Spatial Variability of the Subglacial Hydrology in West Greenland from Airborne Radar Data and Simple Drainage Models
Abstract:Sliding at the interface between ice and bed governs ice discharge for glaciers, including outlet glaciers in Greenland. Sliding is dependent on water pressure and the distribution of water across the bed. Both of these water qualities are sensitive to basal topography and roughness. Here, we characterize the subglacial environment beneath the ice sheet that feeds several outlet glaciers in West Greenland. These glaciers have time varying velocities that appear to be sensitive to surface water drainage and subglacial water pathways. Pathways are dependent on basal topography that we characterize using AGAP, Operation IceBridge and IcePod ice-penetrating radar data. These topographic data are then fed into a steady-state water flow algorithm to characterize the subglacial water pathways. These inferred pathways are sensitive to the input of water pressure, which we adjust to understand the sensitivity of the drainage network. Our results show that subglacial water pathways beneath outlet glaciers can evolve over time, following a combination of the ice surface slope and the bed slope. The degree to which water flow follows the surface slope or the bed slope depends on the magnitude and spatial heterogeneity of water pressures within the catchment. We find that water pathways are most sensitive to water pressure when the surface and bed have similar slopes. Because individual glaciers have different surface and bed topography, seasonal evolution of the size of the subglacial water catchment can vary considerably between neighboring glaciers.
To evaluate the spatial heterogeneity of subglacial water, we examine bed reflectivity using the range-migrated, SAR-focused airborne radar data collected by IceBridge and IcePod. To correct the observed bed reflectivity for attenuation, we use a radar attenuation model that accounts for the ice temperature. The reflectivity analysis allows us to identify water pathways independently of the inferred pathways from the algorithm. We compare both the identified and inferred pathways to yield information on the broad scale water pressure variations, which is important to constrain in order to understand the influence of subglacial hydrology on seasonal ice velocity.