Environmental Calculators and Models

Policy makers need to be able to model environmental impacts, from climate change to floodplain management, and beyond. Models are used to simulate different environmental conditions based on data collected through observation and research and can be used to direct many municipal policy decisions.

These models are not infalliable, but are updated as new information becomes available, and are only presented as a guide to better inform the public and policy makers. WSU Puyallup Research and Extension Center does not endorse nor does it qualify the data used in any of these models.

EPA National Stormwater Calculator

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.

EPA Announces Climate Adjustment Tool for the Storm Water Management Model

EPA's Storm Water Management Model (SWMM) is widely used throughout the world and is considered the "gold standard" in the design of urban wet-weather flow pollution abatement approaches. It is a dynamic hydrology-hydraulic water quality simulation model used for single event or long-term (continuous) simulation of runoff quantity and quality from primarily urban areas, and allows users to include any combination of low impact development (LID)/green infrastructure controls to determine their effectiveness in managing stormwater and sewer overflows. The new Climate Adjustment Tool (SWMM-CAT) is a simple to use software utility that applies monthly climate adjustment factors onto historical precipitation and temperature data to consider potential impacts of future climate on stormwater.
Click here to learn more and access the tool.
Click here to download the SWMM-CAT user's guide.

GSI Calculator Software

GSI-Calc was developed through a Washington State Department of Ecology Grant of Regional or Statewide Significance and administered by Kitsap County. GSI-Calc allows sizing of LID BMPs, or “green stormwater infrastructure” (GSI), as a function of contributing impervious area, prevalent soil types in the region, representative site infiltration rates, and mean annual precipitation.


AgWeatherNet provides access to current and historical weather data from Washington State University's automated weather station network along with a range of models and decision aids. This includes 151 automated weather stations located mostly in the irrigated regions of eastern Washington state. The weather data, advisories, weather data products and decision support systems provided by AgWeatherNet and WSU can help improve production and product quality, optimize resource use and reduce environmental impact.

Updated EPA Low Erosivity Waiver (LEW) Calculator

EPA has published an updated calculator to help construction sites calculate their rainfall erosivity factor (“R” in the Revised Universal Soil Loss Equation, or RUSLE) at http://water.epa.gov/polwaste/npdes/stormwater/Welcome-to-the-Rainfall-Erosivity-Factor-Calculator.cfm. The updated calculator uses data from the Natural Resources Conservation Service’s (NRCS) Revised Universal Soil Loss Equation, Version 2 (RUSLE2) database. Many NPDES permitting authorities waive stormwater permitting requirements for small construction sites with a low erosivity potential. If allowed by the NPDES permitting authority, this calculator can be used by small construction sites to determine their eligibility for a low erosivity waiver (LEW). Operators of small construction sites in areas where EPA is the NPDES permitting authority qualify for the waiver if the “R” factor is less than 5 during the period of construction activity. For more information about the construction rainfall erosivity waiver, see http://water.epa.gov/polwaste/npdes/stormwater/upload/fact3-1.pdf.

Urban Water Blueprint map from the Nature Conservancy

Ever wondered where urban areas all over the world receive their water? This interactive map allows you to explore the origins of drinking water for cities all over the globe. While aquifers are not heavily impacted by stormwater, surface sources such as rivers, lake, and reservoirs can be.

To learn more about the map, visit the Nature Conservancy website.

iTree Hydro (BETA)

i-Tree Hydro is designed for users interested in watershed scale analyses of vegetation and impervious cover effects on hydrology. The most recent version of Hydro provides users with several key improvements including a more user-friendly interface with integrated help, executive summary reporting and qualitative modeling capabilities at the city scale.