Surface displacement of a Himalayan debris-covered glacier derived from automated cross-correlations using imagery acquired with unmanned aerial vehicles

Abstract

Glacial melt water is a source of drinking water for the 1.3 billion people living in downstream area of the Hindu Kush Himalayan region. Himalayan glaciers function as a fresh water buffer as they constantly release water, making it an important resource for people during the dry season. In this research, surface displacements are studied in order to understand more of the melting behaviour of debris-covered glaciers. Understanding these processes will help predict future water levels in rivers. In situ, glaciological measurements are limited due to the inaccessibility of the glacier and need to be interpolated to assess the full surface dynamics. In this study, imagery of the Lirung glacier in Nepal is used that is derived from an unmanned aerial vehicle (UAV). The dataset is ortho-rectified and covers almost a full year including a summer and a winter season. The flight data are May 2013, October 2013 and May 2014. Based on a Structure from Motion process digital elevation models and ortho-rectified mosaic images are created. Some of the glaciers in the Himalaya are covered with debris. This allows tracking the rocks on top of the glacier, where rocks move upon with the ice. The frequency cross-correlation of Cosi-Corr is using the phase in the Fourier domain to track such debris. The output of this correlation is a continuous raster map containing the surface displacement of the glacier. Fine spatial and general seasonal differences can be found in the surface displacement. During the summer, the maximum average displacement is 6 m a-1 while the winter displacement is 2.5 m a-1 at the upper part of the tongue. The lower part is nearly stagnant throughout the year. It is very likely that basal sliding is the dominating process during the higher summer. It is hypothesized that the increment of both temperature and rainfall during the summer causes a lubrication of the bedrock. Other process on the glacier is a yet unknown depression, which can be found to the north of the terminus and emergence velocities that occur near the bend of the glacier. The high-resolution imagery and elevation models of UAVs have high potential to map surface displacements of debris-cover glaciers and help to understand general glacier dynamics on a small spatial scale