Observed multi-scale variations of subglacial hydro-mechanical conditions at Kongsvegen, Svalbard.

Glacier flow variations are predominantly due to changes at the ice-bed interface, where basal slip and sediment deformation drive basal glacier motion. Determining subglacial conditions and their responses to hydraulic forcing remains challenging due to the difficulty of accessing the glacier bed. In this study, we analyze data series from instruments placed at the base of Kongsvegen glacier (Svalbard) thanks to a 350 m borehole.The borehole was instrumented witha pressure sensor, seismometers, and a ploughmeter to monitor the interplay between surface runoff and hydro-mechanical conditions. Covering the two ablation seasons of 2021 and 2022, , we measured point-scale subglacial water pressure and till strength, and we derived at a kilometre scale the subglacial hydraulic gradient and radius from seismic observations.. Across seasonal, multi-day, and diurnal time scales, we compared these measurements to characterize the variations in subglacial conditions caused by changes in surface runoff. We discuss our results in light of existing theories of subglacial hydrology and till mechanics. We find that during the short, low intensity melt season of 2021, the subglacial drainage system evolves to accommodate runoff variations, increasing its capacity as the melt season progressed. In contrast, during the long and high intensity melt season of 2022, the subglacial drainage system evolved transiently to respond to the abrupt and large water supply. We suggest that in this configuration, the drainage capacity of the hydraulically active part of the subglacial drainage system is exceeded, promoting the expansion of hydraulically connected regions and local weakening of ice-bed coupling, thus enhancing sliding. Our in-situ, multi-method approach provides a unique insight into conditions at the ice-bed interface.