Water isotopes-based investigation of hydrological processes in a climatologically sensitive glacierized catchment in the post-monsoon season

Recent studies in the headwaters of the Ganga River (Central Himalaya) have predicted an increase in post-monsoon temperature and wet precipitation with a decrease in snowfall, which could affect hydrology of glacierized catchment. Therefore, determining the factors influencing water movement and estimating the contribution of glacier-snowmelt is crucial for effective water resources management in the region. This study employed stable water isotopes to investigate key hydrological processes governing movement of water in the Pindar River (a tributary of the Alakananda River) basin and further estimated seasonal (post-monsoon) glacier-snowmelt contribution to the river using two-component mixing models. Isotopic characterisation of rainfall (n=7), glacier-snowmelt (n=18), river (n=41) and groundwater (n=44) were performed to identify sources of water and establish end-members. This ascertains that although the Pindar river's primary source is glacier-snowmelt water, the river exhibits a systematic isotopic enrichment downstream, reflecting the influence of hydrological mixing and environmental exchange. The glacier-snowmelt contribution rapidly diminishes along the river course, decreasing significantly from 96%±36.70% at the glacier snout to 48%±8.90% just 21 km downstream. This spatial trend indicates a declining reliance on cryospheric inputs and a corresponding increasing contribution from isotopically distinct groundwater sources. These results underscore the strong dependence of the Pindar River on glacier-snowmelt during post-monsoon, making the water resources of this central Himalayan region highly vulnerable to the impacts of ongoing climate change and accelerated glacier recession.