Calculating the Impact

By Andy Dinsmore

What happens when all of the rain from a storm can’t soak into the ground on my property?  Where does it go?  Is that runoff causing any problems?

These are questions many people may be asking themselves.  In developed areas, that runoff likely enters into a storm sewer (a pipe) which outlets directly to a stream.  And too much runoff after a storm can cause severe damage to our streams, such as bank erosion and habitat loss.

Screen shot of Stormwater Calculator

Screen shot of Stormwater Calculator

But do I know how much runoff is coming from my property?  A new tool from EPA, the National Stormwater Calculator, has made it much easier to figure out.  It uses a simple, step-by-step process to assess a property (land use, soil features, and rainfall) and give an estimate for runoff.

I used the tool to evaluate my property and found it to be very user friendly.  I just entered my address and a window showed my property location.  Then I selected my soil type and landscape features using the incorporated links and overlays.  I gave my best estimates for the percentage of trees, grass, and impervious surfaces (my roof and driveway).  The calculator then showed me approximately how much runoff my property produces in a year.

The intent of the calculator is to compare current runoff estimates with reductions that would result from some simple practices, like redirecting your downspouts over your lawn, installing rain barrels, or making a rain garden.  More complex practices that may be used by industrial, commercial, or municipal properties are also available, such as curbside planters, green roofs, infiltration basins, and porous pavement.

I already have two rain barrels and a rain garden, so I ran the calculator twice.  The results of my two calculator runs (one without the rain barrels and rain garden and one with them) showed that I’m capturing an estimated 1.77 inches of runoff each year.  When I convert that to volume, it becomes 6,343 gallons of runoff that does not leave my property – enough to fill more than 100 average-sized bathtubs.  You can see the output from the calculator here.

So if you’ve asked yourself where your runoff goes, check out the calculator.  And then consider taking some basic steps to reduce your impact.

Maybe my next blog will have to be about how easy it is to build your own rain barrel and install it yourself.   But for now, be sure to check out our Regional Administrator Shawn Garvin’s recent blog about rain barrel art.

About the Author: Andy Dinsmore has been with EPA since 1997 and is currently the Mid-Atlantic Region’s Stormwater Team Leader, responsible for ensuring that regulated entities, such as construction sites, certain industrial facilities, and urbanized municipalities, properly manage their stormwater runoff and comply with their NPDES permits. Outside of the office, Andy enjoys gardening, playing racquetball, and spending time with his two children.

Editor's Note: The views expressed here are intended to explain EPA policy. They do not change anyone's rights or obligations. You may share this post. However, please do not change the title or the content, or remove EPA’s identity as the author. If you do make substantive changes, please do not attribute the edited title or content to EPA or the author.

EPA's official web site is Some links on this page may redirect users from the EPA website to specific content on a non-EPA, third-party site. In doing so, EPA is directing you only to the specific content referenced at the time of publication, not to any other content that may appear on the same webpage or elsewhere on the third-party site, or be added at a later date.

EPA is providing this link for informational purposes only. EPA cannot attest to the accuracy of non-EPA information provided by any third-party sites or any other linked site. EPA does not endorse any non-government websites, companies, internet applications or any policies or information expressed therein.