Inferring basal plasticity in a temperate ice cap from observationally constrained ice-flow models

Despite the importance of glacier bed mechanics to understanding glacier flow and projecting the future glacier states, our knowledge of some fundamental concepts concerning the coupling of basal shear stress, water pressure, and slip rate remains incomplete. Here we infer the existence of a plastically deforming bed beneath Hofsjökull, a temperate ice cap in central Iceland, using numerical ice flow models constrained by surface velocity observations. Basal plasticity is evident by the independence of basal shear stress and basal slip rate, both of which we inferred using higher-order flow models in the Ice Sheet Systems Model (ISSM). We derived seasonal surface velocity fields from airborne interferometric synthetic aperture radar (InSAR) data collected with NASA's Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) in winter and during the early melt season. Using velocity fields from different seasons and applying a simple plastic model for basal stress, we present estimates of surface-melt-driven changes in basal water pressure that correspond to observed variations in surface velocity. Our data and an idealized basal slip model show that the sensitivity of basal slip to basal water pressure variability scales as the inverse of the ice surface slope, making glaciers with shallow surface slopes more sensitive to basal water pressure fluctuations than steep-sloped glaciers.