Role of Groundwater in Sustaining Northern Himalayan Rivers.

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. Plain Language Summary: Major rivers flowing from the Himalayan Plateau provide critical water resources for 2 billion people who live downstream and rely on Himalayan rivers for water supply, irrigation, hydropower and in‐stream flows for aquatic habit. While groundwater discharge to rivers is known to contribute to available water resources, how much Himalayan rivers depend on groundwater is poorly known. This study models the spatial groundwater flow and discharge volume to the Yarlung Zangbo, the largest river of the Himalayas in China, based on long‐term climate and streamflow observations. We map the patterns of groundwater flow paths, water table elevations, recharge, and discharge to rivers. We estimate that ∼55% of the total river discharge in the upstream reaches in the central Himalayas is sustained by groundwater and ∼27% in the downstream reaches in the eastern Himalayas. Groundwater aquifer recharge totals at least ∼9 km3 during the monsoon season (May–October) and this sustains river low flows during dry season (November–April). This study fills a knowledge gap on subsurface flow processes in the northern Himalayas and provides a baseline for comparing changes in the stream water flow under climatic warming over vulnerable mountainous headwater regions. Key Points: Groundwater supports dry season streamflow, providing 55%–27% annual water to central and eastern Himalayan rivers, respectivelyLow flows between November and April are sustained by groundwater storage resulting from recharge between May and OctoberThe percentage groundwater contribution is inversely related to steepness of the topography and total precipitation, in combination [ABSTRACT FROM AUTHOR]