Modelling rock glacier ice content based on InSAR-derived velocity, Khumbu and Lhotse valleys, Nepal

Active rock glaciers are viscous flow features embodying ice-rich permafrost and other ice masses. They contain significant amounts of ground ice and serve as potential freshwater reservoirs as mountain glaciers melt in response to climate warming. However, current knowledge about ice content in rock glaciers has been acquired mainly from in situ investigations in limited study areas, which hinders a comprehensive understanding of ice storage in rock glaciers situated in remote mountains over local to regional scales. This study proposes a novel approach for assessing the hydrological value of rock glaciers in a more quantitative way and presents exploratory results focusing on a small region. We develop an empirical rheological model to infer ice content of rock glaciers using readily available input data, including rock glacier planar shape, surface slope angle, active layer thickness, and surface velocity. The model is calibrated and validated using observational data from the Chilean Andes and the Swiss Alps. We apply the model to five rock glaciers in the Khumbu and Lhotse valleys, northeastern Nepal. The velocity constraints applied to the model are derived from interferometric synthetic aperture radar (InSAR) measurements. The volume of rock glacier is estimated based on an existing scaling approach. The inferred volumetric ice fraction in the Khumbu and Lhotse valleys ranges from 70 +/- 8% to 74 +/- 8 %, and the water volume equivalents lie between 1. 4 +/- 0. 2 and 5. 9 +/- 0. 6 +/- 10(6) m(3) for the coherently moving parts of individual rock glaciers. Due to the accessibility of the model inputs, our approach is applicable to permafrost regions where observational data are lacking, which is valuable for estimating the water storage potential of rock glaciers in remote areas.

Automated summary

This study proposes a novel approach for assessing the hydrological value of rock glaciers and presents results from a small region. They develop an empirical rheological model using readily available input data to infer ice content of rock glaciers. The model is validated using observational data and applied to five rock glaciers in Nepal. The inferred volumetric ice fraction ranges from 70 +/- 8% to 74 +/- 8%, with water volume equivalents between 1. 4 +/- 0. 2 and 5. 9 +/- 0. 6 +/- 10(6) m(3) for the coherently moving parts of individual rock glaciers.