A model of water extraction from the subglacial hydrologic system under idealized conditions

Abstract. Subglacial water modulates glacier velocity across a wide range of space and time scales by influencing friction at the glacier bed. Observations show ice acceleration due to supraglacial lake drainage and water draining through moulins, where both configurations involve water inputs to the bed. Here we consider the reverse: water extraction from the subglacial hydrologic system, which is a proposed intervention method intended to slow the flow of glaciers and reduce the associated sea-level rise. Removing subglacial water results in different dynamics than injecting water, and we hypothesize that understanding these processes will allow for improved characterization of the physics of subglacial hydrology. We set up these model experiments in the Subglacial Hydrology And Kinetic, Transient Interactions (SHAKTI) model coupled with the Ice-sheet and Sea-level System Model (ISSM). By analyzing the problem of an isolated borehole in a background pressure field to determine the region of extraction influence, we find an approximate analytical solution which shows that the water pressure returns to the background value approximately as a logarithm with distance. The benefit of the analytical solution is that the dependence of uncertain parameters is clear and may be used alongside data to constrain subglacial hydrology models. We find good agreement between this analytical result and numerical SHAKTI simulations. Using the coupled SHAKTI-ISSM model, we perform transient model experiments on Helheim Glacier, Greenland and Thwaites Glacier, Antarctica, to determine the effects of water extraction on glacier velocity. With continuous pumping, we simulate a velocity decrease on the order of 1 %, which depends on the site location. The response time to pumping initiation and the recovery time, following cessation, scale according to effective pressure, with typical times on the order of hours to days. These results demonstrate that water extraction is a method of probing the subglacial hydrologic system to better constrain the uncertain physics, and that further research is required to assess its effectiveness as an intervention method.