Around the Water Cooler: Measure Runoff with EPA’s Stormwater Calculator

By Lahne Mattas-Curry

It’s been raining for what seems like weeks straight this summer. Each day as I leave the office, it’s not unusual for the skies to open up and let loose a torrential downpour. I have watched many people struggle to find their umbrellas in their bags or skip over the water pooling around street corners while running to the metro. While Washington, D.C., is one of the most beautiful cities in the country, when it rains, you just can’t escape the water flowing rapidly across the pavement and other impervious surfaces that make up our nation’s capital. Interested in water research, I wondered, “How much water actually runs off into the Potomac and the Anacostia rivers during each rainfall?”

Now, thanks to EPA scientist Lew Rossman, we can measure runoff amounts for specific locations. After answering some questions about a particular site, such as percent of impervious surfaces and soil composition, Rossman’s National Stormwater Calculator can estimate the amount of runoff and inform decisions about how to reduce runoff. The Calculator is a tool that can help developers, urban planners, landscapers, and other professionals determine what green infrastructure elements could best reduce the runoff.

Adding green infrastructure (we’ve talked a lot about green infrastructure here and here) is both environmentally and economically beneficial. From trees and plants to green roofs, rain barrels, and cisterns, these changes can help decrease the amount of pollutants threatening our waterways. With heavy rains increasing and continued development, runoff has become one of the fastest growing sources of water pollution around the country.

The Calculator is just phase I of the Stormwater Calculator and Climate Assessment Tool package announced in the President’s Climate Action Plan in June. An update to the Calculator will be released at the end of this year that links to several future climate scenarios.

You can access the National Stormwater Calculator here:

About the Author: Lahne Mattas-Curry works with EPA’s Safe and Sustainable Water Resources Program and is a frequent “Around the Water Cooler” contributor. Besides playing in puddles after a rain, she spends a lot of time adding plants to her rain garden to reduce the runoff, and quite frankly, add beauty and value, to her own property.


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