The Hydrology of High Mountain Asian Headwater Catchments via Mechanistic Modelling

Abstract

The headwater catchments of High Mountain Asia (HMA) provide freshwater to large populations from the semi-arid lowlands of Central Asia to the densely populated agricultural plains and metropoles of Southern and Eastern Asia. Climatic conditions vary greatly between HMA sub- regions, with contrasting seasonality and amounts of precipitation, controlled by the interactions of several large-scale circulation systems. Climate change is altering the boundary conditions under which these systems are operating, with cascading effects on all components of the energy and water balances: Seasonal snow accumulation and glacier masses are decreasing, and hydrological regimes are shifting gradually towards less snowmelt runoff and more rainfall runoff. Changes in the land surface energy budget and properties of the atmosphere are altering vegetation productivity and evaporative fluxes, with largely unknown consequences for runoff. Projections of future high-mountain hydrology are highly relevant to society, however, they are currently limited by our understanding of the multiple and entangled physical processes at play, and a lack of appropriate modelling tools for representing their spatial and temporal integration. In this thesis, we apply a modelling framework that combines a state-of-the-art model of snow, glacier, and ecohydrology, Tethys-Chloris (T&C), with glaciological, hydrological, and meteorological in-situ observations and gridded data products, to investigate the hydrology of headwater catchments in HMA. For this purpose, we collected in-situ data in one seasonally dry, westerly controlled catchment in the Pamir mountains (Kyzylsu) with a winter/spring accumulation regime, one monsoonal, summer-accumulation catchment in the Nepalese Himalaya (Langtang), and one monsoonal catchment on the Southeastern Tibetan Plateau (24K) with an intense spring/summer accumulation regime. We implemented new components in the model, and updated existing ones: an energy-balance scheme for ice melting under supraglacial debris, a multi-layer snow pack and advanced schemes for precipitation partitioning and albedo. We use this modelling framework and new in-situ data to address three primary research gaps: First, we aim to understand the energy-balance controls on glacier ablation at multiple sites in the Central and Eastern Himalaya. We conduct point scale simulations at seven automatic weather stations installed in glacier ablation zones. Constrained by local measurements, we use the simulations to understand the impact of monsoonal conditions on the surface energy balance, ice ablation, snow accumulation and melting, evaporation and sublimation. On debris-covered glaciers, variations in the radiative budget are offset by changes in turbulent heat fluxes, leading to minimal or no changes in the melting of ice beneath debris, between the pre-monsoon and peak monsoon seasons. In contrast, clean-ice melt is directly and primarily influenced by variations in the radiative budget while turbulent fluxes remain small. Over thin debris however, turbulent fluxes act to enhance melting during the monsoon. Second, applying the model in a distributed way to the three main study sites, we focus on the current partitioning of water fluxes and in particular on the role of evaporative fluxes, including all- surface evaporation, transpiration and sublimation. We use downscaled reanalysis data, bias- corrected against weather station data, as meteorological forcing for the model. Evaporative fluxes account for 28%, 19% and 13% of the water losses from Kyzylsu, Langtang and 24K, respectively. Sublimation and evapotranspiration are both most important in the water balance of Kyzylsu, with sublimation returning 15% of snowfalls to the atmosphere, and evapotranspiration corresponding to 76% of total rainfall, while the largest evapotranspiration flux occurs at the wettest site, 24K with 413 mm yr-1. Compensatory effects in the runoff generation between evaporative fluxes and ice melt runoff under warmer conditions are indicated, which motivates further experimentation under altered meteorological forcings. Third, we investigate the impact of future climate warming on energy and mass fluxes and the potential hydrological response of the three catchments. We re-run the simulations under experimental conditions, representative of mid-21st century temperature increase under the shared socio-economic pathways SSP2-4.5 (W4.5) and SSP5-8.5 (W8.5). Warming increases the annual amount of water transferred through the catchment, but compensations between ice melt and evapotranspiration result in only moderate or no changes in runoff, depending on the hydroclimatic context: Changes in the water yield range from -1% in 24K under W4.5 to +14% in Kyzylsu, under W8.5. While the runoff sensitivity is higher in Kyzylsu, the sensitivity of glacier mass balances is greater at the monsoonal sites: shifts from solid to liquid precipitation are most pronounced in 24K, where up to 76% of additional ice melt (+130%) under W8.5, can be attributed to losses of glacier-protecting monsoon-season snow cover. Increases in vegetation productivity together with the pronounced increases in evapotranspiration manifest in the lengthening of the growing season, ranging between +9 days in Langtang (+26% evapotranspiration, W4.5) and +31 days in 24K (+ 50% evapotranspiration, W8.5). Sublimation shows a relatively insensitive and mixed response across sites, with seasonally varying increases and decreases, determined by the local temperature and vapour regimes. In this thesis, we use a physically-based model consistently, to investigate the hydrological functioning of headwater catchments across the spectrum of HMA climates, and assess their sensitivity to warming. Integrating in-situ observations and remote sensing, our modelling framework allows us to study in a fully distributed way energy and mass fluxes in mountain catchments. This is, to our knowledge, the first time that these methods are applied to glacierized catchments in HMA, in order to disentangle the processes of runoff generation. This thesis clarifies the role of the cryosphere and evaporative fluxes in these environments, and in particular, the roles of debris-covered glaciers, evapotranspiration, sublimation and runoff compensations under warming. The presented work improves the scientific basis for the modelling of mountain- water-fed river basins, which will aid the reduction of uncertainty in future water resources projections.