Calculator estimates the annual amount of rainwater and frequency of runoff from any U.S. site
The U.S. Environmental Protection Agency’s (EPA) National Storm Water Calculator is a desktop application that estimates the annual amount of rainwater and frequency of runoff from a specific site anywhere in the U. S. 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
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 seven green infrastructure practices:
- 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. Green infrastructure also has the added benefit of beautifying neighborhoods and increasing property values.
The storm water calculator (SWC), like any model, estimates an outcome based on available information such as soil type, landscape and land-use information, and historical weather. These estimates can be affected by limitations on site-specific information and uncertainties about future climate. To better inform decisions, it is recommended that the user develop a range of results with various assumptions about model inputs such as percent of impervious surface, soil type, and sizing of green infrastructure. An update to the SWC, which will include the ability to link to several future climate scenarios, will be released by the end of 2013. Climate projections indicate that heavy precipitation events are very likely to become more frequent as the climate changes. Green infrastructure can increase the resiliency of storm water management approaches to a changing climate, and this update will allow users to consider how runoff may vary based both on historical weather and potential future climate. Check with local authorities about whether and how use of these tools may support local storm water 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.