Trees and Stormwater Runoff

April showers bring May flowers…and a lot of urban stormwater runoff.  There is plenty of published research that shows that with increased urban development comes increased stormwater runoff.  Trees and urban forest systems (trees, groundcover, and soil) are natural “green” infrastructure that help to manage stormwater runoff at its source.  The parts of this system work together as a “treatment train” or series of practices designed to mitigate stormwater runoff to provide considerable stormwater volume and pollution control through rainfall interception and intensity reduction, stormwater infiltration, and nutrient uptake.

The canopy formed by urban trees intercepts rain as soon as it starts to fall.  That rainfall remains in the canopy where it eventually evaporates back into the atmosphere.  Analysis of all research looking at urban tree canopy interception shows that deciduous trees retain about 20% of the rain falling on its crown while conifers retain close to 30%.  The rest of the rain either falls through the crown to the surface below or trickles down the stem where it infiltrates into the soil around the tree.  With these numbers stormwater engineers can begin to calculate stormwater benefits of existing tree cover.

When the leaf and branch surface area in the upper part of the tree canopy is filled and cannot hold additional rainfall, excess water drips from these surfaces to those lower in the canopy. This reduces rainfall intensity by the time it drips from the canopy and delays runoff to storm drains or other stormwater control measures. This, in effect, allows the stormwater drainage system to work more efficiently and reduces the chances of it becoming overwhelmed thus helping to reduce surface flooding.

Soils provide the bulk of stormwater volume control. Macro- and micro-pores within the soil allow for temporary water storage from which trees acquire water and nutrients. Tree roots condition the soil through mechanical, biological, and chemical means, increasing its ability to store greater volumes of water. Stormwater runoff not intercepted in the canopy is directed to the soil at the base of a tree. Stemflow, or excess water traveling down the stem of the tree to the soil at its base, can penetrate deep into the soil profile as water moves along the root surfaces. READ FULL ARTICLE HERE.

This article, written by Eric Kuehler, is an excerpt for an upcoming research manual for urban foresters. Eric is Science Delivery/Technology Specialist for Urban Forestry South, US Forest Service. 

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