Monitoring the surface mass balance and dynamic evolution of a partially debris-covered glacier using multi-temporal visual and thermal UAV imagery

The rapid retreat of glaciers in the last decades highly impacts the regional hydrological cycle and water supply and increases the potential for cryosphere-related hazards. Expansion of debris-cover and changes in glacier dynamics are indicators of mass loss. While debris-cover has an insulating effect, ice-cliffs exhibit fast backwasting and therefore represent melt hotspots in the debris-covered areas. Their relative contribution has only been quantified for few glaciers and for global and regional glacier models they have therefore not yet been parameterized. The number and distribution of moulins impacts the (seasonal) glacier dynamics. Monitoring alpine glaciers is essential to quantify ongoing mass loss and project their future evolution and water supply. However, the small-scale characteristics and dynamic nature of these features cannot be captured by traditional approaches such as satellite remote sensing. Mass loss can be quantified reliably with ablation stakes at selected locations, but to investigate spatio-temporal variations across the glacier, a distributed approach is needed.

With repeated UAV photogrammetry, the  annual variability of elevation change, velocity and mass balance can be studied. On the debris-covered area, thermal imaging is employed for spatial surface temperature and debris thickness mapping. In this study, we use a structure-from-motion and multi-view stereo approach to create high-resolution orthophotos and DEMs from visual imagery of the partially debris-covered tongue of Kanderfirn in the Swiss Alps. UAV surveys have been conducted on a seasonal to annual basis since 2017 and cover the glacier tongue, which features debris-free and debris-covered surfaces. After coregistration of image pairs, we derive annual surface velocity and elevation change maps. Based on additional ice thickness information, the mass continuity method and an inversion technique is applied to investigate the glacier's dynamical evolution. The result is a SMB time-series that portrays surface processes with and without debris-cover. Additionally, the surface roughness and brightness are examined. The debris thickness maps provide a basis for estimating subdebris melt rates. Our UAV-based monitoring approach provides detailed insights into characteristic mass balance patterns and dynamic processes of (partially) debris-covered glaciers that impact their evolution and response to climatic changes. It will help with parameterizing glacier models for future mass loss projections on alpine and debris-covered glaciers.