High resolution modelling of the seasonal evolution of surface water storage on the Greenland Ice Sheet

Seasonal meltwater lakes form when surface runoff gets trapped in topographic depressions on the surface of the Greenland Ice Sheet (GrIS). The development of such lakes affects the surface energy balance (and hence the mass balance) of the ice sheet, (as the lower albedo of surface water increases radiation-driven melting), and the ice sheet dynamics, (as sudden drainage of such lakes has been linked to observed increases in ice velocity). The areal extent, the depth, and duration of lakes have been inferred in an increasing number of studies from satellite imagery of the ice sheet. Such observational studies suffer from the limited temporal availability of satellite imagery which means that not all lakes may be observed, and also cannot be used to forecasting purposes. Here, we adopt a different model-based strategy to estimate the seasonal evolution of surface water storage for a ~3600km2 area of the western Greenland Ice Sheet. Our approach combines a high resolution surface mass balance model (used to calculate surface melt) with a novel model for supraglacial water routing which calculates the time-dependent filling and draining of supraglacial lakes, using a water volume threshold to trigger lake drainage. The model shows very good agreement between modelled lake locations and volumes and those observed in 10 LANDSAT images from 2001, 2002 and 2005. We have used the model to investigate the water volume needed to trigger lake drainage events, and the impact that this threshold volume has on the proportion of melt which runs off the ice surface supraglacially, the proportion stored in lakes on the ice surface, and the proportion which enters the subglacial drainage system following lake drainage events. Model performance is maximised with lake volume thresholds set to between 4000 and 7500 times the local ice thickness. For these thresholds, lakes transiently store up to 40% of melt at the beginning of the melt season, but this decreases to ~5 to 10% by the middle of the melt season. Between 40 and 50% of meltwater runs off the surface of the ice directly, and the remainder (50 to 40%) enters the subglacial drainage system through the bottom of drained lakes.