Efficient meltwater drainage through supraglacial streams and rivers on the southwest Greenland ice sheet

Abstract

Thermally incised meltwater channels that flow each summer across melt-prone surfaces of the Greenland ice sheet have received little direct study. We use high-resolution WorldView-1/2 satellite mapping and in situ measurements to characterize supraglacial water storage, drainage pattern, and discharge across 6,812 km(2) of southwest Greenland in July 2012, after a record melt event. Efficient surface drainage was routed through 523 high-order stream/river channel networks, all of which terminated in moulins before reaching the ice edge. Low surface water storage (3.6 ± 0.9 cm), negligible impoundment by supraglacial lakes or topographic depressions, and high discharge to moulins (2.54-2.81 cm⋅d(-1)) indicate that the surface drainage system conveyed its own storage volume every <2 d to the bed. Moulin discharges mapped inside ∼52% of the source ice watershed for Isortoq, a major proglacial river, totaled ∼41-98% of observed proglacial discharge, highlighting the importance of supraglacial river drainage to true outflow from the ice edge. However, Isortoq discharges tended lower than runoff simulations from the Modèle Atmosphérique Régional (MAR) regional climate model (0.056-0.112 km(3)⋅d(-1) vs. ∼0.103 km(3)⋅d(-1)), and when integrated over the melt season, totaled just 37-75% of MAR, suggesting nontrivial subglacial water storage even in this melt-prone region of the ice sheet. We conclude that (i) the interior surface of the ice sheet can be efficiently drained under optimal conditions, (ii) that digital elevation models alone cannot fully describe supraglacial drainage and its connection to subglacial systems, and (iii) that predicting outflow from climate models alone, without recognition of subglacial processes, may overestimate true meltwater export from the ice sheet to the ocean.

Keywords: Greenland ice sheet; mass balance; meltwater runoff; remote sensing; supraglacial hydrology.

Fig. 1.

Supraglacial river networks represent an important high-capacity mechanism for conveying large volumes of meltwater across the GrIS surface, as illustrated by (A) 23 July 2012 field photo (see authors in the image for scale), and (B) same-day WV2 satellite image. Both images were acquired ∼55 km inland of the ice edge near Kangerlussuaq, southwest Greenland.

Fig. 2.

Five hundred twenty-three supraglacial river networks, their terminal moulin locations, and moulin discharges were mapped from 32 multispectral WV2 images, with calibration from contemporaneous field measurements collected on the ice sheet (18–23 July 2012). An additional 102 moulins were mapped from panchromatic WV1 imagery (black circles). Downstream discharges from the proglacial river Isortoq were also observed (yellow triangle). All mapped supraglacial rivers terminated in moulins (green circles, with diameters proportional to estimated meltwater flux), with negligible water impoundment in depressions and supraglacial lakes on the ice sheet surface.

Fig. 3.

Comparison of 2012 simulated meltwater production M (magenta) and runoff R (blue) from the MAR regional climate model, observed proglacial discharge (outflow) exiting the ice sheet in the Isortoq river (vertical gray bars, spanning measurement uncertainty), and total supraglacial river moulin discharge estimated for ∼52% of the Isortoq ice watershed during 18–23 July 2012 (dark gray box, spanning measurement uncertainty). Observed proglacial outflows are lower than MAR modeled runoff, especially in June. A record melt on the ice sheet occurred 11–13 July 2012.