| Summary: | The subglacial drainage system is one of the main controls on basal sliding but remains only partially understood, constituting one of the most significant sources of uncertainty in glacier dynamics models. Increasing the accuracy of such models is of great importance to correctly forecast the availability of water in glaciated basins and the global sea level rise. While current glacial hydrology models are successful in reproducing the general seasonal change in surface speed and the structure of the subglacial drainage system, they fail to reproduce significant features observed in boreholes. Here we use an eight-year dataset of borehole observations on a small, alpine polythermal valley glacier in the Yukon Territory, to assess which missing physical processes in current glacier hydrology models can explain borehole observations. Our primary tool to analyze the borehole dataset and make inferences about the structure and evolution of the subglacial drainage system is a custom methodology to cluster water pressure time series according to their similarities. We find that the standard picture of a distributed drainage system that progressively channelizes throughout the melt season explains many features of the dataset. However, our observations underline the importance of hydraulically disconnected parts of the bed. Different regions of the bed are generally either hydraulically well-connected or disconnected, and the transition between the two states is abrupt in time (minutes to a few hours) and space (<15 m), and the diffusivity at the bed has a significant fine structure at scales smaller than our minimum borehole spacing of 15 m. We found that some regions of the bed are more likely to become hydraulically well-connected than others, and some areas can remain hydraulically disconnected year-round, with a significant portion of these disconnected areas experiencing pressure variations due to normal stress transfers from hydraulically connected areas. Using GPS measurements of surface speed, we found that the ratio between connected and disconnected regions of the bed seems to have a greater influence on basal sliding than the effective pressure within the connected drainage system, suggesting that a significant modification has to be made to the accepted ideas about basal sliding. Science, Faculty of Earth, Ocean and Atmospheric Sciences, Department of Graduate
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