Spring and stream intermittency in an instrumented steep Himalayan Mountain catchment

The pathway for rainfall into stream flow in mountain catchments can be fast via surface run-off or short-lived storage in the weathered zone, or slow via the deep fractured bedrock groundwater system. In mountainous topography, springs can be found at almost all elevations, suggesting that groundwater storage occurs at all elevations. There is however uncertainty as if this storage is short lived, confined to the weathered zone, or longer lived and is part of the groundwater system. Intermittent streams and springs might reflect the storage of water within the subsurface. To measure stream intermittency and the migration of the associated headwater springs we installed intermittency loggers based on repurposed HOBO luminosity loggers along five gulleys within the Kahule Khola catchment in central Nepal.

The intermittency loggers measure an electric current when the circuit is closed by surface moisture and flowing water. The loggers were installed in spring 2023 before the pre-monsoon and were removed in November 2024. At low elevation, three series of loggers were installed in gullies below the village of Listi. These below Listi loggers had perennial springs at their lowest elevation. Furthermore, one series of loggers ended at an ERT repeat survey that showed evidence of year-round shallow subsurface saturation. At high elevation, two series of loggers were installed near the village of Bagham, below an open meadow where ephemeral springs were mapped (we call these the meadow loggers). A coincident ERT repeat survey showed evidence of lateral flow of groundwater within this region.

The loggers recorded three distinct phases: (1) The pre-monsoon, where individual storm events can be registered along each gulley as separate wetting events. (2) Monsoon, where there is a continuous and high conductivity measurement for all loggers, representing continuous flow of surface water. (3) The dry season, which starts with a recession in the electric current observed, followed by sparce wet events. The below Listi systems dried completely within the dry season, while the meadow gulleys recorded low but non-zero electric currents even throughout the dry season. The loggers did not record any evidence of spring migration down the gulleys, rather a uniform drying after rainfall events at all locations, with prolonged wetness post monsoon only seen for loggers that were situated just above known perennial springs. The observations would therefore suggest that intermittent run-off comes from the temporary storage in the weathered zone that dries out at the same rate across the catchment, while persistent flow is from points where the topography intersects with the deeper groundwater reservoir. Run-off within the steep catchment therefore operates through two coexisting systems, (1) an intermittent system that is fed from temporary storage of water in the weathered zone, where there is no distinct headwater spring, and (2) perennial streams fed by groundwater springs.