Subglacial models represent moulins as cylinders or cones, but field
observations suggest the upper part of moulins in the Greenland Ice
Sheet have more complex shapes. These more complex shapes should cause
englacial water storage within moulins to vary as a function of depth, a
relationship not currently accounted for in models. Here, we use a
coupled englacial--subglacial conduit model to explore how moulin shape
affects depth-dependent moulin water storage and water pressure dynamics
within a subglacial channel. We simulate seven different moulin shapes
across a range of moulin sizes. We find that the englacial storage
capacity at the water level is the main control over the daily water
level oscillation range and that depth-varying changes in englacial
water storage control the temporal shape of this oscillation. Further,
the cross-sectional area of the moulin within the daily oscillation
range, but not above or below this range, controls pressures within the
connected subglacial channel. Specifically, large cross-sectional areas
can dampen daily to weekly oscillations that occur in the surface
meltwater supply. Our findings suggest that further knowledge of the
shape of moulins around the equilibrium water level would improve
englacial storage parameterization in subglacial hydrological models and
aid predictions of hydro-dynamic coupling.