Hydrologic constraints on supraglacial canyon evolution in Greenland using high-resolution DEMs, ICESat-2 altimetry, and a new numeric model

Abstract:

Supraglacial streams modulate the Greenland Ice Sheet’s mass balance by delivering meltwater to the englacial, subglacial, and proglacial environment. Large, deeply-incised canyons likely convey high meltwater fluxes across the ice sheet, yet they remain an understudied feature within the supraglacial landscape. Key unknowns include the conditions required for canyon development, meltwater fluxes through canyons, and potential feedbacks between canyon size and effective albedo. An emerging literature on supraglacial canyons demonstrates important discrepancies between observations and extant model results, highlighting the need for model development alongside remote and field data constraints. We respond to this need with a new numeric model for supraglacial canyon evolution that unifies thermo-fluvial incision with solar ablation of the canyon banks, two essential processes of canyon formation that have so far been treated separately. The model is calibrated for canyons on Vibeke Glacier, East Greenland, which has two primary, highly sinuous canyons longitudinally extending >10 km down the glacier. We measure the valley depth and width along these canyons using 2m WorldView DEMs for the 2013, 2015, and 2017 melt seasons as well as ICESat-2 photon returns from the 2019 melt season. We find an increase in the north canyon’s mean incision rate from 0.87 m a^-1 for the 2013-2015 period to 2.90 m a^-1 for the 2015-2017 period and an increase in the south canyon’s mean incision rate from 0.55 m a^-1 for the 2013-2015 period to 2.83 m a^-1 for the 2015-2017 period, as well as a maximum valley depth of 30.20 m in both canyons. Consistent with previous work, both canyons have asymmetric banks. We compare these observations with results from our model driven by different representations of stream discharge: a sinusoid, one based on channel geometry, one based on watershed area, and the MERRA-2 surface runoff field. Using WorldView imagery, we find that a substantial portion of the south canyon’s watershed rerouted to the north canyon sometime between the 2014 and 2015 melt seasons and incorporate this step change into our model forcing. This research offers first-order constraints on supraglacial canyon evolution that inform our understanding of surface meltwater transport across the Greenland Ice Sheet.