Estimating ice content of rock glaciers in Khumbu Valley, Nepal, using a surface-velocity-constrained model

Abstract:

Rock glaciers (RG), are an integral component of the alpine permafrost realm, contain significant amount of ground ice and serve as important freshwater resources as mountain glaciers melt in response to climate warming. Both RG and glaciers abound the Nepalese Himalaya, and many glaciers are transitioning into RG. In this study, we develop a surface-velocity-constrained model to infer ice content of RG situated in Khumbu valley, eastern Nepal.

We adopt a rheological model based on adaptations of Glen’s flow law and assume a homogeneous two-layer structure for RG that consists of an ice-free active layer and an ice-rich permafrost core. Ice content affects rheological properties of the permafrost core and thus influences RG creep. To determine empirical relationships between RG creep and ice content, we calibrate the model using a comprehensive existing RG dataset. We apply several regression models to depict possible relationships between effective viscosity and ice fraction. We obtain three candidate parameterization schemes and validate the model using observational data; providing two reasonable candidate parameterization schemes.

40 interferograms generated from ALOS-1 PALSAR and ALOS-2 PALSAR-2 images (2006-2020) were used to measure RG movements in our study area. We identified active parts of five RG based on InSAR-derived downslope velocities and quantified their geometric parameters using Google Earth and SRTM DEM. Active layer thickness was determined from the ESA Permafrost CCI Product. We used downslope velocities of the active part to model ice content.

The two parameterization schemes produce similar results (+/-5%). The inferred ice fraction of the five RG in Khumbu Valley ranges from 80-90% (water volume equivalents 1.6 to 7.1 million m³). Considering previous estimations and extrapolating from our findings in Khumbu Valley, the total amount of water stored in RG could be ~10 billion m³ over the Nepalese Himalaya.

The model adopted in our study provides an effective estimation of RG water storage. Combined with the observations of RG kinematics, it is a promising approach that can be applied over large permafrost regions, providing a comprehensive assessment of the significance of RG as vital freshwater resources in a warming future.