EPA’s National Stormwater Calculator is a downloadable application (requires a computer running Microsoft© Windows operating system) for estimating 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. A climate change component is included to help users explore how climate change impacts may affect the amount of stormwater runoff produced by a site, and how it is managed. The tool 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 green infrastructure controls they would like to use. The user can choose the following seven green infrastructure practices:

• Disconnection
• Rain harvesting
• Rain gardens
• Green roofs
• Street planters
• Infiltration basins
• Porous pavement

Green infrastructure promotes the natural movement of water, instead of allowing it to wash into streets and down storm drains. The Calculator also allows users to consider how runoff may vary based on historical weather and potential future climate.