Patchy subglacial drainage systems: observations and modelling

Subglacial water pressure time series from instrumented boreholes provide widespread evidence for the formation of hydraulically disconnected regions at the bed. These do not exhibit the typical diurnal pressure oscillations that indicate a connection to the melting glacier surface during summer. Importantly, the spatial extent of hydraulic disconnection can evolve over time. This is a feature of borehole data sets from both, sub-Arctic glaciers where a cold surface layer quickly seals water-filled boreholes from the surface through freezing, and from temperate mid-latitude glaciers. Current two-dimensional subglacial drainage models do not allow for the connectivity of the drainage system to evolvein time, and in fact, do not allow for the complete shut-down of the system anywahere. In this presentation, we review the observational evidence, focusing on a data set obtained at a small polythermal valley glacier in the southern Yukon Territory, Canada. We document the rapid switching of sizeable (~ 1 ice thickness in extent) parts of the bed from disconnected to connected and back again, driven by the resumption of surface melt resumes following periods of summer snow cover. We show how current drainage models need to be modified to account for suc switching behaviour, and discuss the wider implications of these modifications on drainage system behaviour and glacier dynamics. In particular, we show that switching behaviour can explain the typically observed high water pressures under glaciers during winter, and how this creates conflict with widely used friction laws. We also show how hydraulic switching may limit the ability to pump water out of the bed, which has been suggested elsewhere as a technically plausible mechanism for artificially slowing glacier flow.