Subglacial channel evolution observed in western Greenland from moulin water level and GPS-derived surface velocities

Fluctuations in supraglacial melt input to the ice sheet bed, delivered by moulins, drive changes in surface velocities. Here we present contemporaneous local measurements of the supraglacial, englacial, and subglacial hydrological system of the Greenland Ice Sheet, with the goal of understanding how the subglacial hydrological system evolves over the course of the melt season. Between 18 July and 22 August 2011, we measured moulin water level, stream water level, and GPS-derived sub-daily surface velocity in the Pakitsoq region of Greenland. In addition, we drilled boreholes in the vicinity of the moulins and measured subglacial water pressure. These data demonstrate clearly that daily peaks in moulin water levels occur simultaneously with peak surface velocities throughout the measurement period, while peak borehole water levels consistently lag peak velocities by several hours. However, the magnitude of the surface-velocity relationship with moulin water levels varies on multiple timescales. On a daily timescale, the relationship between surface velocity and moulin water level displays hysteresis; moulin water levels are associated with higher velocities before the diurnal peak in meltwater input. During periods of prolonged surface melt, the moulin water level eventually drops below the sensor depth where it sometimes remains for several days. These periods are correlated with decreases in surface velocity to the winter background rate. In addition, we observe a general decrease in surface velocity magnitude over the observation period despite a relatively constant daily peak in moulin water level. We suggest that these observations are direct indicators of the evolution of the drainage efficiency of the subglacial hydrological system on seasonal and diurnal timescales.