Supraglacial lakes on debris-covered glaciers: high resolution observations of subseasonal lake expansion, drainage, and the controlling processes

Supraglacial lakes and ponds are important features on debris-covered glaciers (DCGs) that can increase ablation rates through increased radiation absorption, ice cliff calving, and efficient transfer of heat to the englacial and subglacial environments. Further, they present a potential hazard to downstream communities through glacial lake outburst floods, particularly in cases where smaller ponds coalesce into large lakes at the glacier terminus. The rugged topography of DCGs has limited the feasibility of in situ investigations of supraglacial ponds to relatively small spatial and temporal extents, while their rapid temporal changes and wide range of spatial scales has limited the ability of remote sensing techniques to capture the full variability in their characteristics. In this study we present observations of the evolution of supraglacial ponds in unprecedented spatial and temporal resolution using PlanetScope satellite imagery. A fully automated workflow was used to map supraglacial pond extent on eight DCGs in 1500 PlanetScope images over the 2017-2022 period. A strong seasonality in total ponded area was observed, with an annual minimum at the end of the monsoon season (~September), followed by a gradual increase throughout the winter and early spring before reaching their maximum extent at the beginning of the monsoon season (~June). This seasonality shows that caution should be used when interpreting long-term trends from intermittent observations, as the seasonal variability is larger than any long-term trend observed. The observed September-to-June increase in ponded area is driven primarily by the appearance and expansion of small ponds and supraglacial streams at higher elevations, while ponds near the glacier terminus tended to have smaller seasonal variations. These findings agree with and expand upon previous work that suggests pond formation and size are controlled by the interplay between surface gradients, glacier velocity patterns, and seasonal climate. Further, we show that supraglacial ponds are associated with areas of increased glacier thinning rate, with ponded areas thinning up to 1 m/yr faster than nearby pond-free areas. These observations present an important step forward in quantifying the impact that supraglacial lakes have on the long term evolution of DCGs.