Figure 1. Conceptual model of three-component subglacial hydrologic system for the GrIS.

(a) Onset of the melt season: a large fraction of the bed is composed of weakly connected cavities at a higher water pressure than the surrounding distributed system. (b) Middle of melt season: meltwater draining from the surface through moulins is largely accommodated by the formation of efficient channels (dashed grey outline). Concurrently, some of the weakly connected cavities have leaked water, lowering their water pressure, due to increasing connectivity with the rest of the system initiated during periods of pressurization. (c) End of melt season: channels collapse within days after melt inputs cease, but the partially drained weakly connected cavities take months to recharge by basal melting, leaving higher integrated basal traction than before summer began.