In situ measurements of meltwater flow through snow and firn in the accumulation zone of the SW Greenland Ice Sheet

Abstract

Abstract. The Greenland Ice Sheet is losing mass, part of which is caused by increasing runoff. The location of the runoff limit, the highest elevation from which meltwater finds its way off the ice sheet, plays an important role in the surface mass balance of the ice sheet. The recently observed rise in runoff area might be related to an increasing amount of refreezing: ice layer development in the firn reduces vertical percolation and promotes lateral runoff. To investigate meltwater flow near the runoff limit in the accumulation zone on the southwestern Greenland Ice Sheet, we carried out in situ measurements of hydrological processes and properties of firn and snow. The hydraulic conductivity of icy firn in pre-melt conditions measured using a portable lysimeter ranges from 0.17 to 12.8 m h−1, with flow predominantly occurring through preferential flow fingers. Lateral flow velocities of meltwater on top of the near-surface ice slab, measured at the peak of the melt season by salt dilution and tracer experiments, range from 1.3 to 15.1 m h−1. With these lateral flow velocities, the distance between the slush limit, the highest elevation where liquid water is visible on the ice sheet surface, and the runoff limit could be roughly 4 km in regions where near-surface ice slabs are present. These measurements are a first step towards an integrated set of hydrological properties of firn on the SW Greenland Ice Sheet and show evidence that meltwater runoff may occur from elevations above the visible runoff area.

Greenland Ice Sheet Ice Slab Expansion and Thickening

“…Accretion below existing ice slabs (Figures 4b and 4d) was primarily associated with the very large PDH sum during summer 2012. We propose that abundant meltwater-which may have originated from higher elevations (Clerx et al, 2022)-was able to exploit local areas of higher permeability despite the overall role of ice slabs as near-impermeable aquitards. Although previous observations show that deep percolation (>10 m) can occur in firn without homogeneous wetting front advance (Humphrey et al, 2012;Machguth et al, 2016;Samimi et al, 2020), these processes are less likely to occur through several meters thick ice slabs.…”

50 years of firn evolution on Grigoriev Ice Cap, Tien Shan, Kyrgyzstan

“…We have no direct evidence of surface runoff on the summit of Grigoriev, but studies from other firn areas show that lateral runoff starts in the subsurface and becomes visible only after substantial distances (e.g. several kilometers on the gently sloping surface of the Greenland Ice Sheet: Holmes, 1955;Clerx et al, 2022). Lateral runoff would mean that less latent heat gets released at depth, inside the firn.…”

A Thermodynamic Nonequilibrium Model for Preferential Infiltration and Refreezing of Melt in Snow

“…Additionally, when melt interacts with a subfreezing snowpack, it can readily refreeze as ice and decrease local snow porosity. This refreezing process reduces the effective infiltration rate by both consuming liquid water available for transport and by lowering the hydraulic conductivity of snow, which hinders vertical percolation and promotes lateral runoff (Clerx et al., 2022; Culberg et al., 2021; Eiriksson et al., 2013). The resulting heterogeneous porosity structures, such as ice pipes or ice lenses, play an important role in snow hydrology and geohazard assessment, but the mechanism of their formation remains poorly captured by models.…”

“…While the generation of melt at snow surface can be relatively uniform in space, meltwater infiltration through the underlying snowpack is known to be highly heterogeneous in nature, forming (a) vertical preferential flow pathways that channelize meltwater (e.g., ice pipes) and (b) lateral flow pathways guided by horizontal low permeability zones (e.g., capillary barriers or ice lenses). Both types of preferential pathways have been observed in the field directly or indirectly (Campbell et al., 2006; Clerx et al., 2022; Culberg et al., 2021; Evans et al., 2016; Humphrey et al., 2012; Kinar & Pomeroy, 2015), but systematic investigation and mechanistic understanding of these phenomena are limited (Eiriksson et al., 2013; Webb, Williams, & Erickson, 2018). In particular, laboratory experiments in 3D samples (Avanzi et al., 2016; Katsushima et al., 2013; Waldner et al., 2004) have shown the percolation of meltwater into 3D snowpack/columns to be intrinsically unstable, analogous to gravity‐driven water infiltration through dry soil (Glass et al., 1989; Glass & Nicholl, 1996; Selker et al., 1992).…”