Influence of the debris cover on the mass balance of glaciers in High Mountain Asia

Debris-covered glaciers of High Mountain Asia (HMA) are major contributors of the water cycle. However, the influence of the debris cover on the ablation processes remain poorly constrained at both regional and glacier scales.

At the scale of HMA, we combined the 2000-2016 glacier-wide mass balance of more than 6 500 glaciers with maps of glacier surface state (i.e. maps of debris coverage). We found that the percentage of debris-cover was a weak predictor of the glacier-wide mass balance. Maybe counter intuitively, we found that debris-covered glaciers have significantly more negative mass balances for four regions out of twelve, a significantly less negative mass balance for one region and non-significantly different mass balances for the remaining seven regions.

Beyond this statistical analysis at regional scale, studies at the glacier scale are required to better constrain the contribution of the different ablation processes on debris-covered tongues. For the debris-covered tongue of Changri Nup Glacier, Everest region, Nepal, we used a combination of very high resolution DEMs derived from Pléiades images and an unmanned aerial vehicle and found that ice cliffs contributed to 23 ± 5 % of the total net ablation of the tongue, over two contrasted years, while occupying only 7 to 8 % of its area. Although they are hot spot of melt, cliffs cannot alone explain the debris cover anomaly, i.e. the fact that debris free and debris covered tongues have similar thinning rates. This anomaly is probably due to smaller emergence velocity over debris-covered tongues than over debris-free tongues.