Urban Forest’s water retention, infiltration, and transpiration

NORTH CAROLINA – Raindrops that land on trees may never hit the ground. “Trees intercept a significant amount of rain,” says Forest Service science delivery specialist Eric Kuehler.

“Once intercepted by a leaf or branch, some rain evaporates, and some trickles down the tree’s stem and seeps into the soil. Soil is the best place to store water,” says Kuehler. “Trees slow water down and give it a chance to infiltrate into the soil.”

Trees are part of the water cycle and can be a very effective component of green infrastructure. However, civil engineers need numbers. How much rain do trees intercept, and how much is retained?

Kuehler and his colleagues surveyed the academic literature for answers. Their review study was published in the journal Ecohydrology.

“We looked for information on urban forests as a part of green infrastructure,” says Kuehler. “We also looked for research on three major components of the system: water retention, infiltration into the soil, and transpiration out of the soil.”

Together, retention, infiltration, and transpiration can mitigate up to seven percent of a city’s annual stormwater runoff – depending on rainfall volume and intensity. The effect is greatest when rains are short and low-intensity. Kuehler and his colleagues also cite research describing that:

• Trees planted over open, impervious surfaces such as parking lots could reduce stormwater runoff by as much as 20 percent.
• Branches and stems can capture and store up to 15 percent of total rainfall.
• A large tree can capture and retain as much as 332 gallons of water. The estimate assumes the widest part of the tree’s crown is 33 feet.
• Up to 75 percent of the water output from a parking lot rain garden was due to tree transpiration.
• Rain clings to leaves. In some species the effect is miniscule, in others the film is relatively thick – up to a thousandth of an inch (2.24 mm).

Although trees capture and store some water, soil is best. “It’s well established that soil provides greater stormwater storage than tree canopy cover,” says Kuehler. “Trees bring more natural hydrologic processes back to urban watersheds. Tree roots condition the soil. Roots help soils infiltrate, store, and percolate stormwater runoff.”

Urban trees have many other benefits as well. Trees help improve air quality, increase home and business values, and make communities livable. Trees also offer food and shelter to wildlife, and trees and other green spaces help sustain human health.

It’s a timely message. The South’s population is increasing, and cities are expected to grow. “If we can use our natural resources as a mitigation practice rather than an impediment to growth, we could lower drinking water treatment costs, lower utility bills, increase air quality, and increase property values,” says Kuehler. “The Forest Service knows how much potential benefit forest systems can provide to a city. We want to provide more information to stormwater managers and design engineers through our research.”