Debris-covered ice in the high-mountain hydrological cycle: From field scale processes to catchment scale interactions

A significant amount of mountain glacier ice on the globe - between 10 and 30% of the total ice areas and volume - is covered in debris material. There are large differences between mountain chains and the debris extent is generally larger for high-mountain Asia. Here, extensive ablation tongues at relatively low elevations are common and especially favorable for continuous and oftentimes thick debris cover.

In this thesis, I present a number of field investigations from a single catchment in the Central Himalaya, where debris cover is especially prevalent and a number of measurement setups over recent years are available. This allows us to investigate processes in detail and finally evaluate the significance of debris cover on the catchment scale with regard to melt water production.

Investigations of the surface boundary layer of a debris-covered glacier show that the rocks heat up well above the air temperature during the day. During the night, the surface cools significantly. We also find that the lapse rate generally applied on off-glacier terrain needs to be adapted over a debris-covered glacier as it is generally warmer than the surrounding environment. The steep and fast changing gradient between the surface and air temperature drives a complex turbulent process that contributes to a general cooling of the surface.

While the debris surface itself is already very hummocky, some features appearing on in make it even more complex. At numerous locations, ice breaks through the surface and cliffs that expose the otherwise insulated ice mass to a rapid melting process appear. Such ice cliffs become strong hotspots for local melt and increase the disintegration of debris-covered tongues. However, these cliffs cover only 1 to 4% of all debris-covered tongues in the catchment, and hence their overall contribution to the water budget remains limited.

Incorporating all our measurements from multiple years on the glacier surface allows us to drive a full energy balance model, allowing us to reproduce the mass loss observed from geodetic mass balance from repeat UAV flights. This is encouraging, as it gives us some confidence in the computation of energy fluxes and energy transfer through the debris. Solar radiation remains the most important driver in space, but other fluxes that contribute to radiative cooling are also essential to reproduce melt rates accurately. What seems most important is, however, that we better understand the composition of the debris pack. It holds moisture that influences its energy transfer capabilities and retains melt water before it is released to actual streamflow. This has a potential modulating effect on the catchment water balance.

This thesis highlights a few hotspots that are important to understand how debris-covered glaciers melt and how important these processes are to understand their role in the high mountain water balance. Future research will need to investigate how these processes develop with a changing climate and how their importance shifts as the overall area and volume of ice changes.