While 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.
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.”
We found that the dewpoint temperature is going up with increased irrigation.
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.
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.
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.
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 be increasing irrigation applications and when applications are at their peak, Mahmood says.
The changes connected to irrigation can impact temperatures
as well as cloud formation and regional winds.
They collected data from multiple sources, using flux towers at 12 different sites, focusing on latent, sensible and ground heat flux, as well as incoming and outgoing solar radiation. For the irrigated and nonirrigated areas, each had six towers. They also put down 75 weather stations (Environmental Monitoring, Economical Sensor Hubs developed at the University of Alabama in Huntsville) and launched radiosonde weather balloons every two hours totaling about 1,200 launches during the period of the study. The group also used a Doppler on Wheels unit for the project. The study covered two locations, one near Lincoln and the other near York, Nebraska.
When Mahmood and his colleagues started to look at the data, they could see how latent heat flux goes up starting at the end of June and early July and peaks in the middle of that month. “With that, the temperature goes down,” he says. “We found that the dewpoint temperature is going up with increased irrigation.”
Irrigation also affects the planetary boundary layer, the lowest part of the atmosphere that increases and decreases in height across night and day. For irrigated areas, the depth of the planetary boundary layer is lower compared to nonirrigated areas. In rainfed areas, there’s a drier atmosphere, which means a higher temperature and sensible heat flux, which causes the atmosphere to become more turbulent. Because of that turbulence, the planetary boundary layer becomes deeper over rainfed areas as compared to irrigated areas. Those irrigated areas have less sensible heat flux and more latent heat flux, which keeps the lower atmosphere slightly cooler as a result.
They found that lifting condensation levels are also lower overirrigated areas. “That means clouds can form more easily over irrigated areas with a lower height compared to rainfed areas,” he says. The level of free convection is also lower over irrigated areas.
The changes in temperature impact weather, such as regional winds. As the lower part of the atmosphere and land surface cools, it weakens the upslope wind toward the eastern side of Colorado. That can influence cloud formation and potentially the Great Plains low-level jet, which brings moisture up from the Gulf of Mexico.
They found that the dewpoint temperature could be about two degrees higher compared to rainfed conditions and that air temperature could be about 1.5 degrees cooler due to irrigation. “Anecdotally, when you talk to people in the region, they can feel that change in humidity as irrigation is applied in the middle of July, as a high dewpoint combines with high heat,” Mahmood says.
When it comes to precipitation, while this study hasn’t covered long-term rainfall on its own, other studies have shown that there are ongoing changes to precipitation when irrigation is used, though it isn’t entirely conclusive.
In terms of precipitation, it’s a small amount. “But it’s creating a situation where one water manager is getting less water than they’re used to while another is getting more than they’re used to getting,” he says. While applying irrigation on an agricultural scale might not seem like it could make that large of an impact, it can change the precipitation pattern going forward.
“Our main focus for this project now is to continue to analyze the data and looking for patterns due to these two different land uses,” says Mahmood.