Groundwater monitoring along a Trans-Himalayan transect using seismic velocity changes

The Himalayas represent one of the world’s major water towers, sustaining downstream populations across Asia. However, hydrological processes in this region remain poorly constrained due to extreme topography, limited accessibility, and sparse in situ observations. Consequently, most existing studies rely on modeling and remote sensing, with a strong emphasis on glacier melt. Yet water-budget estimates suggest that up to two-thirds of discharge originates from groundwater, underscoring the need to better understand subsurface water dynamics.To anticipate future water availability in a progressively glacier-free Himalaya, ground-based observations are required. Here, we exploit the dense Hi-CLIMB seismic array to investigate hydrological variability along a Trans-Himalayan transect. The array spans ~250 km from the Terai plains to the Tibetan Plateau, with an average station spacing of ~5 km. Most stations operated between late 2022 and 2024, capturing at least one full monsoon cycle.We derive temporal seismic velocity changes from ambient noise interferometry as a proxy for groundwater storage variations, and analyze seismic noise amplitudes as an indicator of river activity. These seismic observations are compared with climatological datasets, regional geology, glacier cover from the Randolph Glacier Inventory, and geomorphic features extracted from GIS analysis. Our results reveal strong spatial contrasts in monsoon-driven hydrological responses and identify distinct zones and phases contributing to runoff along the transect, highlighting the potential of seismic monitoring for resolving groundwater dynamics in high-mountain environments.