The Seasonal Evolution of the Supraglacial Hydrologic Network at Humboldt Glacier, Northwest Greenland.

<p>Supraglacial channels and lakes across the Greenland Ice Sheet form a key component in the routing and storage of surface meltwater during the summer melt season, yet to-date, their temporal evolution remains poorly mapped and quantified across the Greenland Ice Sheet. We produce the first high-resolution record of the seasonal supraglacial drainage network (channels and lakes) at Humboldt Glacier, northwest Greenland, spanning the melt seasons of 2016 to 2020. Using an automatic detection algorithm applied to Sentinel-2 imagery, which utilises a satellite-derived normalised difference water index (NDWI) and morphological operators, supraglacial channels and lakes were effectively mapped and quantified across the study region, including meltwater area, meltwater area fraction, channel drainage density (Dd), lake density and comparisons of satellite-derived drainage mapping to regional climate model (MAR) runoff.  </p><p>Results reveal the migratory behaviour of the supraglacial drainage network at Humboldt Glacier as surface runoff increases and the snowline recedes through the melt season. In the high melt years of 2016, 2019 and 2020, Dd commonly peaks in early-July as the ‘inefficient’ system (with hydrologically-connected saturated slush zones) advances inland, before decreasing as the system reaches maximum ‘efficiency’ and inland extent (late-July) before waning as the melt season ends. During these years, this annually-expansive network consistently reaches elevations of ~1450 m (~80km inland), with surface meltwater area covering between 7.2 - 11.6% of the area <1500 m. Retardation to this behaviour is notably observed within the low melt years of 2017 and 2018, when Dd and melt area do not peak until August.</p><p>Supraglacial lakes are also widely present within the drainage system, with their number and size also shown to peak with Dd before decreasing as the melt season progresses, attributed to increased moulin connectivity and subsequent lake drainage events. The combined meltwater area fraction (%) of both supraglacial channels and lakes is strongly correlated with MAR modelled surface runoff.</p><p>This work provides important advancements in the mapping, quantification and understanding of the expansive yet ephemeral supraglacial hydrologic system that exists across northwest Greenland Ice Sheet and its influence on the routing and evacuation of meltwater from these systems and beyond.</p>