How Do Trees Reduce Stormwater and Flooding?

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- [Vincent] Have you ever wondered why communities are working hard to reduce stormwater?

This video will help you answer that question.

This is just one of many questions you can find answers to as part of the Penn State Extension stormwater basics education series.

As rainfall washes across streets, parking lots, driveways and lawns, it picks up gas and oil, heavy metals, sediment, pesticides, fertilizers and other chemicals that end up polluting our streams, rivers and lakes.

Those same paved surfaces do not allow rainfall to infiltrate into the soils leading to local stream, street and home flooding during rain events.

As we try to reduce the amount of stormwater that pollutes our streams and rivers and floods our communities, trees can be one of the cheapest and greenest solutions.

So how do trees reduce runoff and the pollutants in stormwater?

First, trees act like giant green umbrellas when it rains.

There are large leafy canopies intercept rainfall that would otherwise fall to the ground and wash down paved surfaces.

And depending on the size of the tree a large amount of rainfall never even reaches the ground but instead evaporates off the leaf surface following the storm.

The trees canopy also slows down rainfall which reduces local flooding.

Research has shown that large deciduous trees can capture over a thousand gallons of stormwater each year in their canopies.

Large evergreens or conifers can capture two to three times that amount because they are in leaf year round and have more leaf surface area.

The second way that trees reduce stormwater is that their deep and extensive woody root systems improve rainfall penetration into soils or what we call infiltration.

Here we see how extensive a root system can be on even a small tree.

The large woody root channels and organic matter in the soil allow for water to percolate into the soils.

Recharging groundwater and reducing surface runoff.

Think of soil as a sponge that absorbs and holes water quickly.

In one study, soils with trees allowed for 10 inches of rain to infiltrate per hour, while nearby soils converted to lawn could only infiltrate four inches of rain per hour.

Trees also act like large water pumps removing what was stormwater from the soils by extends of root systems and using that water for growth and photosynthesis.

Ultimately, putting that water back into the atmosphere through transpiration in the leaves.

During the summer growing season on average, large trees can transpire between 80 and a 100 gallons of water per day while some swamp species, such as bald cypress, can pump up to 800 gallons per day.

As trees and plants absorb water through their roots, they remove pollutants found in the runoff either using the nutrients for growth or storing heavy metals and chemicals in their wood.

We call this process phytoremediation, green plants cleaning up contaminated soils.

Without the plants those contaminants would wind up polluting our water.

These photos show some innovative designs to capture runoff for phytoremediation.

Trees prevent soil erosion by holding soils in place.

Eroded soils are a major source of sediment in our waterways.

Extensive tree root systems help keep soils in place.

This is especially true along streams where trees are essential to stream health providing shade, leafy food for the aquatic food chain and removing pollutants.

All trees provide us with multiple benefits including and stormwater reduction, shade and energy conservation, air pollution removal, increased property values and beauty but some provide us more benefits than others.

The larger the tree canopy, the more benefits the tree provides, including stormwater interception and evapotranspiration.

The combined effect of a community's trees has tremendous positive impact on stormwater runoff as well as aesthetics.

A recent study found that 1,800 street trees were able to intercept over 4 million gallons of stormwater each year.

Trees growing in yard and along streets and parking lots provide more than just shade.

They are an important factor in the effort to reduce polluted stormwater runoff.

Ultimately, the best plan is to have these large green umbrellas over paved surfaces like streets and parking lots where falling rain quickly turns into runoff that is collecting pollutants as it moves towards our waterways.

Managing polluted storm water is a huge problem for our communities, but we can all help by taking simple and local actions like protecting and preserving existing trees or planting new trees around our homes, streets and along streams.

The payoff is huge and they will do much more for you and your property than just manage stormwater.

If you have additional questions about storm water or you are just interested in learning more, you can find a full series of videos and fact sheets on the Penn State Extension website.

Just search for stormwater basics.