Five years of change of two debris-covered glaciers monitored by unmanned aerial system

Glaciers in the Himalaya are often covered by debris, which affects melt rates and causes high spatial heterogeneity in surface elevation changes. In the past decade years, unmanned aerial system (UAS) data have been shown to be indispensable in mapping and monitoring this type of glacier and catalysed new research focusing on process understanding of ablation processes at unprecedented detail. In this study, we present the results of a five-year biannual UAS monitoring campaign in the Langtang Catchment in the Nepalese Himalaya in which we surveyed Lirung Glacier (9 surveys, 2013–2018) and Langtang Glacier (7 surveys, 2014–2018). Optical UAS imagery was processed into high-resolution image mosaics and elevation data, accurately positioned using ground control data and co-registered using tie points. Derived glacier surface velocities and modelled bed topography were used to perform fully distributed corrections for ice flow and emergence velocity. The resulting flow-corrected surface changes of the glacier were analysed and used to evaluate supraglacial ice cliff evolution, glacier retreat, and melt. Results show that on average the surveyed areas of Lirung Glacier and Langtang Glacier had comparable surface velocities ranging from about 1.0 to 3.5 m a^-1 and a melt of −1.40 ± 0.05 m a^-1 and -1.22 ± 0.08 m a^-1, respectively, with both glaciers having strong spatial heterogeneity and temporal variability. Supraglacial ice cliffs on both glaciers exhibit variable (rates of) change in morphology, largely irrespective of aspect. The terminus of Lirung Glacier, which is characterized by a debris-free ice cliff, experienced very fast retreat of 41 m a^-1. The five-year time series of UAS data presented in this study has provided unique insights in surface changes of debris-covered glaciers. UAS surveys are and continue to remain highly valuable tool to study such glaciers, with potential still to be unlocked.