservoirs on the propagation from meteorological to hydrological extremes under climate change in the Ruzizi river basin: historical assessment and future projection

This research examines how climate change affects the propagation of meteorological to hydrological extremes in the Ruzizi River Basin (RRB), a transboundary catchment in Central Africa with existing reservoir infrastructure. Using the hydrological model (SWAT) driven by bias-corrected outputs from three CMIP6 models under SSP2-4.5 and SSP5-8.5, streamflow was simulated for historical (1983-2020) and future (2040-2100) periods. Extremes were assessed using the Standardized Precipitation Index (SPI) and Standardized Streamflow Index (SSI). We found that the basin showed decreasing rainfall and temperatures, with subtle decadal shifts during the historical period. Mid-century SSP2-4.5 projects increased rainfall, while SSP5-8.5 shows little change, though both project substantial Tmax and Tmin rises, leading to more uniform seasonal temperatures. Hydrometeorological extremes are projected to become more variable, particularly under SSP5-8.5. The historical SPI-SSI lag of 3 months shortens to 0.19 months under SSP5-8.5, indicating faster hydrological responses. SPI12-SSI3 correlation strengthens to 0.74 mid-century but drops to 0.19-0.44 by century's end, suggesting decoupling between precipitation and streamflow extremes. These shifts imply accelerated, unstable transitions between wet and dry conditions, narrowing early warning times, and underscore the need for adaptive, climate-resilient water management in transboundary systems like the RRB.