TECH CORNER
Irrigation applications could play a role in understanding ongoing changes in weather
By Kyle Brown W
hile growers constantly monitor their water use, they generally stop tracking it once it hits the soil. But that’s where
Rezaul Mahmood, professor at the University of Nebraska-Lincoln School of Natural Resources in Lincoln, Nebraska, has been focused on picking up the trail for the past several years. In his most recent study, with the final results yet to be published, he and other researchers have found that ag irrigation applications could have some effect on a region’s rainfall.
EARLIER STUDY In the time after he finished his doctorate, Mahmood became curious about the amount of agricultural irrigation and thought that it must have some effect on the lower atmosphere. He followed it up by looking into evapotranspiration rates over irrigated and nonirrigated areas and over grasslands. With this model-based work, he found that with overirrigated areas compared to standard grassland, ET rates could be as much as 36% higher.
He thought that if those rates were that much higher, then there must be a response in the atmospheric temperature. As solar energy enters the atmosphere, some is partitioned into latent heat flux. “If there’s a lot of water available, the amount of energy directed toward latent heat flux will be
higher, and sensible heat will be lower because there’s a fixed amount of energy coming in every day,” he says. “That air temperature will be slightly cooler.”
Using data from weather stations that had been collecting for a longer period of time, Mahmood and colleagues did an analysis. The group released a paper in 2006 finding that in the locations that had been irrigated, temperatures had declined during the second half of the 20th century. They looked at the data from multiple viewpoints and determined that where there wasn’t irrigation, the temperature actually increased.
“We found that the dewpoint temperature is going up with increased irrigation.”
Following up on this project, Mahmood thought to look at the dewpoint temperature because of its relation to atmospheric moisture content. “We found that dewpoint temperatures go up quite a bit over irrigated locations during the growing season and peaks as growers are applying more water in months like July,” he says.
WEATHER TRACKING These studies led the way for the proposal to the National Science Foundation for the Great Plains Irrigation Experiment, which brought together researchers from six institutions with a $2 million grant. The project included collecting weather data in Nebraska in 2018, and the paper providing the complete results is still being developed.
The Great Plains Irrigation Experiment used multiple types of equipment to collect weather data, including a Doppler on Wheels unit. Photos: Rezaul Mahmood
28 Irrigation TODAY | Winter 2023
They took a 60-mile by 60-mile area in eastern Nebraska where rain-fed corn transitions to irrigated corn so they could get side-by-side data collection. The data collection was done for two 15-day periods, one starting on May 30, 2018, and another starting on July 15, 2018. Those dates were chosen because they represent time periods when farmers tend to
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