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Lookup NU author(s): Dr Ian Stevens
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A glacier's surface is characterised by a <2 m deep porous near-surface layer termed the `weathering crust' (WC). The WC acts as a perched aquifer yet remains a poorly described and overlooked component of the supraglacial hydrological system. To augment contemporary understanding of supraglacial hydrology, it is necessary to quantify the hydraulic functionality of the WC. Here, to estimate glacier WC permeability (K), we used standard techniques more commonly associated with groundwater science. Shallow 5 cm diameter `boreholes' were drilled to depths of <47 cm and the borehole water column was evacuated. Borehole water recharge was then recorded at 2 s intervals using an innovative logging piezometer. To provide a variety of glacier surface characteristics and climatic settings, field data were collected at 4 valley glaciers throughout the northern hemisphere and at a site on the Greenland Ice Sheet during 2014 and 2015. A mean K of 0.51 ± 1.6 m d-1, with a range of 0.004 - 18.4 m d-1 and relative standard deviation of 309%, was defined from 328 WC borehole recharge curves. These results indicate the WC generally exhibits a semi-pervious nature akin to sandstone, but with high local variability. Such variation implies the dominant influence of controlling factors including antecedent melt rates, cumulative radiation receipt, time of day and instantaneous melt production; interestingly, latitude did not appear to directly influence K. We suggest ice structure and associated impacts on crystal size and morphology, and micro-scale topography, also play an important role in governing local permeability. With supraglacial rills and channels typically separated by <102 m length scales, and widespread saturated supraglacial ice surfaces being uncommon, our data suggest that in-situ meltwaters may be stored or delayed in the WC for periods of several days. Therefore, bulk supraglacial stream discharge may comprise a proportion of `old' meltwater. Consequently, the WC is shown as an important regulator of meltwater egress from the supraglacial environment, with secondary influences on the supraglacial energy balance, the meltwater and suspended particle budget at a range of timescales, and the supraglacial ecosystem through the transport of microbes, fine mineral grains, and associated nutrients.
Author(s): Stevens IT, Irvine-Fynn TD, Porter PR, Smart M, Moorman BJ, Hodson AJ, Cook J, Edwards A, Mitchell AC
Publication type: Conference Proceedings (inc. Abstract)
Publication status: Published
Conference Name: AGU Fall Meeting
Year of Conference: 2016
Online publication date: 16/12/2016
Acceptance date: 01/08/2016
Publisher: American Geophysical Union
URL: https://agu.confex.com/agu/fm16/meetingapp.cgi/Paper/186282
