Evaporation: friend or foe?

Vegetation in stormwater swales and other green infrastructure allow natural processes - like evapotranspiration and infiltration – to manage stormwater where it lands.

Vegetation in stormwater swales and other green infrastructure allow natural processes – like evapotranspiration and infiltration – to manage stormwater where it lands.

by Jennie Saxe

I was mulching my garden recently, trying to remember why I had decided to spend my weekend this way, and I thought about how much mulch helps out plants during the hot, often dry summer.

In addition to keeping weeds at bay without chemicals, mulch provides the additional benefits of holding moisture in soil and preventing wide fluctuations in soil temperature. I also recently read about how evaporation can affect water supplies by causing significant losses during storage and transmission of drinking water supplies.

So, does that make evaporation the enemy?

Not necessarily: evaporation can also be a good thing. The process of transpiration by the plants is another key ingredient in and keeping our waters clean. Plants transpire by drawing water up from the soil, through roots, throughout the plant, and eventually releasing water to the air through the leaves.

Using green infrastructure mimics natural processes like infiltration and allows communities to reap the benefits of evapotranspiration, the combination of evaporation and transpiration processes in plants.

Green infrastructure utilizes plants for intercepting, capturing and reusing rainwater. For example, water that lands in the canopy of trees may evaporate before it comes in contact with pollutants which reduces the amount of water and pollution that would otherwise end up in sewers and streams.

Although trees can clearly make a huge impact, all types of vegetation in curb bump-outs, stormwater planters, green roofs, and rain gardens can use evapotranspiration to help keep stormwater and pollutants out of our sewer systems and waterways.

This is important because a heavy storm, especially in an urbanized area, can result in rapid runoff of stormwater from roofs, across sidewalks and streets, and many times into combined sewer systems, where it can contribute to sewer overflows – or directly into waterways where it can load streams with pollutants and sediment.

Rapid stormwater runoff can also lead to flooding and property damage. Green infrastructure techniques are one way to slow the flow of stormwater runoff, keeping huge volumes of stormwater out of sewer systems, reducing flooding, and preventing pollution from entering waterways.

So, while evaporation can be a friend or a foe, understanding when it can be helpful is critical to protecting our water resources.


About the author: Dr. Jennie Saxe joined EPA in 2003 and is currently a Water Policy Analyst in the Water Protection Division of EPA Region 3 in Philadelphia. When not in the office, Jennie enjoys tending to a vegetable garden.


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