Melissa Pearson Oklahoma Department of Agriculture chats with Al Sutherland Oklahoma Mesonet at the Oklahoma Irrigation Conference

Melissa Pearson, Oklahoma Department of Agriculture, chats with Al Sutherland, Oklahoma Mesonet, at the Oklahoma Irrigation Conference.

Evapotranspiration 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.

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 (, 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.”

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