By Jenny Molloy
Most people have a vague recollection, perhaps from a brief fourth grade poetry unit, of the opening lines to Joyce Kilmer’s poem Trees: “I think that I shall never see a poem as lovely as a tree.” Less well remembered is Kilmer’s characterization of a tree as one “who intimately lives with rain.”
An often overlooked fact about trees is that they also do a great job of preventing and reducing stormwater runoff. Depending on the type of tree and the intensity of rain events, trees can intercept as much as 30% of total annual rainfall before it even reaches the ground.
When precipitation does reach the ground, trees’ extensive root systems drink it up. The transpiration rates of trees (or how much water evaporates from the trees) vary notably, but some of the thirstier species can transpire dozens or even hundreds of gallons per day.
That adds up. A New York City study estimated that one tree reduces stormwater runoff by 13,000 gallons per year. That means the 500,000 existing trees in the city reduce runoff by 6.5 billion gallons per year, and 300,000 new trees could remove another 3.9 billion gallons from the overburdened NYC sewer systems.
In Washington, D.C. a similar study estimated that simply using larger tree boxes could reduce annual stormwater runoff by 23 million gallons, and that increasing the use of trees could provide reductions of 269 million gallons per year.
Trees also provide other benefits: shading and cooling that ultimately provide energy savings; carbon sequestration; wildlife habitat; enhanced property values; air quality improvements; community health and safety. And of course, as Kilmer noted, there are aesthetic advantages as well.
Example output from the National Tree Benefit Calculator
Communities who do the math now see trees as a win-win in wet weather management. While trees require capital investment and maintenance, compared to other stormwater controls (which are costly to build and maintain but don’t provide benefits beyond stormwater), trees are often an obvious component of the solution.
The U.S. Forest Service public domain i-Tree family of tools now provides now standard approaches to quantifying the benefits. So for municipal planners, utility managers, regulators and anyone else with a role in controlling the consequences of wet weather, trees no longer need be considered supplemental or boutique elements. They are on the A-list of options.
You too can estimate the value of that oak or poplar in your yard with the National Tree Benefit Calculator: Enter your zip code, choose from a drop-down list of over 200 species, enter the tree diameter and voila! The calculator provides an estimate of overall annual value in dollars, and also breaks that down into specific benefits: stormwater, property value, electricity, natural gas, air quality and carbon dioxide.
Which brings to mind another childhood standard: Shel Silverstein’s The Giving Tree, an allegory illustrating not only the multiple benefits of trees, but also conveying that with a little care those benefits can be realized for a very long time.
Jenny Molloy has been working in Clean Water Act wet weather programs at the state and federal level for nearly 20 years, and has been at EPA for the last 9. She was EPA’s first Green Infrastructure coordinator, and just completed a 2-1/2 year detail to Region III and the Chesapeake Bay Program Office focusing on the stormwater permitting program. She’s an active member of her son’s high school band program just so she can fulfill a life-long dream of being able to say “I’m with the band”.