TECH CORNER


Kyle Brown


be increasing irrigation applications and when applications are at their peak, Mahmood says.


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.”


ONGOING STUDY 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 over irrigated areas. “That means clouds can form more easily over irrigated areas


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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.


Kyle Brown is the editor-in-chief of Irrigation Today and can be reached at kylebrown@irrigation.org.


The changes connected to irrigation can impact temperatures as well as cloud formation and regional winds.


One set of results from the study has found that irrigation can affect latent heat flux, impacting the dew point temperature.


Winter 2023 | Irrigation TODAY 29


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