Dissecting the subseasonal and altitudinal water balance of a high-elevation Himalayan catchment using a land surface model

The snow and glacier reservoirs of High Mountain Asia play a key role in sustaining water supply to mountain communities and downstream ecosystems, populations and economic activities. However, little is known about how rain, snow- and ice melt vary sub-seasonally and along the altitudinal gradient in high-elevation watersheds.

We generate detailed simulations of catchment hydrology using a land surface model that constrains energy and mass fluxes using advanced physical representations of both cryospheric and biospheric processes in high detail at 100 m spatial resolution. We use the model to study how snow and glacier processes affect the hydrological cycle and how vegetation can mediate water yield from the high mountains of a glacierized Himalayan catchment downstream. This bridges the modelling gap between snow- and glacier dynamics, which generate the runoff, and vegetation processes, which interfere with runoff production and water uses at lower elevations.

We study the upper Langtang catchment (~4000-7000 m a.s.l.) in the Nepalese Himalayas, and simulate catchment runoff for two hydrological years (2017-2019), revealing the relative importance of precipitation, snow, ice, soil moisture and vegetation for different elevations and seasons. The land surface model is forced with hourly meteorological input data based on the main weather station in the basin and air temperature and precipitation were spatially distributed using observed elevational gradients. 

We calibrate a minimal set of parameters (physical properties of supraglacial debris) and use integrative variables such as catchment runoff or glacier mass balance only for validation. The availability of a rich dataset of field- and remote sensing observations allows validation of numerous physical processes simulated by the model and drastically reduces the probability of internal error compensation.

The model provides detailed insights into the importance of each of the energy and mass balance components in the catchment water budget and shows that evaporative fluxes are non-negligible contributors to mass loss at very high elevations (especially from snow) and in the lower part of the catchment (transpiration from vegetation). Often neglected or derived as a bulk quantity in simpler model approaches, evaporation accounts for about 15% of the water leaving the basin. We show precipitation to be the major source of uncertainty in the simulations and that vegetation is relevant in determining the amount of runoff transferred further downstream even for high elevation, extensively glacierized Himalayan catchments.