Losing Street Trees Increases Stormwater Runoff

Urban trees reduce stormwater runoff through interception (rain caught by the canopy), evapotranspiration (water moved from plants into the air) and infiltration (rain soaking into the soil), but their role has been largely overlooked. Early research relied on rough assumptions about canopy cover and design storms. A 2017 review by the Center for Watershed Protection found only six studies of how well urban trees cut runoff, three based on measurements from a single plot and three on models, and another 2017 review called for studies scaling the local effects of trees up to whole sewer catchments.

The experiment

The U.S. Geological Survey, with the U.S. Environmental Protection Agency, the U.S. Forest Service and the University of Wisconsin, measured what happens to stormwater when street trees are removed from a medium-density residential area. They compared rainfall and runoff in two similar neighborhoods in Fond du Lac, Wisconsin, from May through September in 2018 through 2020.

The city's aggressive removal of trees infested with emerald ash borer made the test possible. With street trees still standing, the researchers first established the relationship between runoff from the test and control neighborhoods. Then, in March 2020, 31 street trees were removed from the test neighborhood, eliminating 2,990 square meters of canopy over streets, driveways, sidewalks and lawns.

The control and test catchments in Fond du Lac, Wisconsin, in the Lake Michigan drainage basin. U.S. Geological Survey.

Umbrella or funnel?

Street trees usually grow in the open with little competition for water and light, so they develop large crowns and leaf areas, making them important parts of the urban water cycle. A canopy acts as an umbrella, catching rain until its leaves can hold no more; then water drips through or runs down branches and trunks. Branches can also funnel water from their "elbows" onto the pavement or ground below.

Light rain intercepted by a street tree's canopy in Fond du Lac. U.S. Geological Survey.

What changed

The study measured 134 warm-season storms of more than 0.5 millimeter over 15 months, 92 before the trees came down and 42 after. Runoff volume rose after the trees were removed. Breaking storms into size ranges showed where the change was statistically significant:

In those two significant ranges, runoff rose by 45 cubic meters and 153 cubic meters, a combined 198 cubic meters, equal to 4% of all runoff measured in the test neighborhood after the trees were removed. That means the lost canopy had been holding back about 66 liters of runoff per square meter over the 42 storms from May through September 2020, the combined effect of interception, evapotranspiration and infiltration.

Why it matters

The test neighborhood's street trees were maples and ashes, the two most common street trees in the Midwest. The measured benefit reflects only green ash, but by leaf area, the ash genus is a good average stand-in for the Midwest's common street trees. With an estimated 17 million trees at risk of removal in the central and eastern United States because of infestation, more stormwater is likely to reach streams and lakes as street canopy disappears. Knowing what tree canopy is worth for stormwater can help cities weigh the effects of removing and replanting street trees.

Sources

• U.S. Geological Survey, "Loss of street tree canopy increases stormwater runoff," Fact Sheet 2022–3074. https://pubs.er.usgs.gov/publication/fs20223074/full
• Full study: Selbig and others (2022). The fact sheet also cites the Center for Watershed Protection (2017), Kuehler and others (2017), Ma and others (2020), Xiao and McPherson (2002) and Kovacs and others (2010).
• The fact sheet's second table labels the larger significant range "2.46–25.15" mm; its first table and text give 12.46–25.15 mm, which this page uses.