Abstract. The Greenland Ice Sheet is losing mass, part of which is caused by increasing runoff. The location of the runoff limit, the highest elevation from which meltwater finds its way off the ice sheet, plays an important role in the surface mass balance of the ice sheet. The recently observed rise in runoff area might be related to an increasing amount of refreezing: ice layer development in the firn hinders vertical percolation and promotes lateral runoff. To investigate meltwater flow near the runoff limit in the accumulation zone on the southwest Greenland Ice Sheet, we carried out in situ measurements of hydrological processes and properties of firn and snow. The hydraulic conductivity of icy firn in pre-melt conditions measured using a portable lysimeter ranges from 0.17 to 12.8 m hr-1, with flow predominantly occurring through preferential flow fingers. Lateral flow velocities of meltwater on top of the near-surface ice slab at the peak of the melt season measured by salt dilution- and tracer experiments range from 1.3 to 15.1 m hr-1. With 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 could be up to 4 km in regions where near-surface ice slabs are present. These measurements are a first step towards an integrated set of hydrological properties of firn on the SW Greenland Ice Sheet, and show evidence that meltwater runoff might occur from elevations above the visible runoff area.
Abstract. The Greenland Ice Sheet is losing mass, part of which is caused by increasing runoff. The location of the runoff limit, the highest elevation from which meltwater finds its way off the ice sheet, plays an important role in the surface mass balance of the ice sheet. The recently observed rise in runoff area might be related to an increasing amount of refreezing: ice layer development in the firn reduces vertical percolation and promotes lateral runoff. To investigate meltwater flow near the runoff limit in the accumulation zone on the southwestern Greenland Ice Sheet, we carried out in situ measurements of hydrological processes and properties of firn and snow. 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 the 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. With 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 could be roughly 4 km in regions where near-surface ice slabs are present. These measurements are a first step towards an integrated set of hydrological properties of firn on the SW Greenland Ice Sheet and show evidence that meltwater runoff may occur from elevations above the visible runoff area.
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<p>The Greenland ice sheet is losing mass. Thereby, the location of the runoff limit, the highest elevation from which meltwater finds its way off the ice sheet, plays an important role. Above the runoff limit all meltwater refreezes and does not contribute to mass loss. In recent years surface runoff has increasingly occurred from higher elevations, thereby expanding the area of mass loss: between 1985 and 2020, the maximum runoff limit rose by on average 194 metres, expanding the visible runoff area by around 29%.</p> <p>The observed rise in the runoff limit might be related to intensive meltwater refreezing within the firn which leads to the formation of thick ice layers, also called ice slabs. Our field experiments, carried out at around 1750 m a.s.l. on the K-Transect, have shown that meltwater generated over ice slabs is generally forced to flow laterally: initially through a near-surface slush matrix and then forming streams and rivers. It remains unclear, however, how much of the meltwater contributes to runoff, and which percentage refreezes and contributes to ice slab formation or expansion.</p> <p>Here we present a conceptual quasi 2D-model of runoff, that simulates lateral meltwater flow on top of an ice slab using firn hydrological properties measured on the southwest Greenland ice sheet. We adapted a gridded linear-reservoir runoff routing model to calculate (i) the distance meltwater can travel within one melt season, and (ii) when meltwater breakthrough at the snow surface (i.e. slush formation) occurs. First results provide insight into the evolution of the water table height over time that matches observations made during our summer field campaign. We are exploring ways to incorporate meltwater refreezing, to better understand ice slab evolution and their impact on the fate of meltwater between vertical percolation, refreezing and lateral runoff.</p>
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