Mapping debris thickness on alpine glaciers using UAV thermography and photogrammetry

Supraglacial debris covers the tongue of many mountain glaciers. In the course of ongoing climate change and the rapid melting of glaciers, debris extent and thickness will continue to increase. The thickness and other inherent properties of the debris layer control sub-debris melt rates and influence how glaciers respond to climate change. It is therefore essential to consider the impact of supraglacial debris on ablation in glacier surface mass balance models and glacier evolution models. However, this requires detailed knowledge on the debris thickness distribution. As debris thickness is spatially very variable, it remains a challenge to map debris thickness across the entire ablation zone of a glacier. Here we present the preliminary results of a feasibility study on the Kanderfirn in the Swiss Alps, where we deployed an Unoccupied Aerial Vehicle (UAV) with a visible and thermal infrared camera to map and analyse spatial variations in debris surface temperature, debris thickness, and sub-debris melt rates. Two independent approaches originally developed for satellite data were tested and compared to map debris thickness in high resolution. First, we used the statistical relationship between spatial UAV observations and in-situ point measurements (mapped surface temperature vs. measured debris thickness) to derive spatial debris thickness variations from mapped surface temperature variations. Second, we calculated distributed sub-debris melt rates from UAV-based elevation-change maps and quantified debris thickness through the inversion of a sub-debris ice melt model. Both methods deliver promising results. Despite the remaining challenges, the results emphasise the potential of UAVs equipped with visible and thermal infrared cameras for glacier-wide debris thickness mapping.