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Flow resistance in subglacial conduits regulates the basal water pressure and sliding speeds of glaciers by controlling drainage efficiency and conduit enlargement and closure. Flow dynamics within subglacial conduits, however, remain poorly understood due to limited accessibility. Here we report the results of the first computational fluid dynamics simulations of flow within a realistic subglacial conduit beneath Hansbreen, a polythermal glacier in Svalbard, Norway. The simulated friction factor is 2.34 +/- 0.05, which is around 5 to 230 times greater than values (0.01-0.5) commonly used in glacier hydrological modeling studies. Head losses from sinuosity and cross-sectional variations dominate flow resistance (similar to 94%), whereas surface roughness from rocks and ice features contributes only a small portion (similar to 6%). Most glacier hydrology models neglect head losses due to sinuosity and cross-sectional variations and thus severely underestimate flow resistance, overestimating the conduit peak effective pressure by 2 times and underestimating the conduit enlargement area by 3.4 times, respectively.

Plain Language Summary Subglacial conduits drain meltwater from polar ice sheets, thus directly regulating the ice sheet sliding speed through basal flow resistance and water pressure inside the conduits. Despite their importance, our understanding of subglacial conduits is extremely limited due to difficulties of observing them and their interiors with either remote sensing or in situ exploration. Simplified models have been proposed for the hydraulics inside these conduits. A key problem in these models is the lack of scientific support in parameterizing the flow resistance. Currently, the resistance is parameterized but has not been validated due to the accessibility issues. To narrow this knowledge gap, we performed three-dimensional computational fluid dynamics simulations based on a millimeter-scale resolution model of an actual subglacial conduit in the Arctic. For the first time we give a direct and physics-based estimation of the flow resistance in an actual subglacial conduit and highlight the important contributions of the cross-sectional variations and longitudinal sinuosity. We further demonstrate the impacts of our simulated flow resistance on subglacial hydrodynamics and ice sheet dynamics.