Evapotranspiration: The Oft-Forgotten Outflow

The hardship of drought gives cause for legitimate concern for all stakeholders in the Savannah River Basin, up and downstream. Drought’s resulting lower lake levels draw sharp attention to the one subject to blame for our disappearing water: outflows.

For most people in the basin the term outflow is defined as the conscious release of water through the gates of Thurmond Dam. But there are two silent and invisible culprits to lower lake levels often not taken into consideration, which are truly worthy of the term outflow. These two forms of outflow are called evaporation and transpiration. Together they are called evapotranspiration, which is a combination of evaporation and plant transpiration (water being transpired from vegetation into the atmosphere).

While evaporation accounts for the movement of water into the air from sources such as soil and bodies of water, transpiration involves the evaporation of water from plants. These two processes combined can result in a significant water loss from the reservoirs. When it rains, plants take water from the soil through their roots. They in turn release water vapor to the atmosphere through thousands of small holes (called stomata) on the backs of their leaves during transpiration. At the same time, water evaporates from the ground surface and from the lakes.

In the Savannah River Basin, evaporation alone accounts for anywhere between 200 and 1,200 cubic feet of water loss – every second. Transpiration is much more difficult to measure, but it isn’t out of the question to conclude that in the summer more water is lost from transpiration than evaporation. The most tangible way these two phenomena can be recognized is observation of lake-level trends in the summer. Despite significant amounts of above-average rain in the summer, this warmer season is almost always associated with falling lake levels: Evapotranspiration at work.

Evapotranspiration, along with other natural factors, tends to create seasonal cycles in lake levels. During the summer months, the reservoirs typically decline, even with normal rainfall, because they lose a significant amount of water to transpiration and evaporation. Because air temperatures are warm and the trees are fully foliated during this time of year, transpiration occurs at large rates. According to the U.S. Geological Survey website, a large oak tree can transpire 40,000 gallons per year.

However, during the winter months when plants lose their leaves, transpiration rates are lowest. As a result, the winter snow and rainfall help recharge the lakes for spring, because the lakes are not competing with the trees for precipitation.  According to Todd Hamill, a meteorologist with the National Weather Service, transpiration increases dramatically between the end of March and the beginning of June as foliage progressively increases during that time.  Once summer arrives, evapotranspiration usually wins the water wars and lakes begin to fall.

The single most important take way from the phenomenon of evaporation and transpiration is that refill periods take place in the winter, and we can expect water levels to decrease in summer months even with normal rainfall.

Read more about Evapotranspiration on the U.S. Geological Survey website.

About US Army Corps of Engineers Savannah District

The U.S. Army Corps of Engineers Savannah District oversees a multimillion dollar military construction program at 11 Army and Air Force installations in Georgia and North Carolina. We also manage water resources across the Coastal Georgia region, including maintenance dredging of the Savannah and Brunswick harbors; operation of three hydroelectric dams and reservoirs along the upper Savannah River; and administration of an extensive stream and wetland permitting and mitigation program within the state of Georgia. Follow us on Twitter @SavannahCorps and on Facebook.com/SavannahCorps
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