Trees help protect water quality by capturing, storing and using rainfall. This reduces the amount of runoff that carries pollution off of the landscape and into nearby rivers and lakes. It also decreases the rate and volume of stormwater flowing through local stormsewers.
A 2005 study found that in Minneapolis, a single tree intercepts an average of 1,685 gallons of rainwater each year. Multiply this effect by the city’s estimated 200,000 street trees, 400,000 park trees and hundreds of thousands of privately owned trees, and you’ll get a sense of how much water is intercepted by the urban tree canopy — and that’s just in one city.
A healthy and robust tree canopy is especially important in an urban environment, where trees provide other environmental and health benefits. These include filtering air pollutants, reducing the urban heat island effect, helping heat and cool buildings, and improving residents’ physical and mental health.
Here’s a more detailed explanation of how trees mitigate stormwater runoff:
Interception — Rain falls on the tree’s leaves, branches and trunk. Some of this is absorbed by the tree, some evaporates back into the atmosphere, and the rest falls through to the ground — but at a much slower rate than it would otherwise. This helps reduce “peak flows” during rain events and also helps prevent soil erosion.
Infiltration — Water that falls through the tree canopy soaks into the ground and gets absorbed by the tree’s roots. By soaking up water from the ground, trees add capacity for the soil to store even more stormwater. As they grow, the roots also help break up compacted soil, which allows water to more easily soak downward into the groundwater table.
Evapotranspiration — Trees act like pumps, drawing water out of the ground and releasing it back into the atmosphere in the form of water vapor. This process is called evapotranspiration. It is a key part of the water cycle, and has a side benefit of helping to cool the air and reduce high temperatures in the summer.