Supraglacial River Forcing Of Subglacial Water Storage And Diurnal Ice Sheet Motion

Surface melting impacts ice sheet sliding by supplying water to the bed, but subglacial processes driving ice accelerations are complex. We examine linkages between surface runoff, transient subglacial water storage, and short-term ice motion from 168 consecutive hourly measurements of meltwater discharge (moulin input) and GPS-derived ice surface motion for Rio Behar, a similar to 60 km(2) moulin-terminating supraglacial river catchment on the southwest Greenland Ice Sheet. Short-term accelerations in ice speed correlate strongly with lag-corrected measures of supraglacial river discharge (r = 0.9, tau = 0.7, p < 0.01). Though our 7 days record cannot address seasonal-scale forcing, diurnal ice accelerations align with normalized differenced supraglacial and proglacial discharge, a proxy for subglacial storage change, better than GPS-derived ice surface uplift. These observations counter theoretical steady state basal sliding laws and suggest that moulin and proglacially induced fluctuations in subglacial water storage, rather than absolute subglacial water storage, drive short-term ice accelerations.Plain Language Summary The importance of surface meltwater runoff to Greenland ice sheet subglacial hydrology and ice sliding dynamics is widely recognized but poorly constrained by field observations. We present 168 consecutive hours of rare in situ discharge measurements in a large supraglacial river draining the ice sheet surface, just upstream of where it plummets into a major moulin. GPS measurements of ice surface motion record brief accelerations in ice sliding speed that follow daily cycles in meltwater entering the moulin. By comparing these measurements with proglacial river discharges leaving the ice sheet, we identify daily fluctuations in subglacial water storage that track shortterm accelerations in ice motion. These findings affirm the importance of supraglacial rivers to subglacial water pressure and ice dynamics, even in relatively thick ice >40 km inland from the ice terminus.