Groundwater dynamics in a steep Himalayan catchment: the role of a widespread weathering layer in water storage and transfer

The Himalayan region is crucial for providing water resources to millions of people in downstream regions across Asia. However, the processes governing groundwater storage and flow in steep mountain catchments remain poorly understood, particularly regarding the interplay between monsoonal rainfall, infiltration, and groundwater recharge in these highly dynamic landscapes. This study investigates the Kahule Khola watershed in central Nepal, combining field-based approaches encompassing Electrical Resistivity Tomography (ERT), infiltration measurements, and hydrogeochemical analyses, to investigate the pathways and storage mechanisms of groundwater across pre-, during, and post-monsoon seasons. Our findings highlight the critical role of a laterally extensive weathering layer, 10–25 m thick, in regulating hydrological processes. The weathering layer exhibits high infiltration capacities (<24.1 cm/h) that exceed even intense monsoonal rainfall rates (<162.8 cm/h), allowing surface water to rapidly penetrate the subsurface and replenish groundwater stores. The 2D ERT profiles reveal seasonal variations in the saturation of this layer, with significant vertical and lateral flow dynamics linking it to deeper fractured bedrock aquifers. Hydrogeochemical analyses of spring water further demonstrate a bi-compartmentalized flow regimes, where fast and shallow pathways dominate during the monsoon, while slower and long-term storage within the fractured bedrock sustains perennial spring discharge and stream baseflow throughout the dry season. This study enhances our understanding of the hydrological functioning of steep mountain landscapes, emphasizing the dual role of the weathering layer as both a temporary water reservoir and a conduit for deeper aquifer recharge, demonstrating heightened efficiency during monsoon season. By proposing a conceptual model of water transfer and storage in Himalayan catchments, this research provides critical insights into groundwater processes that are fundamental for sustainable water resource management under increasing pressures from climate variability and tectonic activity.