Drivers and magnitude of evapotranspiration in a high-altitude Himalayan catchment: insights from eddy covariance observations

While the role of High Mountain Asia (HMA) as water tower is well established, the evaporative loss component of the high-altitude water balance remains poorly constrained. Quantifying evapotranspiration (ET) in these environments is complicated by extreme topographic relief and the severe scarcity of in-situ observations. Consequently, the interplay between atmospheric demand, soil moisture, and alpine vegetation remains a significant source of uncertainty in assessing current and future mountain water resources. In this study, we characterize the temporal variability and controlling mechanisms of ET in the Nepal Himalayas using data from a unique high-altitude hydrometeorological observational setup. In addition to measurements of temperature, precipitation, relative humidity and soil moisture, we continuously monitored turbulent fluxes using an eddy covariance system installed at 4214 m a.s.l. in the Langtang Valley in Nepal for a full year (November 2023 – October 2024), providing an exceptionally detailed dataset for understanding temporal ET dynamics and model evaluation. We observed high ET rates in the pre-monsoon season, driven by vegetation transpiration when there is sufficient soil moisture in a water-limited regime. With the increased precipitation during the monsoon season, the system shifts to a largely energy-limited regime. ET is suppressed during precipitation, but rebounds rapidly during multi-day dry spells. In the post-monsoon season, when precipitation is mostly absent, evapotranspiration is dominated by receding soil moisture and exceeds the precipitation input. Over the entire year, ET returned 44% of the precipitation input to the atmosphere. This substantial fraction indicates that these high-altitude headwaters are not merely passive runoff generators but active ecohydrological systems that contribute substantially in regulating catchment yield. To support robust climate adaptation strategies, future hydrological projections in HMA should therefore explicitly account for vegetation dynamics and soil moisture coupling to avoid significant misinterpretations of the water balance under global change.