Measuring and modelling firn hydrology in the accumulation zone on the southwestern Greenland Ice Sheet
Authors:   , Clerx Nicole 
Published: June 2024

The Greenland Ice Sheet (GrIS), storing about one-tenth of the world’s freshwater in the form of snow and ice, has the potential to raise global sea levels by ∼7.4 metres. Since the early 1900s it has contributed 40 mm, or 15%, to global mean sea level rise, and its influence is increasing due to Arctic air temperatures rising faster than the global average. Projections for GrIS-induced sea level rise by 2100 range from 4 to 27 cm, emphasising the need for accurate projections of the ice sheet’s response to further climate change. More than 30% of the GrIS mass loss results from increased meltwater generation and surface runoff. Below the runoff limit, meltwater contributes to mass loss, while above it, melt percolates into porous firn and refreezes in place. This percolation does not directly influence mass loss, but has a strong effect on firn structure, leading to the formation of thick, impermeable near-surface ice layers (ice slabs) that force lateral meltwater runoff. Projections of future GrIS runoff loss vary, depending on model representation of hydrological processes in the accumulation zone. Regional climate models, used to predict future evolution of GrIS mass loss, notably simplify meltwater flow due to the inherent complexity and computational intensity of detailed hydrological simulations. Despite progress in remote sensing, the availability of in situ data, and hence our ability to calibrate GrIS hydrological models, remains limited. This thesis presents observations and a lateral meltwater flow model that enhance our understanding of hydrological processes around the runoff limit. First, we describe in situ measurements of vertical percolation and lateral meltwater flow around 1750 m a.s.l. in the accumulation zone of the SW Greenland ice sheet. The hydraulic conductivity of icy firn in pre-melt conditions measured using a portable lysimeter ranges from 0.17 to 12.8 m h-1, with flow predominantly occurring through preferential flow fingers. Lateral flow velocities of meltwater on top of a near-surface ice slab, measured at the peak of the melt season by salt dilution and tracer experiments, range from 1.3 to 15.1 m h-1. Based on these lateral flow velocities, the distance between the slush limit (the highest elevation where liquid water is visible on the ice sheet surface) and the runoff limit is in the order of roughly 4 km in regions where near-surface ice slabs are present. Using the field measurements as input parameters, a quasi-2D physics-based model was developed which is presented in this thesis. The model simulates lateral meltwater flow and the formation of superimposed ice onto ice slabs using an Eulerian Darcy flow scheme, predicting meltwater emergence at the surface. The water table height and the slush limit depend on the melt season’s evolution, and on changes in ice slab gradient. Lower amounts of melt at higher altitudes can delay meltwater appearance compared to when substantial melting occurs at high elevations, but even in colder melt seasons the slush limit may manifest due to brief yet intense melt events. Results highlight the efficiency of lateral flow as runoff mechanism, with lateral outflow exceeding the locally generated meltwater as much as more than 30 times at specific locations. Superimposed ice formation can retain up to 40% of the available meltwater, thereby delaying visible runoff. Expansion of the model to incorporate snowpack warming and surface meltwater runoff throughout the melt season, mimicking river-like surface discharge, shows that snowpack warming delays the onset of melt, and highlights the model’s sensitivity to both meltwater quantity and timing. In high melt years, surface runoff evacuates up to 99% of meltwater due to insufficient refreezing rates and by melt completely removing the annual snowpack. In low melt years, superimposed ice formation retains up to 33% of the water, but surface runoff exceeds retention in all simulated years except one. Model comparisons with MODIS satellite imagery demonstrate the model’s ability to produce realistic estimates of the visible runoff limit, particularly in gradually evolving melt seasons. Despite variable performance when melt occurs in short, intense melt pulses, the model proves to be a valuable tool with potential for expansion to quantify future increases in meltwater runoff associated with changes in firn properties. It not only addresses existing gaps in understanding, but also lays the groundwork for refining future projections of the GrIS’s surface mass balance. Persisting challenges involve quantifying meltwater fluxes in the accumulation zone to improve surface mass balance projections and integrating recent insights on ice slab formation into models for accurate assessments of their impact on surface runoff. The findings in this thesis highlight the importance of describing lateral meltwater flow using an integrated approach, combining field observations and modelling, with support from remote sensing data, to advance our understanding of the Greenland Ice Sheet’s future mass changes within the context of projected climate change.