Controls on the relatively slow thinning rate of a debris-covered glacier in the Karakoram over the past 20 years: evidence from mass and energy budget modelling of Batura Glacier

Abstract. The influence of supraglacial debris cover on glacier dynamics in the Karakoram is noteworthy. However, understanding of how debris cover affects the seasonal and long-term variations in glacier mass balance through alterations in the glacier's energy budget is incomplete. The present study applied an energy-mass balance model coupling heat conduction within debris layers on debris-covered Batura Glacier in Hunza valley, to demonstrate the influence of debris cover on glacial surface energy and mass exchanges during 2000–2020. The mass balance of Batura Glacier is estimated to be -0.262 ± 0.561 m w.e. yr^-1, with debris cover reduced 45 % of the negative mass balance. Due to the presence of debris cover, a significant portion of incoming energy is utilized for heating debris, leading to a large energy emission to atmosphere via thermal radiation and turbulent sensible heat. This, in turn, reducing the melt latent heat at the glacier surface. We found that the mass balance exhibits a pronounced arch-shaped structure along the elevation gradient, which primarily attributes to the distribution of debris thickness and the impact of debris cover on the energy budget within various elevation zones. Through a comprehensive analysis of the energy transfer within each debris layer, we have demonstrated that the primary impact of debris cover lies in its ability to modify the energy flux reaching the surface of the glacier. Thicker debris cover results in a smaller temperature contrast between debris layers and the ice-contact zone, consequently reducing heat conduction. Over the past two decades, Batura Glacier has maintained a relatively small negative mass balance, owing to the protective effect of debris cover. The glacier exhibits a tendency towards a smaller negative mass balance, with diminishing dominance of ablation in areas with thin debris cover and debris-free parts of the ablation area.