Unfolding cascading impacts of changing South Asia monsoon on a Hindu Kush Himalayas basin

Indrawati River Basin (IRB) in Nepal. We employ the Soil and Water Assessment Tool model, calibrated and validated with historical data, to assess the hydrological responses of the IRB to future climate scenarios. These scenarios are projected using an ensemble of five bias-corrected CMIP6 climate models under Shared Socioeconomic Pathways 2-4.5 and SSP 5-8.5 for three-time frames: Near Future (2025–2050), Mid Future (MF) (2051–2075), and Far Future (2076–2100). The bias correction was done using Robust Empirical Quantiles for precipitation and linear parametric transformation functions for temperature. The study reveals significant shifts in the IRB's hydrological patterns, including an extended rainy season—most pronounced under SSP 5-8.5 in the MF scenario—and an increase in annual discharge, especially during the South Asian Monsoon. Notably, there is an amplified flood risk due to heightened extreme flow events. Additionally, the analysis indicates increasing risks of wildfires and landslides given the rise in pre-monsoon dry spells, which exacerbates the cascading devastation such as debris flow. These insights underscore the urgent need for tailored water resource management, resilient infrastructure, and forward-looking policies to mitigate the impending hydro-meteorological challenges of climate change. • Bias-corrected streamflow projection using SWAT model. • Prolonged rainy season, more extreme discharge, and severe pre-season aridity. • Resilient infrastructure and proactive policies are needed.