Abstract
Nearly all meltwater from glaciers and ice sheets is routed englacially through moulins, which collectively comprise approximately 10–14 % of the efficient englacialâsubglacial hydrologic system. Therefore, the geometry and evolution of moulins has the potential to influence subglacial water pressure variations, ice motion, and the runoff hydrograph delivered to the ocean. We develop the Moulin Shape (MouSh) model, a time-evolving model of moulin geometry. MouSh models ice deformation around a moulin using both viscous and elastic rheologies and models melting within the moulin through heat dissipation from turbulent water flow, both above and below the water line. We force MouSh with idealized and realistic surface melt inputs. Our results show that variations in surface melt change the geometry of a moulin by approximately 30 % daily and by over 100 % seasonally. These size variations cause observable differences in moulin water storage capacity, moulin water levels, and subglacial channel size compared to a static, cylindrical moulin. Our results suggest that moulins are significant storage reservoirs for meltwater, with storage capacity and water levels varying over multiple timescales. Representing moulin geometry within subglacial hydrologic models would therefore improve their accuracy, especially over seasonal periods or in regions where overburden pressures are high.
Highlights
Surface-sourced meltwater delivered to the glacier bed influences the evolution of the subglacial hydrologic system and associated subglacial pressures (e.g., Iken and Bindschadler, 1986; Müller and Iken, 1973)
These lengths are not insignificant and suggest that moulin geometry and evolution may be important to subglacial processes
In contrast to the above parameters, we find that moulin geometry is strongly sensitive to the choice of basal ice softness and the friction factors used within the moulin
Summary
Surface-sourced meltwater delivered to the glacier bed influences the evolution of the subglacial hydrologic system and associated subglacial pressures (e.g., Iken and Bindschadler, 1986; Müller and Iken, 1973). The efficiency of the subglacial 25 system, in turn, changes the flow patterns of the overlying ice on daily, seasonal, and multi-annual timescales (e.g., Hoffman et al, 2011; Iken and Bindschadler, 1986; Moon et al, 2014; Tedstone et al, 2015; Williams et al, 2020). Glacial hydrology is a crucial factor in short-term calculations of mass loss on glaciers and ice sheets (Bell, 2008; Flowers, 2018). On the Greenland Ice Sheet, surface meltwater can take multiple paths, depending on its spatial origin. Meltwater may percolate through snow and firn, remaining liquid (Forster et al, 2014) or refreezing (MacFerrin et al, 2019). 30 In the ablation zone, meltwater runs over bare ice, coalesces into supraglacial streams, and pools into supraglacial lakes
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