Near-surface hydraulic conductivity of Northern Hemisphere glaciers

The hydrology of near-surface glacier ice remains a neglected aspect of glacier hydrology despite its role in modulating meltwater delivery to downstream environments. To elucidate the hydrological characteristics of this near-surface glacial “weathering crust”, we describe the design and operation of a bespoke capacitance-based piezometer that enables rapid and economical deployment across multiple sites and provides an accurate, high resolution record of near-surface water level fluctuations. Piezometer tests were conducted on the surface of ten northern hemisphere glacier sites. Through application of standard terrestrial hydrology bail-recharge techniques, we derive hydraulic conductivity (K) values that varied from 0.003 to 3.519 m d-1, with a mean of 0.185 ± 0.019 m d-1. These results are comparable to those obtained in other discrete studies of glacier near-surface ice and firn, and indicate that the weathering crust represents a hydrologically poor, impervious aquifer. K was negatively correlated with altitude and cumulative short-wave radiation since the last synoptic period of either negative air temperatures or dominance of turbulent energy fluxes; K was positively correlated with water table height. The large range of K observed suggests complex interactions between meteorological influences and differences arising from variability in ice structure and crystallography. Our data demonstrate a greater complexity to the hydrology of near-surface ice than hitherto appreciated and support the notion that the weathering crust can regulate and delay the supraglacial discharge response to melt production. This has implications not only for the accuracy of predictive meltwater runoff models, but also for biogeochemical processes and transfers that are strongly conditioned by water residence time and the efficacy of the cascade of sediments, contaminants, microbes and nutrients to downstream ecosystems. Since continued atmospheric warming will lead to rising snowline elevations and glacier thinning, the supraglacial hydrological system may assume greater importance in many mountainous regions and, consequently, detailing hydraulics of the weathering crust represents a research priority since the complexity of the flow-path it represents is poorly constrained.