Groundwater pathways and storage dynamics in steep mountain topography

The Himalayan Mountain range is considered as a sustainable large water reservoir, often termed as “water towers of Asia”. This important reservoir of water is replenished annually by monsoon precipitation and is slowly drained in dry season. However, the processes that govern this water budget: the connectivity between perched aquifers situated high in the topography and the underlying fractured bedrock, is not well understood. In this study we investigate the surface-subsurface coupling and characterize the water pathways on a watershed scale. This will help to better understand where and how water is stored within the steep Himalayan topography. The study focuses on the unglaciated Kahule Khola watershed (~33 km²) situated north of Kathmandu in the central Himalayas (ranging from ~1000 to ~3500 m asl). During two field campaigns, we mapped the location of springs before (in May 2022) and after (in November 2022) the monsoon season. We characterized the surface infiltration capacity, soil permeability and carried out multiple ERT surveys covering the first 3000 m elevation profile. All these measurements were made on the major landforms (ridges, V-shaped gullies, and debris filled gullies) and different land use types (terraces, forest, meadow, and landslide debris), giving a clear picture of the landscape structure within this catchment. Infiltration rates and soil permeability are high with an average over 1 m/d, which suggests that infiltration dominates over surface runoff during the monsoon. ERT surveys show low resistivity (from ~100 to ~1000 Ω.m) at shallow depth in line with a weathered upper soil layer. Below this layer the ridges have a higher resistivity (from ~1000 to ~50000 Ω.m) while the gullies have very low resistivities suggesting saturated perched aquifers systems close to the surface. We found that spring heads move up or down slope to the seasonal water table fluctuations, tracing the topographic intersection of the groundwater with the surface. These observations suggest that water storage is substantial but not uniformly distributed within the landscape over time and space. We propose that besides the fractured bedrock, filled gullies and landslide deposits form perched water pockets with an important role in storing and distributing water, especially in the higher parts of mountain landscape.