The conceptual view of runoff used by SWMM is illustrated in the figure below.
Each subcatchment surface is treated as a non-linear reservoir. The inflow comes from precipitation and runoff from any designated upstream Subcatchments. The outputs consist of infiltration, evaporation and surface runoff. The capacity of this "reservoir" is maximum depression storage, which is the maximum surface storage provided by waterlogging, surface wetting, and interception. Runoff, Q, occurs only when the water depth d in the "reservoir" exceeds the storage maximum depression, ds, in which case runoff is given by Manning's equation. The water depth over the subcatchment ( d) is continuously updated over time, numerically solving a water balance equation over the subcatchment.