Snowmelt dynamics and stream flow response to climate change in a mountainous basin, western Himalaya.

Snow is a critical component of the Himalayan hydrological cycle, sustaining streamflow, recharging groundwater, rejuvenating springs, maintaining soil moisture, and supporting various water-dependent sectors. However, climate warming is reducing seasonal snow cover and altering snowmelt timing, thereby increasing uncertainty in the relative contributions of snowmelt, rainfall, and groundwater to streamflow and limiting reliable prediction of future water availability. To address this scientific question, we integrated 24 years of satellite-derived snow-cover observations with field-based stable isotope measurements (δ18O and δ2H) collected during 2020-2023 from rainfall, snowfall, groundwater, and streamwater, together with the Snowmelt Runoff Model (SRM). This integrated approach enabled us to investigate long-term snow-cover dynamics and streamflow generation processes in a mountainous catchment of the western Himalaya under both current and projected future climate scenarios. Long-term trend analysis (2000-2023) revealed a significant decline in snow-cover area (-3.31 km2 yr-1), exceeding reported rates from several other Himalayan regions. Bayesian mixing analysis coupled with SRM indicated that snowmelt dominated streamflow during summer (61 ± 3.2%), groundwater predominantly controls winter flow (62 ± 2.5%), and rainfall is the major contributor during spring months (37 ± 2.3%). On an annual basis, snowmelt remained the primary source of streamflow (52-60%), followed by groundwater (24-28%). Future streamflow simulations forced with downscaled Coupled Model Intercomparison Project Phase 6 (CMIP6) climate projections under different Shared Socioeconomic Pathways (SSP2-4.5 and SSP5-8.5) indicate an advance in snowmelt timing and a pronounced seasonal redistribution of discharge from summer to spring. Peak flows are projected to increase by ∼8-15% under SSP2-4.5 by the late twenty-first century and by ∼20-40% under SSP5-8.5, accompanied by a decline in late-summer discharge. The findings highlight the vulnerability of snowmelt-dependent, Himalayan catchments to climate warming and underscore the need for adaptive water-resource management and climate-resilient agricultural planning to mitigate seasonal water-supply risks.