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

Madhusmita Nanda , Archana M Nair 

The Brahmaputra river system is one of Asia's largest and most dynamic transboundary rivers. Yet, significant investigations on the isotopic signature of its water are limited and not documented. This study aims to address the limitations in datasets and enhance the knowledge of the river's complex hydrological processes and its climatic influence using the stable isotope technique. Over 1-2 years, a comprehensive dataset of isotopes was gathered from multiple locations within the basin to study the continuous changes in precipitation, groundwater, and surface water. Seasonal observations were also conducted along different parts of the lower Brahmaputra river system, from its entry point in Indian territory to its lower reaches in the Assam valley. The isotopic composition reflects contributions from various sources such as glacial melt, snowfall, rainfall, and groundwater that flow into the river water. Seasonal isotopic variations in precipitation show depleted signatures during the post-monsoon due to intense rainout, resulting in enrichment in the remaining water vapour. Enriched values of precipitation during the pre-monsoon indicate significant evaporation. Spatial variation in river water points to isotopic depletion in upstream regions due to snowmelt and glacial contributions, while downstream reaches display isotopic enrichment from heavy precipitation inputs and tributary influence. The result of distinct isotopic signatures in different reaches of the river is due to variations in basin characteristics. It helps to understand water sources, evaporation-precipitation dynamics, and regional hydrological processes. These findings would contribute to developing a better knowledge of the hydrological behaviour of the Brahmaputra River and help in taking appropriate measures for crucial management of water resources and predicting climate change impacts on river systems.