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National Stormwater Calculator

Tool to help control runoff and promote the natural movement of water

EPA’s National Stormwater Calculator (SWC) is a software application that estimates the annual amount of rainwater and frequency of runoff from a specific site. Estimates are based on local soil conditions, land cover, and historic rainfall records. Users supply information about the site’s land cover and then select the low impact development (LID) controls they would like to use. The LID controls include seven green infrastructure practices. The SWC is designed to be used by anyone interested in reducing runoff from a property, including site developers, landscape architects, urban planners, and homeowners.

Fact Sheet: National Stormwater Calculator
 

Software Downloads, Platform, and User's Guides

The SWC is available as a mobile web-based application or as a desktop program—both versions require an internet connection. The mobile web-based app can be used on desktop devices and mobile devices, such as smartphones and tablets, and is compatible with all operating systems. It works best with the following browsers: Microsoft Edge, Google Chrome, Mozilla Firefox, and Apple Safari. The Windows-based desktop program runs on any version of Microsoft Windows with Version 4 or higher of the .Net Framework installed.

Mobile Version

Date Title
4/5/2019 Version is undergoing maintenance and is not available at this time. We apologize for any inconvenience.

Desktop Version

Date Title 
4/10/2019 National Stormwater Calculator Version 1.2.0.3 (ZIP)(50 MB, 04/10/19)
  1. If you have a previous version installed, you must uninstall it before installing the update.                 
  2. Download the ZIP file and move it to a desired folder and then unzip the file.
  3. Right click on 'StormwaterCalculator.exe' and then select 'Create Shortcut'.
  4. Drag the shortcut onto your desktop and use it to launch the software.
  5. For installation problems, contact your system administrator.                                                           

User's Guides

Date Title
4/10/2019

National Stormwater Calculator User's Guide-Desktop Version 1.2.0.3                                                                  

Disclaimer: Any mention of trade names, manufacturers, or products does not imply an endorsement by EPA. EPA and its employees do not endorse any commercial products, services, or enterprises.

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Modeling Capabilities

Hydrology Analysis. The SWC allows users to analyze site hydrology for small- to medium-sized (less than 12 acres) locations within the United States, including Puerto Rico, using LID controls. It estimates the amount of stormwater runoff generated from a site under different development and control scenarios over a long-term period of historical rainfall.

Cost Module. An LID cost estimation module within the application allows planners and managers to evaluate LID controls based on comparison of regional and national project planning level cost estimates (capital and average annual maintenance) and predicted LID control performance. Cost estimation is accomplished based on user-identified size configuration of the LID control infrastructure and other key project and site-specific variables. This includes whether the project is being applied as part of new development or redevelopment and if there are existing site constraints.

Climate Scenarios. The SWC allows users to consider how runoff may vary based on historical weather and potential future climate conditions. To better inform decisions, it is recommended that users develop a range of results with various assumptions about model inputs. Please check with local authorities about whether and how use of these tools may support local stormwater management goals.

Note: The SWC uses the 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. The SWC accesses several national databases that provide soil, topography, rainfall, and evaporation information for a chosen site.

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Real-World Applications

The SWC is most appropriate for performing screening level analysis of small footprint sites up to several dozen acres in size with uniform soil conditions. Its primary focus is informing site developers and property owners on how well they can meet a desired stormwater retention target. It can be used to answer questions such as the following:

  • What is the largest daily rainfall amount that can be captured by a site in predevelopment, current, or post-development condition?
  • To what degree will rainfall from storms of different magnitudes be captured on site?
  • What mix of LID controls can be deployed to meet a given stormwater retention target?
  • How well will LID controls perform under future meteorological projections made by global climate change models?
  • What are the relative planning level costs (capital and maintenance) differences for various mixes of LID controls?

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Green Infrastructure as LID Controls

Green infrastructure practices, which are the LID controls used in the SWC, promote the natural movement of water, instead of allowing it to wash into streets and down storm drains. Having less water runoff into storm drains and roadways can help prevent contamination of waterways, infrastructure degradation, flooding, and overwhelming of treatment plants. This allows stormwater to be used as a resource rather than a waste product, and can add aesthetic and economic value to a community. The following green infrastructure practices are included in the SWC:

Rooftop downspout disconnectionRooftop (Downspout) Disconnection. This practice allows rooftop rainwater to discharge to pervious landscaped areas and lawns instead of directly into storm drains. It can be used to store stormwater and/or allow stormwater to infiltrate into the soil.


 

CisternRainwater Harvesting (Rain Barrels or Cisterns). Containers that collect roof runoff collect runoff from rooftops and convey it to a tank where it can be used for non-potable water uses and onsite infiltration. Cisterns may be located above or below ground and have a greater storage capacity than rain barrels.


 

SWMM rain gardenRain Gardens. Hallow depressions filled with an engineered soil mix that supports vegetative growth. They provide opportunity to store and infiltrate captured runoff and retain water for plant uptake. They are commonly used on individual home lots to capture roof runoff.


 

green roofGreen Roofs (also known as vegetated roofs). Bioretention systems placed on roof surfaces that capture and temporarily store rainwater in a soil medium. They consist of a layered system of roofing designed to support plant growth and retain water for plant uptake while preventing ponding on the roof surface.


 

Street PlanterStreet Planters. Consist of concrete boxes filled with an engineered soil that supports vegetative growth and are typically placed along sidewalks or parking areas. Beneath the soil is a gravel bed that provides additional storage as the captured runoff infiltrates into the existing soil below.


 

Infiltration basinInfiltration Basins. Shallow depressions filled with grass or other natural vegetation that capture runoff from adjoining areas and allow it to infiltrate into the soil. They provide storage volume and additional time for captured runoff to infiltrate the native soil below. 


 

Permeable PavementPorous Pavement. Excavated areas filled with gravel and paved over with a porous concrete or asphalt mix or with modular porous blocks. Normally all rainfall will immediately pass through the pavement into the gravel storage layer below it where it can infiltrate at natural rates into the site's native soil.

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Support

For comments or questions, email us at SWC@EPA.gov.

Resources

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