Digital elevation models of the bed and surface of Unteraargletscher, Switzerland, are used to reconstruct the theoretical pattern of basal water drainage for the years 1927, 1947, 1961 and 1997, ...during which period the glacier was thinning and receding. The theoretical drainage pattern for 1997 compares well, in a broad sense, with the locations of active moulins and the hydraulic connection status of boreholes drilled to the glacier bed. Changes in the basal water-flow pattern over the period 1927–97 that are revealed by the theoretical reconstructions of the subglacial drainage system structure are likely to have resulted from changes in glacier geometry. Concurrent with the retreat and thinning of the glacier, the height of medial moraines increased, probably due to the insulating effect of the debris cover reducing the melt of the underlying ice. This increase of moraine heights has led to the formation of hydraulic barriers at the glacier bed such that water flow has become channelized beneath the ice along drainage axes that parallel the course of the medial moraines on the glacier surface.
The sensitivity of Vatnajoekull ice cap to future climate change is examined using spatially distributed coupled models of ice dynamics and hydrology. We simulate the evolving ice cap geometry, mass ...balance, velocity structure, subglacial water pressures and fluxes, and basin runoff in response to perturbations to a 1961-1990 reference climatology. For a prescribed warming rate of 2'C per century, simulated ice cap area and volume are reduced by 12-15% and 18-25% within 100 years, respectively. Individual outlet glaciers experience 3-6 km of retreat in the first 100 years and a total retreat of 10-30 km over 200 years. For the same applied warming our results suggest a maximum increase in glacier-derived runoff of 625% after 130 years. Ice cap thinning and retreat alters Vatnajoekull's subglacial hydraulic catchment structure in the simulations, with up to several kilometers of local hydraulic divide migration. This serves to redistribute water among the major outlet rivers and, in extreme cases, to isolate river basins from glacially derived runoff. Glacier discharge from northern and northwestern Vatnajoekull (distal from the coast) appears to be the most robust to climate warming, while discharge from Vatnajoekull's southern margin (proximal to the coast) is particularly vulnerable. The latter reflects pronounced changes in the geometry of the southern outlet glaciers and has implications for glacier flood routing and frequency.
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