ET - Moving Water Back into the Atmosphere

If you Google "ET" or "E-T" the first several results returned are for the 1982 movie E.T. - The Extra-Terrestrial, about a boy who befriends an extra-terrestrial stranded on Earth.  The "ET" that is the subject of this article isn't about visitors from outer space, but it is a concept that may be a little "alien" to many people.

ET stands for evapotranspiration, the process in which water vapor moves back into the atmosphere.  Evapotranspiration is the sum of evaporation from ground surfaces and the transpiration of water to the atmosphere from plant leaves.  ET is a function of temperature, wind speed, relative humidity, and solar radiation. Transpiration occurs when the roots of a plant draw moisture from the soil where it moves up through the plant to be released as water vapor from the leaves. On average more than half the precipitation that falls is returned to the atmosphere through ET.  Studies show that transpiration by vegetation accounts for about 10 percent of the moisture in the atmosphere.  An acre of corn can transpire about 3,000 to 4,000 gallons of water per day.  The transpiration from agricultural crops is often significant enough contribute to the higher dew point temperatures that create muggy conditions during the summer and may enhance the formation of showers and thunderstorms.  Conversely, the reduction in transpiration, such during drought, reduces the return of moisture to the atmosphere which in turn inhibits the development of showers and thunderstorms.

The E-T Gauge

CoCoRaHS observers measure what falls out of the atmosphere, precipitation. Beginning last year, however, a number of CoCoRaHS observers have been measuring evapotranspiration, or what is going back into the atmosphere, the "return" side of the water cycle.  Evapotranspiration measurements actually began in mid-2011 with a few volunteers as a pilot project. Last spring the opportunity to make ET measurements was opened up to the observers at large. At present there are about 90 observers in 38 states making ET measurements across the country along with their rainfall (or in many cases, lack of rainfall) measurements.   ET does not vary to the same extent as precipitation (it's more similar to temperature), so multiple measurements in the same general area are usually not needed, unlike precipitation. ET measurements are only made during the warm season, since freezing temperatures can damage the gauge.



The measurements are made using a special ET gauge developed for this purpose.  The gauge consists of a water reservoir, with a cap consisting of a ceramic evaporator surface with a green fabric cover.  In our case the fabric simulates evaporation over turf, so the gauge needs to be sited in a sunny, exposed area and preferably over grass.  We are measuring "reference ET" which is defined as "the ET from an extensive surface of clipped grass… that is well-watered, and fully shades the ground."  This reference ET is referred to as ETo . Another cover is available which simulatesevaporation over alfalfa (ET1). The cap is connected to a supply tube which extends the length of the reservoir. There is a sight tube on the exterior of the gauge which measures the water level in the gauge.  The difference in water level from one observation to the next represents the evapotranspiration.


The measurement of both precipitation and evapotranspiration allows us to calculate an atmospheric water balance.  Water balance charts are available on the CoCoRaHS web site (I also had one in my last blog post about New Mexico). This plots precipitation, ET, and the accumulated difference over time.

Water balance chart for ME-CM-3, located about 20 miles north of Portland, Maine.

Water balance chart for TX-ER-4, about 60 miles WSW of Fort Worth, Texas

Water balance chart for UT-ML-1, about 80 miles SSW of Salt Lake City

 
These charts provide a good sampling of the relationship between ET and precipitation and the water balance. When viewing ET charts, keep in mind that the charts do show multi-day ET values (total E-T over a period of more than a day), but the charts don't plot multi-day precipitation accumulations. Only daily reports are reflected on the charts.

The measuring of ET is relatively easy but there is a commitment of time and money.  The ET gauges cost about $220, and there is a little more to setting up and maintaining them.  However, the ET measurements are very useful and more importantly fill a big data need. Most estimates of ET are calculated, and the deployment of these gauges to CoCoRaHS observers represents the first organized effort to measure ET other than in automated, specialized networks.

For more information on evapotranspiration and the CoCoRaHS measurement program, visit the CoCoRaHS web site and select Evapotranspiration in the Resources menu of the left side of the page.