Melt sustains pre-monsoon flow while groundwater drives the monsoon in the Nepal Himalayas

Himalayan water-security assessments often focus on glacier retreat, yet groundwater may supply much of river flow. We combine seismic observations from the Hi-CLIMB transect across Nepal (2002–2004) with gauged discharge, satellite precipitation, and glacier-cover inventories to resolve when streamflow is sustained by melt versus groundwater. Relative seismic velocity changes track hillslope pore pressure and groundwater recharge, while river-induced seismic noise amplitudes provide a proxy for discharge variability along the Trisuli river. Unglaciated catchments show declining pre-monsoon discharge despite rainfall, consistent with subsurface moisture buffering and delayed runoff generation until monsoon groundwater recharge begins. In contrast, catchments with glacierized headwaters exhibit a pre-monsoon discharge rise attributable to melt, contributing an estimated ~7% of annual discharge at a ~20% glacier-cover outlet and sustaining river flow during the Spring season. During the monsoon, groundwater dominates runoff generation across elevations largely independent of glacier cover, with a sharp transition near ~28.5°N consistent with monsoon penetration, highlighting high Himalayan “water towers” as groundwater-regulated systems with meltwater acting as a critical seasonal buffer.