Abstract

Groundwater flow paths in mountainous regions dictate how water is transferred from the surface, through the weathering zone, and into the bedrock aquifer system. Previous studies have established the critical role of bedrock aquifers in sustaining mountain river flow through the gradual release of water. However, the specific pathways that groundwater takes in the weathering zone to reach deeper aquifers remain poorly understood. In this study, we investigate the compartmentalization and transfer of water from surface rainfall input to a deep groundwater reservoir within a steep Himalayan catchment in central Nepal covering a pre-, during and post-monsoon season. We employed a comprehensive approach, integrating inverted electrical resistivity changes in the upper few tens of meters below the surface with geochemical and hydrological measurements, including infiltration rates. This has helped us to determine groundwater distribution and dynamics. We found a laterally extensive weathering zone with predominantly very high surface infiltration capacity. In steep mountain topography, this permeable mantle plays a key role in regulating catchment hydrology by efficiently absorbing rainfall and transferring surface water into groundwater from the onset of monsoon. In the study catchment, groundwater flows both vertically downward into the mountain bedrock interior, and parallel to the subsurface within the weathering zone and colluvial hollows. Our work enhances understanding of the mechanisms that recharge the mountain groundwater reservoir, govern spring location and activity, and may support water resource management in regions vulnerable to climate variability and change.