The majority of urban hydrological models focus primarily on differences in the rainfall-runoff ratio between pervious and impervious area. Impervious area (as I have mentioned in a previous post), is traditionally assumed to be the major causal source of changes in the hydrological cycle associated with urbanization. Infrastructure-centric urban hydrologic models assume that water that is infiltrated or evapotranspirated before reaching drains and pipes has “exited” the system. As more and more infiltration-based stormwater control measures are implemented however, this assumption needs to be more closely examined. Are urban soils and the urban subsurface truly effectively inexhaustible in capacity? This question is particularly important when we think about multiday rain events and changes in intensity of rainfall associated with climate change.