Assessing controls on mass budget and surface velocity variations of glaciers in Western Himalaya

Abstract

This study analyses spatially resolved estimates of mass budget and surface velocity of glaciers in the Zanskar Basin of Western Himalaya in the context of varying debris cover, glacier hypsometry and orientation. The regional glacier mass budget for the period of 1999-2014 is -0.38 ± 0.09 m w.e./a. Individual mass budgets of 10 major glaciers in the study area varied between -0.13 ± 0.07 and -0.66 ± 0.09 m w.e./a. Elevation changes on debris-covered ice are considerably less negative than over clean ice. At the same time, glaciers having >20% of their area covered by debris have more negative glacier-wide mass budgets than those with <20% debris cover. This paradox is likely explained by the comparatively larger ablation area of extensively debris-covered glaciers compared to clean-ice glaciers, as indicated by hypsometric analysis. Additionally, surface velocities computed for the 2013-14 period reveal near stagnant debris-covered snouts but dynamically active main trunks, with maximum recorded velocity of individual glaciers ranging between ~50 ± 5.58 and ~90 ± 5.58 m/a. The stagnant debris-covered extent, which varies from glacier-to-glacier, are also characterized by ice cliffs and melt ponds that appreciably increase the overall surface melting of debris-covered areas.

Figure 1

False Colour Composite of Landsat image acquired on 17^th August, 1999 showing the boundaries of glaciers (black polygons) in the study area. Glaciers with an area greater than 10 km² are enumerated and highlighted. The location of topographic divide is indicated by the dashed red line. Insets show the location of the study site in context of the physical map of India and the RGI IDs of the highlighted glaciers. (The map was generated in ArcGis 9.3 Master Lab Kit and post processed using Adobe Illustrator Creative Cloud).

Figure 2

Choropleth map of glacier polygons depicting debris cover percentage with respect to total area overlaid on a hillshade map generated from SRTM DEM. Overlaid on the choropleth map is the spatial extent of debris cover on the glaciers within the study area (light pink polygons). (The maps were generated in ArcGis 9.3 Master Lab Kit and postprocessed in Adobe Illustrator Creative Cloud).

Figure 3

Spatial distribution of elevation change rate (dh/dt) (a) and surface velocity (b) overlaid on a hillshade map generated from SRTM DEM. The location of profiles to study A-A’ to P-P’ are depicted in (b). (The maps were generated in Python (https://www.python.org/), Matplotlib based image viewer “imviewer” (https://github.com/dshean/imview) and labelling of profile lines was done in Adobe Illustrator Creative Cloud).

Figure 4

Hypsometric elevation change plots for the (a) entire area and for (b–k) glaciers 1–10 respectively. The blue and grey columns represent total area of clean ice/snow and debris-covered ice respectively, in the corresponding bins. The black line is the median elevation change rate (dh/dt) in each 50 m elevation bin. The red (blue) line is the median elevation change rate for the debris covered ice (clean ice) in the corresponding 50 m elevation bins. The light pink shading along the profile line represents uncertainty in thickness change obtained from equation 5 (Supplementary Section 3). The green line indicates ‘zero’ dh/dt. (The plots were generated using Python 2.7 (https://www.python.org/), Pandas and Matplotlib libraries. The plots were tiled together using Adobe Illustrator Creative Cloud).

Figure 5

Surface velocity variation along profiles A-A′ to P-P′ for the 2013–14. The light colour shading represents the error in surface velocity estimates (68.3^rd percentile, Supplementary Table 2). The profiles have been smoothed using a rolling median filter with a window size of 5 observations (~140 m). Location of the profiles are shown in Fig. 3b. (The plots were generated using Python 2.7 (https://www.python.org/), Pandas and Matplotlib libraries).