Abstract
Abstract.Debris-covered glaciers play an important role in the high-altitude water cycle in the Himalaya, yet their dynamics are poorly understood, partly because of the difficult fieldwork conditions. In this study we therefore deploy an unmanned aerial vehicle (UAV) three times (May 2013, October 2013 and May 2014) over the debris-covered Lirung Glacier in Nepal. The acquired data are processed into orthomosaics and elevation models by a Structure from Motion workflow, and seasonal surface velocity is derived using frequency cross-correlation. In order to obtain optimal surface velocity products, the effects of different input data and correlator configurations are evaluated, which reveals that the orthomosaic as input paired with moderate correlator settings provides the best results. The glacier has considerable spatial and seasonal differences in surface velocity, with maximum summer and winter velocities 6 and 2.5 m a-1, respectively, in the upper part of the tongue, while the lower part is nearly stagnant. It is hypothesized that the higher velocities during summer are caused by basal sliding due to increased lubrication of the bed. We conclude that UAVs have great potential to quantify seasonal and annual variations in flow and can help to further our understanding of debris-covered glaciers.
Highlights
Around 10% of the Himalayan glacierized area is debriscovered (Bolch and others, 2012) and the debris-covered tongues are generally located at the lowest elevation
The accuracy of the surface velocity products is not affected by the true geodetic accuracy of the data, which is indicated by the ground-control points (GCPs) errors for October 2013
The vertical errors, do not contribute much to the accuracy of the surface velocity product determined by feature tracking, as they have little to no influence on the orthomosaic, hillshade and edge-detected digital elevation models (DEMs)
Summary
Around 10% of the Himalayan glacierized area is debriscovered (Bolch and others, 2012) and the debris-covered tongues are generally located at the lowest elevation. Most debris-covered tongues exhibit slower rates of retreat than debris-free glaciers, but they thin at substantial rates (Scherler and others, 2011). The debris, when thicker than a few centimetres, should insulate the ice from melt (Östrem, 1959). Recent work suggests that the debris-covered tongues lose mass at the same rates as debris-free glaciers (Kääb and others, 2012; Gardelle and others, 2013; Pellicciotti and others, 2015). The underlying reason may be the presence of supraglacial lakes and ice cliffs that accelerate melt significantly (Sakai and others, 1998; Benn and others, 2012; Immerzeel and others, 2014). About the behaviour and response of debris-covered glaciers, as they are generally inaccessible and the spatial and temporal resolution of satellite remotesensing products limits our ability to understand the processes governing thinning
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