Using Urban Forests to Manage Stormwater Runoff

Researchers provide an easy method to estimate and compare urban tree impacts on stormwater

Retaining or returning urban forests to cities and suburbs can help mitigate stormwater runoff and prevent pollution. Photo by U.S. Forest Service.
Retaining or returning urban forests to cities and suburbs can help mitigate stormwater runoff and prevent pollution. Photo by USFS.

We’ve read a lot lately about the innumerable human health benefits provided by urban trees and forests. Urban forest systems can also function as part of a city’s stormwater control system by intercepting rainfall and regulating the flow of water to and through the soil. Forests efficiently store stormwater, return water to the atmosphere, and filter pollutants from runoff.

For an article published in the October issue of Stormwater magazine, U.S. Forest Service science delivery/technology specialist Eric Kuehler and co-author Aarin Teague,  an engineer with the San Antonio River Authority, review current research on trees and the urban water cycle, and provide a method practitioners can use to quickly estimate tree impacts on stormwater in planning engineered systems and best management practices for urban and suburban projects.

“Retaining forest systems during construction and returning forest structure to urban environments can help mitigate storm runoff and improve water quality,” says Kuehler, who works with the Forest Service Southern Research Station Integrating Human and Natural Systems unit in Athens, Georgia. “Identifying forest areas to retain before construction and actively conserving them is a good first step. Replacing canopy cover and forest systems where possible after construction can also help regulate runoff.”

As more municipalities move towards low-impact development (LID) and green stormwater practices intended to mimic predevelopment conditions, managers and design engineers need a better understanding of how to manage trees and urban forests to reduce stormwater runoff and water pollution. They also need a quick way to estimate tree impact on stormwater equated to the function of engineered systems

The article reviews findings from the latest research on topics such as the volume of rainfall retained by tree canopy; the impacts of leaf interception on stormwater runoff lag time, peak flow, and velocity; water volume removed from the soil through transpiration (the movement of water from the soil through the tree, then evaporated from leaves and stems); and nutrient uptake, which affects water quality.

The authors provide a method using tree canopy measures to quickly estimate the stormwater function a city's trees are providing. Photo by U.S. Forest Service.
The authors provide a method using tree canopy measures to quickly estimate the stormwater function a city’s trees are providing. Photo by USFS.

The authors also provide a method using tree canopy as a best management practice that allows designers and engineers to quickly estimate tree function in relation to engineered systems. The equations they provide can be used to roughly estimate the stormwater function of forest canopy preservation in terms of volume of water treated that can, in turn, be directly compared to the volume of water treated by an engineered structure.

“The method builds on established methods commonly used in the development design process, which can help improve its acceptance and use within the design community,” says Teague.

“The information in this article is designed to provide stormwater professionals with the basis they need for including urban forest systems in their stormwater management projects,” adds Kuehler.

Read the full text of the article.

For more information, email Eric Kuehler at ekuehler@fs.fed.us.

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