Near-surface permeability in a supraglacial drainage basin on the Llewellyn Glacier, Juneau Icefield, British Columbia

Abstract

Abstract. Supraglacial channel networks link time varying melt production and meltwater routing on temperate glaciers. Such channel networks often include components of both surface transport in streams and subsurface porous flow through near-surface ice, firn or snowpack. Although subsurface transport if present will likely control network transport efficacy, it is the most poorly characterized component of the system. We present measurements of supraglacial channel spacing and network properties on the Juneau Icefield, subsurface water table height, and time variation of hydraulic characteristics including diurnal variability in water temperature. We combine these data with modeling of porous flow in weathered ice to infer near-surface permeability. Estimates are based on an observed phase lag between diurnal water temperature variations and discharge, and independently on measurement of water table surface elevation away from a stream. Both methods predict ice permeability on a 1-10 m scale in the range of 10 −10 -10 −11 m 2 . These estimates are considerably smaller than common parameterizations of surface water flow on bare ice in the literature, as well as smaller than most estimates of snowpack permeability. For supraglacial environments in which porosity/permeability creation in the subsurface is balanced by porous flow of meltwater, our methods provide an estimate of microscale hydraulic properties from observations of supraglacial channel spacing.

Near surface meltwater storage in low-density bare ice of the Greenland ice sheet ablation zone

“…In sum, the weathering crust represents an understudied component of the Greenland Ice Sheet hydrologic system with potential implications for surface mass and energy balance budgets (van den Broeke et al, 2008;Wheler and Flowers, 2011), retrieval of mass changes from surface height change observations (Braithwaite et al, 1998;LaChapelle, 1959;Müller and Keeler, 1969), and controls on the supraglacial hydrograph (Karlstrom et al, 2014;Munro, 2011). Our study presents the first measurements of weathering crust physical properties and documents non-trivial meltwater storage within the low-5 density, near surface ablating weathering crust in a mid-elevation Greenland Ice Sheet supraglacial catchment.…”

“…6 is consistent with conceptual models of the near surface weathering crustcryoconite hole hydrologic system proposed by Irvine-Fynn and Edwards (2014) and Müller and Keeler (1969), and 25 confirms this system is present in the Greenland Ice Sheet ablation zone. The ubiquity of water-filled cryoconite holes, the rapid infilling of drilled holes with liquid water, and the excavation of saturated ice cores to depths of 1.8 m suggests the study area weathering crust acts as a depth-limited aquifer (Irvine-Fynn et al, 2011), storing meltwater in the seasonallytemperate near surface ice and delaying the delivery of meltwater to supraglacial streams and rivers via saturated subsurface flow (Fountain and Walder, 1998;Irvine-Fynn et al, 2011;Karlstrom et al, 2014). 30…”

“…Outside of the Greenland Ice Sheet, however, meltwater storage in a degraded, porous "weathering crust" on the bare ice 5 surface of glaciers delays and dampens meltwater delivery to supraglacial channels via porous subsurface flow (Cook et al, 2016;Fountain and Walder, 1998;Irvine-Fynn et al, 2011;Karlstrom et al, 2014;Munro, 2011;Schuster, 2001), and provides a substrate for internal refreezing of meltwater (Hoffman et al, 2014;Paterson, 1972;Schuster, 2001). Weathering crusts are fractured, disintegrated, or "rotten" ice layers that form on the surface of seasonally temperate glaciers (Brandt and Warren, 1993;Fountain and Walder, 1998;Irvine-Fynn et al, 2011;LaChapelle, 1959;Müller and Keeler, 1969;Nye, 10 1991).…”

“…Other work has focused on the biological and carbon-cycle implications of weathering crust as a substrate for microbial-rich cryoconite holes (Cook et al, 2010(Cook et al, , 2012Hodson et al, 2010;Irvine-Fynn and Edwards, 2014). 25 Recently, a small body of research has addressed the hydrologic significance of the weathering crust on Arctic glaciers and ice sheets (Cook et al, 2016;Irvine-Fynn, 2008;Karlstrom et al, 2014), where weathering crust meltwater storage varies diurnally and seasonally (Cook et al, 2016), and controls the timing and variability of meltwater delivery to supraglacial rivers via porous subsurface flow (Karlstrom et al, 2014;Munro, 2011). However, no studies have reported physical properties or documented subsurface meltwater storage for the Greenland Ice Sheet weathering crust.…”

“…Greenland Ice Sheet ablation zone, the weathering crust would provide a storage reservoir for meltwater and play an unrecognized role in modulating meltwater delivery to moulins and subglacial hydrologic systems (Karlstrom et al, 2014).…”