the Green Bay Lobe, only raising the conductivity values by three orders of magnitude would have allowed all basal melt to be evacuated as groundwater (Cutler et al., 2000). A comprehensive numerical study of the hydrology under the largest European ice cap, Vatnajokull, showed that buried aquifers may only evacuate up to ca. 30% of subglacial water (Flowers et al., 2003). Under Trapridge Glacier, Yukon, the aerial extent of aquifer saturation is 70-90%, with the groundwater drainage system at the edge of its capacity beneath the ablation zone (Flowers & Clarke, 2002b).

Also worth noting is that data from beneath the Whillan's Ice Stream (formerly Ice Stream B), Antarctica, indicate the substratum inefficiency in capturing basal meltwater. Lingle & Brown (1987) suggested that the subglacial water discharge mechanism is advection in the deforming layer, but in the light of a much thinner zone of deformation than originally assumed (Engelhardt & Kamb, 1998) this mechanism is not likely to be efficient. Subglacial ponding and localized ice decoupling are likely, among other effects, owing to the postulated upwards-directed groundwater flow in parts of the bed (Tulaczyk et al., 2000b). Likewise, marine-based ice sheets terminating on shelf sediments that tend to be fine-grained could not have discharged all the melt through the bed (Hindmarsh, 1999).

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