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Evapotranspiration rate is key to irrigation managementEvapotranspiration rate is key to irrigation management

Evapotranspiration (ET) is simply loss of water—evaporation from the soil and transpiration from the plant, Al Sutherland explains.

Ron Smith 1

August 19, 2015

2 Min Read
<p>Melissa Pearson, Oklahoma Department of Agriculture, chats with Al Sutherland, Oklahoma Mesonet, at the Oklahoma Irrigation Conference.</p>

Using evapotranspiration data to manage irrigation decisions requires patience, but will be worth the effort, says Al Sutherland.

Working through the numbers and tables may seem a daunting task, said Sutherland, with Oklahoma Mesonet, during the opening presentation of the second annual Oklahoma Irrigation Conference Tuesday at Fort Cobb.

But the technology provides producers with information that takes a lot of guesswork out of irrigation decisions. “Consider the value,” he says.

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Evapotranspiration (ET) is simply loss of water—evaporation from the soil and transpiration from the plant, Sutherland explains. Many factors affect the rate and timing of moisture loss, including solar radiation, wind speed, wind direction, rainfall, soil moisture, relative humidity, soil temperature, and air temperature. Those factors vary across the region.

Oklahoma Mesonet collects information on those and other factors from 120 stations across the state, offering farmers a useful tool to help make decisions regarding water use.

ET influences

As critical factors change, ET rate also changes, Sutherland notes. When wind speed, solar radiation and air temperature are all lower and relative humidity is higher, the ET rate is lower. When wind speed, solar radiation, and air temperature are all higher and relative humidity is lower, ET rate is higher.

“Mesonet calculations assume a well-watered crop,” he says. “That’s the major shortcoming.” Crop coefficients available on the Mesonet (https://www.mesonet.org/), which indicate when a specific plant needs moisture replenished, also vary with plant species. Cotton, for instance, has a higher rating than corn.

Crops included in the Mesonet system, under an Agriculture tab, include wheat, grass hay, alfalfa, corn, cotton, peanut, sorghum and soybeans. Vegetables and other specialty crops are listed under Horticulture.

Other variables include soil types, which differ in available moisture. Compaction and rooting depth also make a difference.

“No-till cropland has more available water than conventional tilled land,” Sutherland says. “Plant available moisture data can be found on the Mesonet website under the Soil Moisture section. A Learn More section also offers other tips and more information.”

About the Author(s)

Ron Smith 1

Senior Content Director, Farm Press/Farm Progress

Ron Smith has spent more than 40 years covering Sunbelt agriculture. Ron began his career in agricultural journalism as an Experiment Station and Extension editor at Clemson University, where he earned a Masters Degree in English in 1975. He served as associate editor for Southeast Farm Press from 1978 through 1989. In 1990, Smith helped launch Southern Turf Management Magazine and served as editor. He also helped launch two other regional Turf and Landscape publications and launched and edited Florida Grove and Vegetable Management for the Farm Press Group. Within two years of launch, the turf magazines were well-respected, award-winning publications. Ron has received numerous awards for writing and photography in both agriculture and landscape journalism. He is past president of The Turf and Ornamental Communicators Association and was chosen as the first media representative to the University of Georgia College of Agriculture Advisory Board. He was named Communicator of the Year for the Metropolitan Atlanta Agricultural Communicators Association. More recently, he was awarded the Norman Borlaug Lifetime Achievement Award by the Texas Plant Protection Association. Smith also worked in public relations, specializing in media relations for agricultural companies. Ron lives with his wife Pat in Johnson City, Tenn. They have two grown children, Stacey and Nick, and three grandsons, Aaron, Hunter and Walker.

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