Supraglacial streamflow and meteorological drivers from southwest Greenland

Abstract

Abstract. Greenland ice sheet surface runoff is drained through supraglacial stream networks. This evacuation influences surface mass balance as well as ice dynamics. However, in situ observations of meltwater discharge through these stream networks are rare. In this study, we present 46 discrete discharge measurements and continuous water level measurements for 62 d spanning the majority of of the melt season (13 June to 13 August) in 2016 for a 0.6 km2 supraglacial stream catchment in southwest Greenland. The result is an unprecedentedly long record of supraglacial discharge that captures both diurnal variability and changes over the melt season. A comparison of surface energy fluxes to stream discharge reveals shortwave radiation as the primary driver of melting. However, during high-melt episodes, the contribution of shortwave radiation to melt energy is reduced by ∼40 % (from 1.13 to 0.73 proportion). Instead, the relative contribution of longwave radiation, sensible heat fluxes, and latent heat fluxes to overall melt increases by ∼24 %, 6 %, and 10 % (proportion increased from −0.32 to −0.08, 0.28 to 0.34, and −0.04 to 0.06) respectively. Our data also identify that the timing of daily maximum discharge during clear-sky days shifts from 16:00 local time (i.e., 2 h 45 min after solar noon) in late June to 14:00 in late July and then rapidly returns to 16:00 in early August. The change in the timing of daily maximum discharge could be attributed to the expansion and contraction of the stream network, caused by skin temperatures that likely fell below freezing at night. The abrupt shift, in early August, in the timing of daily maximum discharge coincides with a drop in air temperature, a drop in the amount of water temporarily stored in weathering crust, and a decreasing covariance between stream velocity and discharge. Further work is needed to investigate if these results can be transferable to larger catchments and uncover if rapid shifts in the timing of peak discharge are widespread across Greenland supraglacial streams and thus have an impact on meltwater delivery to the subglacial system and ice dynamics.

Brief communication: The Impact of Interannual Melt Supply Variability on Greenland Ice Sheet Moulin Inputs

“…In-situ streamflow observations from a 0.6 km 2 catchment in southwestern Greenland show a statistically significant decrease in the lag time between local solar noon and peak moulin inputs in the first few weeks of the melt season (Muthyala et al, 2022). Our simulations, however, do not support a consistent trend (Fig.…”

“…It is important to note here that our melt forcing data have a temporal resolution of 3 hours (Noël et al, 2019), so it remains difficult to smoothly constrain the time of minimum moulin inputs at sub-hourly resolution. This pattern can not be compared to the observations of Muthyala et al (2022) as their domain does not include a supraglacial lake.…”

“…In the high melt year 2012, lag time ranges from ∼4 to 8 hours, and in 2015 lag time is between 4 and 6 hours. The lack of a consistent trend may, in part, be explained by the larger size of our domain and the number of supraglacial lakes compared to the small, lake-free domain of Muthyala et al (2022).…”

Intra-seasonal variability in supraglacial stream sediment on the Greenland Ice Sheet

“…Once cryoconite holes form, surface albedo may be 0.6 higher than areas with distributed sediment as the bottom of the cryoconite hole is largely shaded from direct sunlight due to the hole geometry and ice lids coving the holes after nighttime freezing (Bøggild et al, 2010). However, sediment granules can flush out of cryoconite holes during heavy melt events (often associated with cloudy conditions or rainfall; Takeuchi et al, 2018;Tedstone et al, 2020;Muthyala et al, 2022) and deposit in supraglacial streams (Leidman et al, 2021b). The similar granular structure and composition of sediment found in supraglacial streams also indicates that it predominantly originates from these cryoconite holes (Bøggild et al, 2010).…”

“…Energy absorbed by supraglacial stream sediment may be partially re-radiated back to the atmosphere or exported from the glacier's terminus before contributing to increased melting. However, the long transport distance between the absorption and export locations and the similar hydraulic properties of these systems to terrestrial rivers (Marston, 1983;Smith et al, 2015;Gleason et al, 2016;Muthyala et al, 2022) suggests that much of the solar energy absorbed by sediment goes towards increasing melt rates. The highly turbulent flow characteristics of supraglacial streams (Gleason et al, 2016) would also facilitate a more efficient transfer of energy to the ice for melting.…”