Spatial and Temporal Variability of the Surface Mass Balance of Debris-Covered Glacier Tongues

Himalayan glaciers often have a supraglacial debris layer that causes spatial heterogeneity in surface changes and mostly reduces melt. Despite this, debris-covered glaciers lose mass at similar rates to debris-free glaciers. Understanding related processes is crucial for assessing impacts on water resources and hazards. This study assesses the spatiotemporal variability of surface changes in debris-covered glaciers by analyzing (parts of) two distinctly different Himalayan glaciers, Lirung Glacier and Langtang Glacier, over 2013-2018 using high-resolution uncrewed aerial system data. Derived multi-annual surface mass balances across the debris-covered tongues had reduced spatial variability compared to (sub) annual surface changes, likely due to topographic feedbacks in surface processes, while at supraglacial ice cliffs both melt amplification and dampening was observed, with an overall net negative effect on mass balance. Increased melting due to higher cliff abundance contributed to the equal surface mass balances between the tongues, despite differences in elevation and air temperatures. Additionally, cliff abundance increased sensitivity to temperature fluctuations, leading to greater surface mass balance variability. In response to the Gorkha Earthquake of April 2015, Langtang Glacier exhibited higher sensitivity of the ice influx, suggesting it has potential sensitivity to climate change. In contrast to Langtang Glacier, which showed no retreat, Lirung Glacier experienced an anomalously fast retreat of a terminal ice cliff, substantially impacting its mass balance. The spatiotemporal response revealed by this study reiterates the significant role of ice cliffs in debris-covered glacier surface mass balance, emphasizing the need for their accurate representation in impact studies.