US EPA - Environmental Protection Agency

National Stormwater Calculator was updated on January 29, 2014

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SWC was updated on January 29, 2014. To get the latest, please load Version 1.1.

EPA’s 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). Estimates are based on local soil conditions, land cover, and historic rainfall records.

It is designed to be used by anyone interested in reducing runoff from a property, including

  • site developers,
  • landscape architects,
  • urban planners, and
  • homeowners.

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 low impact development (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

Green infrastructure promotes the natural movement of water, instead of allowing it to wash into streets and down storm drains. Green infrastructure also has the added benefit of beautifying neighborhoods and increasing property values.

The SWC estimates runoff at a site based on available information such as soil type, landscape and land-use information, and weather. This update to the SWC will allow users to consider how runoff may vary based on historical weather and potential future climate. To better inform decisions, it is recommended that the user develop a range of SWC results with various assumptions about model inputs such as percent of impervious surface, soil type, sizing of green infrastructure, as well as historical weather and future climate change scenarios. Please check with local authorities about whether and how use of these tools may support local stormwater management goals and requirements.

Clean water is essential to keeping our families and the environment healthy. The Calculator helps protect and restore the environmental integrity of our waterways.

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