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Stormwater Calculator Helps Communities Take Action to Reduce Runoff

Updated EPA tool incorporates climate change information to help planners assess green infrastructure for reducing runoff.


Car driving through minor flooding in a city after a storm

Stormwater, particularly from urban and suburban areas, is responsible for flooding, combined sewer overflows, reduced groundwater recharge, and increased pollution reaching the nation's waterways. Incorporating green infrastructure into new development is one way to combat all these issues. Photo Attribution: Anton Oparin / Shutterstock.com

In any downpour, it’s easy to causally observe the difference between a parking lot and a rain garden when it comes to stormwater runoff.  On the pavement, continued rain quickly begins to pool, flow, and in big storms, eventually cascades into adjacent streets and local storm drains. Heavy rains falling on rain gardens or other more natural areas, on the other hand, soak in.

Tapping the natural ability of what watershed managers and others refer to as “green infrastructure” is becoming an increasingly attractive way to reduce the amount of stormwater runoff, along with the pollution it carries, from reaching the nation’s waterways. These types of efficient, economical, and innovative solutions are expected to become increasingly more important in the face of a changing climate, which will spark bigger, more intense rainstorms and other weather systems. 

As part of President Obama’s Climate Change Action Plan, EPA researchers developed and released the National Stormwater Calculator and Climate Assessment tool. The tool is a resource to help city planners, developers, property owners, and others plan for and take action to reduce stormwater runoff. It will help such decision-makers assess the impact of incorporating green infrastructure features such as rain gardens, rain barrels, cisterns and open parks into their projects to prevent current and future challenges associated with stormwater runoff.

The easy-to-use Stormwater Calculator provides estimates of the annual amount and frequency of stormwater runoff from a specific site, based on local soil conditions, land cover, and historical rainfall records.  Users can input any location within the U.S. and select different development and green infrastructure scenarios to see how those changes affect runoff volumes of that particular location.

In January, 2014 EPA released a major update to the tool to include future climate vulnerability scenarios. “Climate change threatens our health, our economy, and our environment,” said EPA Administrator Gina McCarthy when announcing the release of the update. “As part of the President’s Climate Action Plan, this tool will help us better prepare for climate impacts by helping build safer, sustainable, and more resilient water infrastructure”

The calculator now includes climate models that users can incorporate into their calculations, helping them to plan for the resiliency they will need in the future.  

As an example of how the calculator might be used, consider a developer or municipality planning to build a parking lot on what is currently vacant land. When they plug in the location and add data about the parking lot, they may find that runoff volumes greatly increase, perhaps even exceeding local ordinances. They may then decide to trade some parking spots for green space, incorporate permeable pavement into the design, and add rain gardens along the periphery. Running this scenario through the Calculator may reveal that runoff volumes are reduced with those changes, meeting local standards.

By considering how a changing climate might mean the need for more such features in the future, they can incorporate that in their current plans instead of risking flooding, expensive retrofits, or other problems in the near future.

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