Glacio-hydrological modelling of the Aletschgletscher catchment with evolving supraglacial debris since 1900

Supraglacial debris partially covers more than 40% of Earth’s glaciers (excluding Antarctica) and, where present, acts as a major controlling factor for glacier melt. It can enhance melt when the layer is thin by reducing surface albedo (increasing the net radiative flux) and it reduces melt when it is thick by insulating the ice from the atmosphere (dampening the conductive heat flux). Accurately simulating the spatio-temporal evolution of debris over a glacier is still a challenge, because of diverse debris sources and mechanisms of transport on and within the ice that affect long-term debris cover evolution. Previous studies have investigated the evolution of debris from geomorphological and historical data or from debris-tracking ice dynamics modelling. These approaches, however, often do not capture the transient ice melt-debris thickness relationship under a changing climate. To date, no attempt has been made to couple the century-scale evolution of debris extent and thickness with a full surface energy-balance model to evaluate debris-melt feedbacks and assess their impacts on catchment-scale runoff generation.

We apply a distributed land surface-energy balance model to simulate the glacier evolution and surface hydrology of the Aletschgletscher catchment (including glaciated and ice-free areas) in the Swiss Alps from 1900 to 2023. We incorporate the evolution of supraglacial debris, constrained using historical topographic maps and recent debris thickness measurements. We evaluate the impact that time-evolving debris-cover extent and thickness has on the glacier mass balance and hydrology. We also compare results for Grosser Aletsch and Oberaletsch to demonstrate that increasing catchment debris cover influences catchment response to climate and glacier change.

Our results show that the contribution of sub-debris melt to runoff varied substantially over the last century. The catchment-wide sub-debris melt contribution increased until ~1945, then declined until today. However, trends differed between subcatchments. In the sparsely debris-covered Grosser Aletschgletscher, sub-debris melt reached its peak around 1940 (25–30% of total ice melt) before decreasing until present (same pattern as the entire catchment). In contrast, the highly debris-covered Oberaletsch shows a continuous increase, with recent values reaching 40–50% without a clear peak. The catchment trend is primarily explained by the evolution of debris cover on Grosser Aletschgletscher, combined with climate factors. Periods of expanding debris extent (~1910–1930 and ~1940–1965) initially increased the sub-debris melt contribution by enlarging the area where melt is enhanced by thin debris. Once debris area stabilised and average thickness increased (post-~1965), the stronger insulating effect reduced its relative contribution. An early rapid thinning of debris (~1910–1920) further enhanced early melt. Furthermore, climate warming has raised the 0°C isotherm, increasing melt in debris-free areas and thereby relatively reducing the proportion of melt occurring under debris. Therefore, the sub-debris melt contribution is affected by the combination between debris evolution and climate change. In a world with darkening glaciers, it is essential for glacio-hydrological models to consider evolving debris extent and thickness, and to incorporate feedbacks related to these changes, which substantially impact the surface energy balance, in order to accurately project future runoff from these catchments.