EPA National Stormwater Calculator

The U.S. Environmental Protection Agency (EPA) National Stormwater Calculator (SWC) is a desktop application that estimates the annual amount of rainwater and frequency of runoff from a specific site anywhere in the United States (including Puerto Rico). The SWC is a simple to use tool for computing small site hydrology by estimating the amount of stormwater runoff generated from a site under different development and control scenarios over a long term period of historical rainfall. The analysis takes into account local soil conditions, slope, land cover and meteorology. The calculator’s primary focus is informing site developers and property owners on how well they can meet a desired stormwater retention target.

The Calculator accesses several national databases that provide soil, topography, rainfall, and evaporation information for the chosen site. The user supplies information about the site’s land cover and selects the types of LID controls they would like to use. The LID controls that the user can choose are the following seven green infrastructure practices:

  1. Disconnection
  2. Rain harvesting
  3. Rain gardens
  4. Green roofs
  5. Street planters
  6. Infiltration basins
  7. Porous pavement

The calculator is most appropriate for performing screening level analysis of small footprint sites up to several dozen acres in size with uniform soil conditions. The hydrological processes simulated by the calculator include evaporation of rainfall captured on vegetative surfaces or in surface depressions, infiltration losses into the soil, and overland surface flow.

The calculator uses the EPA Storm Water Management Model (SWMM) as its computational engine. SWMM is a well-established, EPA developed model that has seen continuous use and periodic updates for 40 years. Its hydrology component uses physically meaningful parameters making it especially well-suited for application on a nation-wide scale. SWMM is set up and run in the background without requiring any involvement of the user.

Directions to install the Stormwater Calculator are provided on the EPA website. The EPA’s 2014 National Stormwater Calculator User's Guide Version 1.1 is also linked, or can be found at: http://nepis.epa.gov/Adobe/PDF/P100LOB2.pdf

The guide discusses how to install the calculator, how to run it, and how to interpret its output. An example application is presented showing how the calculator can be used to analyze questions related to stormwater runoff, retention, and control. Also, a technical description is given of how the calculator performs its computations and where it obtains the parameters needed to do so.

 If you have any trouble accessing the website link above, please find here an archived page. You may find this has limited use. https://web.archive.org/web/20170125195415/https://www.epa.gov/water-research/national-stormwater-calculator 


Publication Date: 2014

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Resource Types:

  • Modeling tool
  • Tool (general)


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