US Cities Could Be Capturing Billions of Gallons of Rain a Day

Your city is a scab on the landscape: sidewalks, roads, parking lots, rooftops—the built environment repels water into sewers and then into the environment. Urban planners have been doing it for centuries, treating stormwater as a nuisance to be diverted away as quickly as possible to avoid flooding. Not only is that a waste of free water, it’s an increasingly precarious strategy, as climate change worsens droughts but also supercharges storms, dumping ever more rainfall on impervious cities.

Urban areas in the United States generate an estimated 59.5 million acre-feet of stormwater runoff per year on average—equal to 53 billion gallons each day—according to a new report from the Pacific Institute, a nonprofit research group specializing in water. Over the course of the year, that equates to 93 percent of total municipal and industrial water use. American urban areas couldn’t feasibly capture all of that bountiful runoff, but a combination of smarter stormwater infrastructure and “sponge city” techniques like green spaces would make urban areas far more sustainable on a warming planet.

“There really is a substantial amount of stormwater runoff being generated all across the entire country,” says Bruk Berhanu, lead author of the report and a senior researcher at the Pacific Institute. “There really is no reason why stormwater capture shouldn’t be up there on the list of water sources for all communities in the country that are looking to secure their long-term supplies.”

The Pacific Institute did the calculation with the software company 2NDNATURE, which generated a high-resolution model of stormwater runoff for areas in the US with at least 2,000 housing units or 5,000 people. They combined characteristics of cities, such as the amount of impervious surface, with historical rainfall data.

In this map, blue signifies higher amounts of annual stormwater runoff from urban areas, while red is lower. States with relatively large amounts of precipitation and large urban areas, like Texas and Florida, are getting much more stormwater runoff than Montana and Idaho, where there’s less precipitation and less urban coverage. But even if it could, a given state wouldn’t want to capture every drop of stormwater falling on its cities, as rain also needs to replenish nearby rivers and lakes to sustain ecosystems.