The sensitivity of Vatnaj¿kull 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 ~25% after 130 years. Ice cap thinning and retreat alters Vatnaj¿kull'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 Vatnaj¿kull (distal from the coast) appears to be the most robust to climate warming, while discharge from Vatnaj¿kull'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.