Modelling blue-green water fluxes in mountain headwaters at the climatic ends of High Mountain Asia

Mountain catchments receive, retain, transport and release water that determines downstream ecology, landforms, hazards and human livelihoods. The hydrological regimes of such catchments are seasonally governed by the storage and release of water by snow and glaciers, and are modulated by the seasonality of liquid precipitation rates and energy fluxes. The wide range of climatic-topographical situations across High Mountain Asia creates a variety of hydrological regimes in this region.

In this study we apply a sophisticated modelling framework in two heavily glacierized catchments at opposite ends of the climatic spectrum in High Mountain Asia: an arid catchment with winter accumulation glaciers (Vaksh headwaters, Northern Pamir) and  a humid catchment with spring-summer accumulation (the Upper Parlung, South-Eastern Tibet). Both catchments span an elevation range of several thousand metres and a number of vegetation zones. To study the concomitant response of the cryosphere and biosphere, we use a land surface model with a mechanistic and energy-balance-based representation of both the cryosphere and biosphere, at 100m spatial and hourly temporal resolution. We force the model with statistically-downscaled and bias-corrected reanalysis data. For model setup and independent validation, we leverage extensive in-situ observations, collected at both sites. We complement those with spatial datasets, such as ice-dynamics-corrected glacier mass balance, snow cover, and vegetation indices.

We analyse the differences in the catchment water balance and flux partitioning between these two study sites in terms of energy fluxes, snow and glacier accumulation and ablation, vegetation distribution and phenology, and give special attention to patterns of evapotranspiration (ET). Using the model we determine the importance of supraglacial debris cover, widespread in the catchments, and its role in modifying the glacier mass balance under different moisture regimes. We also determine the links between snow melt seasonality, glacier mass balance, plant productivity and the responses in catchment runoff. This work presents one of the first applications of hyper-resolution land surface modelling to understand biosphere-cryosphere-hydrosphere interactions in High Mountain Asia, and will provide insights into the skills and drawbacks of such modelling approaches.