Dynamic Hydraulic Conductivity Reconciles Mismatch Between Modeled and Observed Winter Subglacial Water Pressure

• Published in: Journal of geophysical research. Earth surface 2018-04, Vol.123 (4), p.818-836
• Main Authors: Downs, Jacob Z., Johnson, Jesse V., Harper, Joel T., Meierbachtol, Toby, Werder, Mauro A.
• Format: Article
• Language: English
• Subjects: Boreholes; Computer networks; Conductivity; Drainage control; Drainage systems; Glaciation; Glaciers; Glaciohydrology; Hydraulic conductivity; Hydraulics; Hydrologic models; Hydrology; Hydrostatic pressure; Ice sheets; Meltwater; modeling; Modelling; Moisture content; Numerical experiments; Parameters; Pressure; Stress concentration; subglacial; Subglacial water; Thickness; Upper bounds; water; Water availability; Water content; Water pressure; Winter
• ISSN: 2169-9003; 2169-9011
• DOI: 10.1002/2017JF004522

The link between subglacial hydrology and basal sliding has prompted work on basal hydrology models with water pressure and drainage capacity as prognostic variables. We find that the Glacier Drainage System model, which belongs to a commonly used family of subglacial hydrology models that include both channelized and distributed drainage components, underpredicts winter water pressure when compared to borehole observations from western Greenland given a wide range of plausible parameter values and inputs. This problem, though previously noted by other modelers, has not been addressed. Possible causes for the discrepancy including idealized model inputs or unconstrained parameters are investigated through a series of modeling experiments on both synthetic and realistic ice sheet geometries. Numerical experiments reveal that englacial storage and hydraulic conductivity in the distributed system are the primary controls on winter water pressure in Glacier Drainage System model. Observations of temperate layer thickness and englacial water content from western Greenland imply an upper bound on englacial storage, suggesting that a reduction in hydraulic conductivity is the most plausible cause of high winter water pressure. We conclude that hydraulic conductivity acts as a proxy for the subgrid‐scale connectivity of the linked cavity system and should therefore change seasonally in correspondence with melt water availability. Key Points A commonly used drainage model formulation underpredicts observations of water pressure in winter Englacial storage elevates modeled winter pressure, but observations indicate that storage is limited Decreasing hydraulic conductivity is physically plausible and reproduces winter pressure observations