SWMM is a distributed model designed to intricately analyze runoff generation within diverse landscapes. This involves subdividing study areas into irregular subcatchments to effectively account for spatial variations in topography, drainage patterns, land cover, and soil attributes. Each subcatchment is visualized as a uniform rectangular surface with width (W), a consistent slope, and a singular outlet channel for drainage.

The subcatchment concept further breaks down into three subareas: impervious areas with depression storage, impervious areas without depression storage, and pervious areas with depression storage.

A set of input parameters in SWMM shapes the hydrologic characteristics of these subcatchments:

  • Area: The bounded region of a subcatchment is derived from maps, field surveys, or Update Geometry Parameters if the shapefile is drawn to scale.
  • Width: Calculated as the subcatchment’s area divided by the longest overland flow path length. This method avoids channelized flow inclusion.
  • Slope: Uniform across both pervious and impervious surfaces, defining overland flow paths’ characteristics.
  • Imperviousness: The percentage of subcatchment covered by impermeable surfaces, ranging from 5% in undeveloped areas to 95% in high-density commercial zones.
  • Roughness Coefficient: Reflecting overland flow resistance, it corresponds to Manning’s roughness coefficient n, with separate values for impervious and pervious sections.
  • Depression Storage: Represents initial abstractions like surface ponding, vegetation interception, and surface wetting. Varies for pervious and impervious areas, generally ranging from 0.05 to 0.3 inches.
  • Percent of Impervious Area Without Depression Storage: Accounts for immediate runoff before depression storage is satisfied, often set at 25% by default.
  • Infiltration Model: SWMM offers three methods (Horton, Green-Ampt, Curve Number) to calculate infiltration loss on pervious areas. The Horton model is used here, with parameters including maximum and minimum infiltration rates and decay coefficient.
  • Precipitation Input: The central influencing factor in simulating the quantity of rainfall-runoff is precipitation. The amount and speed of stormwater runoff are intrinsically tied to the size of the precipitation and its temporal and spatial distribution over the catchment. Every subcatchment within SWMM is connected to a Rain Gage element, detailing the manner and origin of the rainfall data input for that specific subcatchment.