Southwest-facing slopes control the formation of debris-covered glaciers in the Bhutan Himalaya

H Nagai,T Nuimura,A Sakai,K Fujita

doi:10.5194/tc-7-1303-2013

Abstract

Abstract. To understand the formation conditions of debris-covered glaciers, we examined the dimension and shape of debris-covered areas and potential debris-supply (PDS) slopes of 213 glaciers in the Bhutan Himalaya. This was undertaken using satellite images with 2.5 m spatial resolution for manual delineation of debris-covered areas and PDS slopes. The most significant correlation exists between surface area of southwest-facing PDS slopes and debris-covered area. This result suggests that the southwest-facing PDS slopes supply the largest quantity of debris mantle. The shape of debris-covered areas is also an important variable, quantitatively defined using a geometric index. Elongate or stripe-like debris-covered areas on north-flowing glaciers are common throughout the Bhutan Himalaya. In contrast, south-flowing glaciers have large ablation zones, entirely covered by debris. Our findings suggest that this difference is caused by effective diurnal freeze–thaw cycles rather than seasonal freeze–thaw cycles, permafrost degradation, or snow avalanches. In terms of geographic setting, local topography also contributes to glacier debris supply and the proportion of debris cover on the studied glaciers is suppressed by the arid Tibetan climate, whereas the north-to-south asymmetric topography of the Bhutan Himalaya has less influence on the proportion of debris cover.

Highlights

Glaciers in the Himalayas and other mountain regions are valuable as seasonally sustainable water reservoirs for downstream agricultural regions (e.g. Immerzeel et al, 2010; Kaser et al, 2010), and they are recognized as sensitive indicators of climate change (e.g. Kaser et al, 2006; Bolch et al, 2012)
These findings, together with those discussed above from other mountain regions, suggest that debris mantles are primarily produced by diurnal freeze–thaw cycles, which are most active on southwest-facing potential debris-supply (PDS) slopes
We have found that the size and shape of debris-covered areas on north-flowing glaciers are linked to the spatial distribution of south-facing PDS slopes, and that the diurnal freeze–thaw cycle is the main source of debris in the Bhutan Himalaya

Summary

Introduction

Glaciers in the Himalayas and other mountain regions are valuable as seasonally sustainable water reservoirs for downstream agricultural regions (e.g. Immerzeel et al, 2010; Kaser et al, 2010), and they are recognized as sensitive indicators of climate change (e.g. Kaser et al, 2006; Bolch et al, 2012). Many of the larger Himalayan glaciers have ablation zones covered by a mantle of unconsolidated sediments, ranging in size from dust to boulders. These are derived from rockfalls and avalanches on mountain slopes, or from subglacial erosion at the ice–bedrock interface Significant lowering rates on debris-covered surfaces have been observed during other remote sensing studies (Berthier et al, 2007; Bolch et al, 2011; Nuimura et al, 2012) and in situ measurements (Nuimura et al, 2011). To understand regionally heterogeneous shrinkage of glaciers throughout the Himalayas (Fujita and Nuimura, 2011), it is necessary to recognize and understand the coexistence of these aspects on debris cover

Results

Discussion

Conclusion