Quantifying runoff in Greenland’s percolation zone with phase-sensitive radar and firn modeling

Surface melting and runoff account for about half of the current mass loss of the Greenland Ice Sheet. Regional climate models (RCMs) project runoff to increase further over the 21st century, but the magnitude of this trend varies strongly between different models. This variability arises because RCMs rely on simplified representations of the complex firn hydrological system in Greenland’s percolation zone. However, key parameters for parametrizing meltwater retention and runoff processes remain poorly constrained due to a lack of time-resolved, in situ observations of firn liquid water content.

We address this gap by demonstrating that the Autonomous phase-sensitive Radio-Echo Sounder (ApRES) can continuously trace the amount of liquid water in the firn. At three automatic weather station sites on the ice sheet (KAN_U, DYE-2 and Camp Century), we acquired hourly ApRES time series between spring 2023 and 2025, covering two melt seasons. By analyzing these observations in combination with a firn model, we quantify rates of lateral meltwater flow. Comparison with runoff simulations from three RCMs shows that all models overestimate local runoff at KAN_U, and that some even predict runoff at DYE-2 (2124 m a.s.l.), where our observations indicate that all meltwater is refrozen. Expanding these observations will support the development of improved representations of Greenland’s firn hydrological system in RCMs and ultimately enhance the accuracy of GrIS mass balance projections.