By Jay Landers

Long seen as a nuisance to be gotten rid of as quickly as possible, urban stormwater increasingly is being viewed as a resource. Although the concept of capturing and using urban stormwater has gained attention in recent years, the full promise of this approach is far from being realized.

This is the assessment of a new report released in early March by global water think tank the Pacific Institute and 2NDNATURE Software Inc., a provider of stormwater management software. Titled Untapped Potential: An Assessment of Urban Stormwater Runoff Potential in the United States, the report finds that U.S. cities, particularly those in coastal regions, could benefit in multiple ways by making greater use of stormwater.

Massive volume

Nationwide, a massive volume of precipitation becomes urban runoff in the U.S. On average, the annual volumetric potential for urban stormwater runoff totals 59.5 million acre-feet per year, equal to 53.1 billion gallons per day, according to the report.

“This volume is equivalent to 93% of the water withdrawals for municipal and industrial uses in 2015, though not all of this runoff is necessarily feasible or desirable to capture,” the report states. That said, the finding “suggests that stormwater capture can make a meaningful contribution to augment and diversify water supplies in more communities than are currently considering it,” according to the report.

Further reading

This estimated total volume “represents all potential stormwater runoff generated from impervious surfaces based on average annual rainfall conditions, without consideration for method of capture or existing uses of stormwater,” according to the report.

Developing the estimate

To arrive at their estimate, the researchers used the Tool to Estimate Load Reduction – an urban runoff and pollutant loading model from 2NDNATURE – to develop spatial estimates of annual stormwater runoff potential for more than 2,600 U.S. cities. “The model leverages simplified analytical techniques and a fully spatially distributed approach using 30-m grid resolution to produce meaningful estimates of stormwater runoff generation,” according to the report.

Key inputs used for the modeling process included interpolated daily historical precipitation data records from 1981 to 2019, hydrologic soil group classifications from the U.S. Natural Resource Conservation Service, and data regarding U.S. land cover and imperviousness from the U.S. Geological Survey’s National Land Cover Database.

Using the data, the TELR model derived annual average precipitation statistics and estimated average annual runoff volumes for each 30-meter grid within urban areas. These volumes were used to estimate runoff volumes for individual urban areas and then spatially aggregated to various scales, including county boundaries, congressional districts, state boundaries, and subbasins, as defined by hydrologic unit code in the USGS’s Watershed Boundary Dataset.

Numbers speak loudly

Across the U.S., the mean stormwater runoff potential for all urban areas is 22,800 acre-feet per year, while the median is 3,900 acre-feet, according to the report. As for individual metropolitan regions, the New York City metro area, which includes portions of New Jersey, was found to have the highest runoff potential with 2.38 million acre-feet per year, followed closely by Houston (2.37 million acre-feet). Chicago and Dallas tied for third (1.62 million acre-feet), followed by Atlanta (1.45 million acre-feet), Philadelphia (1.22 million acre-feet), and Boston (1.01 million acre-feet).

Among states, Texas has the highest estimated annual urban runoff potential at 7.8 million acre-feet, followed by Florida (4.12 million acre-feet), Georgia (2.77 million acre-feet), Louisiana (2.61 million acre-feet), and Ohio (2.5 million acre-feet).

“The results of our analysis clearly show that there’s a vast opportunity to support water resilience in communities by implementing more stormwater capture to supplement water supplies and help offset the negative impacts of urban stormwater runoff,” said Bruk Berhanu, Ph.D., a senior researcher for the Pacific Institute, during a March 19 webinar held by the institute to discuss the report’s findings.

“We need to elevate the role of stormwater capture across the country as part of the toolbox to help alleviate the increasing stress in our water supplies,” Berhanu said.

Coastal basins ideal

Urban areas within coastal subbasins offer ideal locations for capturing and using more stormwater. An estimated 21.9 million acre-feet per year of urban runoff, or about 37% of the national total, occurs in coastal areas, the report found.

Despite generating more than one-third of the national stormwater runoff potential, coastal areas occupy a mere 12% of the total urban land area across the country, Berhanu said. “That is an outsized impact in terms of the amount of stormwater being generated,” he noted.

But what distinguishes such areas further in terms of stormwater capture is that they “typically aren’t constrained by the same requirements for downstream water users as their more inland counterparts,” Berhanu said. For this reason, coastal areas “could have an easier time implementing stormwater capture,” he said.

Whether they are in coastal areas or not, cities interested in pursuing stormwater capture and use need to analyze their local situation carefully, Berhanu said. “The next steps would be for communities to build on these results and to determine how much stormwater capture is feasible for them,” he said.

“That involves looking at local context and considering factors such as potential impacts on downstream water users, including the environment; local stormwater management requirements; allowable uses for stormwater; the specific method of capture that would be the most appropriate and its associated costs; and the potential to realize other co-benefits,” Berhanu said.

Barriers to overcome

Although stormwater offers a readily available water supply in many urban areas, various limitations and challenges can prevent its capture and use. In the American West, the biggest barrier often involves water rights, said Nathan Campeau, P.E., a vice president and senior water resources engineer for Barr Engineering Co. and a member of the project advisory group for the report, during the March 19 webinar. “There are regulatory and legal barriers to using what might be perceived or what legally is somebody else’s water,” Campeau said.

Another common challenge concerns the need for coordination among multiple agencies when attempting to establish programs to capture and use stormwater, Campeau said. “There’s so many agencies that get involved with water in every community.” And the state and local entities often have overlapping jurisdictions. Operations and maintenance of stormwater capture systems also pose challenges, Campeau said. “It’s easier to build it,” he noted. “It’s a lot harder to take care of in the long term.”

Stormwater capture programs must also frequently contend with challenges related to “misaligned” timing between when runoff is available for capture and when demand exists for its use, says Seth Brown, Ph.D., P.E., the executive director of the National Municipal Stormwater Alliance. An alliance of state and local groups that focuses on issues pertaining to municipal sanitary storm sewer systems, NMSA did not participate in the development of the report. “There is just a practical limitation to how much you can store for a certain amount of time,” Brown says.

Funding, guidance needed

The report includes multiple recommendations for promoting the capture and use of stormwater. Among them is a call for states to “strongly consider establishing funding programs to support stormwater capture,” said Shannon Spurlock, a senior researcher for public policy and practice uptake at the Pacific Institute, during the March 19 webinar.

National guidelines for stormwater capture also are needed, the report states. “Currently, a poorly defined patchwork of state and local regulations is being applied to the authorization of stormwater capture projects, often on a case-by-case basis,” according to the report. “Federal agencies should continue to develop and disseminate national guidelines that frame the opportunities for water supply enhancement via stormwater capture and propose strategies for removing barriers to uptake.”

Other recommendations include greater use of regional approaches and interagency collaboration, ensuring that stormwater capture projects have equal access to funding provided by state revolving fund programs, efforts to facilitate public-private stormwater capture projects, and additional research to improve the efficacy of such projects.

“Supportive research can include the advancing of incorporating climate change impacts on precipitation using existing projections, delving deeper into regional and local constraints, and the inclusion of existing stormwater projects and the current volumes that are currently being captured,” according to Spurlock.

Ultimately, the report highlights the extent to which stormwater remains an untapped resource, Brown notes. “I think (stormwater capture) is going to be a rich area of growth in the future,” he says.

This article is published by Civil Engineering Online.