Measuring ET

To set the record straight, no, I’m not running around chasing flying saucers.  ET in this case stands for evapotranspiration.

Evapotranspiration is a combination of two words: evaporation and transpiration. It is a very useful tool for those of us involved in agriculture, and gives a good indication of how much water is lost to the atmosphere every day.

We are all familiar with evaporation. This is the change of liquid water to the vapor, or gaseous state. This water vapor is absorbed into the atmosphere, depending on local atmospheric conditions.

Transpiration is the process where plants take up water through their root systems, move the water up through the plants, and finally send the water in vapor form out through pores in the leaves, again being absorbed into the atmosphere.

One way to think about ET is that it is the opposite of rainfall.  As a CoCoRaHS observer, I measure the precipitation that comes down every day. Now, I’m measuring what returns to the atmosphere every day during the growing season…. ET.

For farmers, ET can provide a valuable tool if they irrigate their crops. Generally speaking, you don’t want to let your soil dry out beyond half of its water holding capacity. If you keep an eye on what rain you are receiving, and then balance that out with daily ET measurements, it can give you an idea of when you need to irrigate.

There are two ways to get those daily ET measurements: calculated and measured.

Many of our MU Ag Experiment Stations have automated weather stations which upload their data to a web site. Some of these have calculated ET measurements, using the Penman-Monteith equation. Local stations in Northwest Missouri giving this calculated ET data include Albany, Brunswick, and Linneus.

The other way to get ET data is to actually measure it. Now that is harder than it sounds… if you could even measure what was going up through a plant, which plant would you use? The best, most consistent way to approach this is to make a mechanical device to measure ET, which is the approach that CoCoRaHS is using. My gauge has a porous ceramic top, covered with a green (leaf-colored) canvas. The company that makes these gauges has spent a lot of effort to insure that these gauges accurately represent true ET. So every morning, I measure how much moisture has been lost over the previous 24 hours.

Since we have finished the month of May, I thought I would compare the precipitation we have received to the ET returning to the atmosphere. In Gallatin, I measured 2.17 inches of precipitation. That was well below the normal for May, which is 5.30 inches. My ET totals for May were 3.53 inches.  That means while we received 2.17 inches, we lost 3.53 inches, for a deficit of 1.36 inches. That certainly didn’t help our drought situation, which was rated as “Abnormally Dry” on the Drought Monitor at the end of May.

For the period of June 1-12, we are starting to see a different picture. I have measured a total of 5.00 inches, with an ET loss of only 1.18 inches. And thanks to that rainfall, Daviess County, and many of the surrounding counties, have been removed from the “Abnormally Dry” status on the Drought Monitor.

If you want to look at ET estimates from the MU Ag Experiment Stations, their web site is http://agebb.missouri.edu/weather/stations

To learn about how to install your own evapotranspiration gauge click here.

(By Tim Baker, Regional Horticulture Specialist, University of Missouri Extension)