Future changes in hydro-climatic extremes of a large transboundary river basin using multi-model bias-corrected CMIP6 projections.

The Upper Meghna Basin (UMB), a critical large transboundary highly responsive catchment shared between Bangladesh and India, is facing escalating hydro-climatic extremes under climate change, impacting millions of livelihoods. This study innovatively assesses the future changes with monthly, seasonal, and annual matrices of near and far future rainfall, temperature, high flows, and low flows including flood frequencies, shifting patterns, and probability of exceedances using the Soil and Water Assessment Tool (SWAT), for the very first time, forced by bias-corrected CMIP6 projections of two unique SSP2-4.5 and SSP3-7.0 pathways unlike any past studies on the UMB. To reduce uncertainty, a performance-ranking ensemble approach was also applied for the first time in the context of UMB. Key findings reveal significant hydrological shifts towards the end of the century. Rainfall is projected to get more intense during the monsoon season, and temperatures are expected to rise during summer. The 100-year return period high flows are projected to increase by 17.5% and 15.3% in the near future (2026-2055) and 28.3% and 29.9% in the far future (2071-2100) through SSP2-4.5 and SSP3-7.0, respectively. Likewise, 100-year return period low flows are expected to decline by 12.8% and 6.5% during the near future (2026-2055), while a 5.3% reduction and a 7.1% increase are projected in the far future (2071-2100) under SSP2-4.5 and SSP3-7.0, respectively. These findings highlight a future risk of both extreme floods and possible seasonal low-flow shortages depending on the climatic conditions. Policymakers should prioritize early warning systems, floodplain zoning, and drought management strategies to buffer communities against future hydrological variability. Furthermore, the diverging outcomes across emission pathways underscore the importance of climate mitigation efforts to limit long-term hydrological disruptions in the UMB.