Climate-Driven Intensification of Flash-Flood Hazards in Transboundary Muhuri River basin: A CMIP6-Based Assessment Using the Tank Hydrological Model

In June 2024, the Muhuri River in eastern Bangladesh reportedly rose by ~7 m within three days, exceeding danger levels by >3 m and affecting more than 3.6 million people across 11 districts. Severe monsoon flooding in August 2024 reinforced the vulnerability of transboundary, data-scarce catchments to extreme hydrological hazards. To assess future flash-flood risks in the Muhuri River Basin, a four-tank conceptual rainfall–runoff model was developed and calibrated against daily discharge at Parshuram station for 2010–2025 (KGE = 0.72; PBIAS = 5.78%; RMSE = 14.30). The calibrated model was forced with an ensemble of 15 bias-corrected CMIP6 General Circulation Models under SSP2-4.5 and SSP5-8.5. Bias correction used Empirical Quantile Mapping applied to precipitation, maximum temperature, and minimum temperature. The projections were analyzed for near- (2026–2050), mid- (2051–2075), and late-century (2076–2100) periods relative to a historical baseline (1985–2014). The results suggest intensification of high-flow hazards: the 95th-percentile daily high flow (Q95) increases by up to 20.7% under SSP5-8.5, and the frequency of Q95 exceedance events increases by 69%. It is also noted that October discharge (post-monsoon) increases by 28.6%, consistent with delayed recession and a higher likelihood of prolonged inundation. The extreme-event analysis further suggests that 100-year flood magnitudes may increase by up to 22%, with substantial inter-model spread. Our data further indicates measurable reorganization of seasonal flow regimes under future forcing, consistent with emerging non-stationary flood behavior. Overall, these findings support climate-informed, impact-based early warning and adaptation planning for vulnerable transboundary basins.