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Mountains are important suppliers of freshwater to downstream areas, affecting large populations in particular in High Mountain Asia (HMA). Yet, the propagation of water from HMA headwaters to downstream areas is not fully understood, as interactions in the mountain water cycle between the cryo-, hydro- and biosphere remain elusive. We review the definition of blue and green water fluxes as liquid water that contributes to runoff at the outlet of the selected domain (blue) and water lost to the atmosphere through vapor fluxes, that is evaporation from water, ground, and interception plus transpiration (green) and propose to add the term white water to account for the (often neglected) evaporation and sublimation from snow and ice. We provide an assessment of models that can simulate the cryo-hydro-biosphere continuum and the interactions between spheres in high mountain catchments, going beyond disciplinary separations. Land surface models are uniquely able to account for such complexity, since they solve the coupled fluxes of water, energy, and carbon between the land surface and atmosphere. Due to the mechanistic nature of such models, specific variables can be compared systematically to independent remote sensing observations – providing vital insights into model accuracy and enabling the understanding of the complex watersheds of HMA. We discuss recent developments in spaceborne earth observation products that have the potential to support catchment modeling in high mountain regions. We then present a pilot study application of the mechanistic land surface model Tethys & Chloris to a glacierized watershed in the Nepalese Himalayas and discuss the use of high-resolution earth observation data to constrain the meteorological forcing uncertainty and validate model results. We use these insights to highlight the remaining challenges and future opportunities that remote sensing data presents for land surface modeling in HMA. [ABSTRACT FROM AUTHOR]

High Mountain Asia (HMA) is among the most vulnerable water towers globally and yet future projections of water availability in and from its high‐mountain catchments remain uncertain, as their hydrologic response to ongoing environmental changes is complex. Mechanistic modeling approaches incorporating cryospheric, hydrological, and vegetation processes in high spatial, temporal, and physical detail have never been applied for high‐elevation catchments of HMA. We use a land surface model at high spatial and temporal resolution (100 m and hourly) to simulate the coupled dynamics of energy, water, and vegetation for the 350 km2 Langtang catchment (Nepal). We compare our model outputs for one hydrological year against a large set of observations to gain insight into the partitioning of the water balance at the subseasonal scale and across elevation bands. During the simulated hydrological year, we find that evapotranspiration is a key component of the total water balance, as it causes about the equivalent of 20% of all the available precipitation or 154% of the water production from glacier melt in the basin to return directly to the atmosphere. The depletion of the cryospheric water budget is dominated by snow melt, but at high elevations is primarily dictated by snow and ice sublimation. Snow sublimation is the dominant vapor flux (49%) at the catchment scale, accounting for the equivalent of 11% of snowfall, 17% of snowmelt, and 75% of ice melt, respectively. We conclude that simulations should consider sublimation and other evaporative fluxes explicitly, as otherwise water balance estimates can be ill‐quantified. Plain Language Summary: The Himalayan mountains are a crucial source of water for local communities and the densely populated lowlands. Snow and glaciers are important for the water cycle in mountains, and many studies have focused on them. However, vegetation can play a central role, too. To better understand the water cycle in a high‐elevation Himalayan basin, we use a computer model that considers physical processes of snow and glaciers, soil, and vegetation in high detail. We simulate the catchment water balance and investigate the importance of single components at different elevations and in different seasons. We find that the transfer of water, in vapor form from the ground, snow and ice and the transpiration of plants are important for the water budget in the study basin, causing more water to return to the atmosphere than ice melt contributing to river runoff. Above 6,500 m above sea level, it is the transformation of snow into water vapor and below 4,000 m the transpiration from vegetation that dominate the water budget. We conclude that models simulating the water cycle in high mountain regions should consider vapor fluxes, as otherwise estimates of the water balance can be misleading. Key Points: Land surface modeling reveals high altitudinal/subseasonal variability in water balance partitioning in a glacierized Himalayan catchmentWater loss through evapotranspiration, dominated by snow sublimation, exceeds water production from glacier melt by 59% at catchment scaleDepletion of cryospheric water budget is dominated by snow melt, but at high elevations is primarily dictated by snow and ice sublimation [ABSTRACT FROM AUTHOR]