Abstract ID: 28.7311 | Accepted as Talk | Talk/Oral | 2025-02-27 14:45 - 15:00 | Ágnes‐Heller‐Haus/Small Lecture Room

Meltwater infiltration in snow and firn has become increasingly significant with rising rain and surface melt in the accumulation zones of glaciers and ice sheets. Yet, the complexities of unsaturated flow, including the downward propagation of wetting fronts and flow through preferential pathways, pose challenges to understanding and modeling infiltration processes. Further, a lack of observations, particularly on glaciers and ice sheets, limits validation studies. Here, we present an analysis of in situ measurements from the Greenland Ice Sheet to partition meltwater fluxes between wetting fronts and preferential flow paths. Our data were collected in boreholes instrumented with temperature sensors, ranging from 10-100 m deep and located at 17 sites along a ~80 km transect. Thermal time series documented evolution of a surface wetting front based on 0° C isotherms. Preferential flow into underlying cold firn was revealed by warming caused by latent heat release during refreezing. The amount of refrozen liquid water associated with each mode of flow was quantified by a thermal scheme that isolated latent heat release from conductive heat transfer. The method differenced a thermal diffusion model, pinned to data for initial and boundary conditions, with observed temperature fields. Our results document wetting fronts that extended from 0 to 5 m depth and persisted up to 100 days during summer. Preferential flow occurred as distinct events that penetrated up to 9 m depth and refroze in a matter of hours. Fluxes between the two modes of flow varied, with preferential flow accounting for 10-95% of refrozen meltwater. Lower melt volumes favored preferential flow dominance, whereas high melt rates approached an even 50/50 partitioning. These findings demonstrate the importance of preferential flow which poses significant challenges to the fidelity of models of meltwater infiltration processes.