Deriving the Østrem curve to quantify supraglacial debris-related melt-altering effects on the Djankuat Glacier, Caucasus, Russian Federation

We have derived the glacier-specific Østrem curve to quantify the influence of a supraglacial debris cover on the mass and surface energy balance components of the Djankuat Glacier, a northwest-facing and partly debris-covered temperate valley glacier in the Caucasus region, which has been selected as a ‘reference glacier’ by the WGMS. A 2D energy balance model, in combination with meteorological data from automatic weather stations and ERA5-Land reanalysis data, are used to assess the melt-altering effect of supraglacial debris on the overall glacier runoff during 1 complete balance year. The main results show that both the surface energy balance and mass balance fluxes are modified significantly due to the presence of debris on the glacier surface. For very thin debris, a slight relative melt-enhancement occurs due to a decreased surface albedo. If debris, however, further thickens, the insulating effect becomes dominant and reduces the melt and runoff of the underlying ice significantly, as thermal conduction becomes the dominant process to induce ice melt beneath such thick debris layers. The above-mentioned effects are modelled to be increasingly pronounced with an increasing thickness of the superimposed supraglacial debris cover, and can be of great importance with respect to future glacio-hydrologic regimes and glacio-geomorphological processes.