Spatio-temporal variability in stable water isotopes of Brahmaputra river system and its implications.

The Brahmaputra river system is one of Asia's largest and most dynamic transboundary rivers. Yet, limited studies are available on the water isotopes of this river. Therefore, this article aims to address this limitation by addressing the river's complex hydrological processes and its climatic influence using the stable isotope technique. A comprehensive dataset of water isotopes in precipitation, river water, and groundwater was generated for multiple locations within the basin from the year 2022-2024. Seasonal observations were also conducted along different parts of the Brahmaputra river system within the Indian territory. Seasonal isotopic variations in precipitation (δD: 93.33 ‰ to -67.28 ‰, δO: 13.37 ‰ to -9.59 ‰) show depleted signatures during the post-monsoon due to intense rainout. Enriched values of precipitation (δD: 97.01 ‰-40.91 ‰, δO: 13.12 ‰-4.09 ‰) during the pre-monsoon indicate significant evaporation. Spatial variation in river water (δD: 112.67 ‰ to -27.68 ‰, δO: 15.45 ‰ to -4.84 ‰) points to isotopic depletion in upstream regions, while downstream reaches display isotopic enrichment. The distinct isotopic signatures in different reaches of the river are due to variations in basin characteristics. The river water isotopic composition reflects the combined effect of hydrological processes such as evaporation and variability in the mixing of water from diverse sources such as glacial melt, snowfall, rainfall, and groundwater. These findings contribute to understand water sources, evaporation-precipitation dynamics, and regional hydrological processes. Consequently, this knowledge helps in taking appropriate measures for the crucial management of water resources in changing climate scenarios.