In recent decades, climate change and other anthropogenic activities have substantially affected groundwater systems worldwide. These impacts include changes in groundwater recharge, discharge, flow, storage, and distribution. Climate-induced shifts are evident in altered recharge rates, greater groundwater contribution to streamflow in glacierized catchments, and enhanced groundwater flow in permafrost areas. Direct anthropogenic changes include groundwater withdrawal and injection, regional flow regime modification, water table and storage alterations, and redistribution of embedded groundwater in foods globally. Notably, groundwater extraction contributes to sea level rise, increasing the risk of groundwater inundation in coastal areas. The role of groundwater in the global water cycle is becoming more dynamic and complex. Quantifying these changes is essential to ensure sustainable supply of fresh groundwater resources for people and ecosystems.

The Himalayas are critical for supplying water for ∼2 billion people who live downstream, and available water is highly sensitive to climate change. The role of the groundwater system in sustaining the northern Himalayan rivers remains unknown, and this compromises Asia's future water sustainability. Here, we quantify the spatiotemporal contribution of groundwater to river flows in the Yarlung Zangbo Basin (upper reaches of Brahmaputra). Our results show that the groundwater recharge represents ∼23% of mean annual precipitation, translating into ∼30 km³/yr of baseflow, which contributes ∼55% of the total river discharge in the upstream reaches to ∼27% in the downstream reaches. The percentage of groundwater contribution is inversely related to topographic steepness and total precipitation, with the steepest topography and highest precipitation in the eastern Himalayas. This study fills a knowledge gap on groundwater in the Himalayas and is a foundation for projecting water changes under climatic warming.