Project strengthens understanding of irrigation’s effects on Great Plains weather

Nebraska is regularly the center of complex Great Plains weather conditions, and University of Nebraska–Lincoln scientists have long studied the dynamics shaping the state’s climate. An ongoing research project, involving Husker collaboration with multiple universities and research institutions, has expanded that knowledge by using an extensive set of sensors in southeast Nebraska to study how agricultural irrigation affects local weather conditions.

The project, known as the Great Plains Irrigation Experiment (GRAINEX), collected data on a range of meteorological factors in a 3,600-square-mile region of southeast Nebraska during the 2018 growing season. Funded by a $2 million grant from the National Science Foundation, GRAINEX compared weather dynamics over irrigated corn acres with those over non-irrigated cornfields.

Researchers recently published their preliminary findings on irrigation’s effects on wind, temperature and humidity in the academic journal Geophysical Research Letters.

Irrigation over the cornfields weakened an afternoon near-surface upslope wind that normally blows from east to west due to the higher slope of terrain as one moves westward across eastern Nebraska, the scientists reported.

“For the first time, we have collected this kind of extensive data that allowed us to investigate the mesoscale (local) wind circulation, and that has not been done before,” said climatologist Rezaul Mahmood, a professor in Nebraska’s School of Natural Resources, director of the High Plains Regional Climate Center and a fellow in the Daugherty Water for Food Global Institute.

The wind system created by the upslope winds “influences storm formation by forcing upward motion and transporting moisture,” Mahmood and his co-authors wrote. “Using the observations from GRAINEX and comparing to a weather model, we find that irrigation weakens this wind system, potentially affecting cloud and rain formation in this region.”

Follow-up study by the GRAINEX researchers will examine additional data sets to deepen understanding of the cloud-related ramifications.

Traditionally, “the observations required to study the mechanisms though which irrigation affects weather and climate are lacking,” Mahmood and his co-authors noted. The GRAINEX project helps fill in the data gaps with observational data on temperature, moisture and air pressure, taking the analysis beyond theoretical modeling.

Strengthening the scientific understanding of irrigation’s effects on climate is especially important because ag-related irrigation is expanding globally and irrigated cropland produces 40% of the world’s food, the GRAINEX researchers noted.

In addition, the GRAINEX data confirmed previous weather modeling studies that irrigated farmland has a lower surface air temperature and higher dew point temperature compared with non-irrigated areas. The dew point temperature is a measure of mugginess.

“The air temperature can be slightly lower over irrigated areas and slightly higher over rainfed areas,” Mahmood said, “because most of the heating over irrigated areas is used for latent heat flux,” referring to the land-to-atmosphere energy partitioning as heat flows upward from the land.

It’s not surprising that an irrigated cornfield would have high humidity, since mature corn releases a major volume of water into the air during the tasseling season at the height of summer. This topic relates to recent news coverage of “corn sweat” — the moisture released into the air by corn plants through a process scientists call evapotranspiration.

The news coverage followed publication in The Washington Post of a column by Barb Mayes Boustead, an Omaha-area meteorologist, about how the large-scale planting of corn and soybeans across the Midwest intensifies humidity in the region as a result of evapotranspiration.

An acre of planted corn can release up to 4,000 gallons of water per day, Boustead noted.

Nebraska corn production involves about 10 million acres of farmland. The state’s soybean crop is harvested from about 5 million acres. Together, that cultivation involves a bit less than a third of Nebraska’s landmass.

In 2021, Nebraska produced 1.85 billion bushels of corn (third-highest in the nation) and 351 million bushels of soybeans (fourth-highest).

The increased local humidity associated with irrigated fields is a well-known aspect of meteorology, and data collected by the GRAINEX project confirmed the phenomenon, Mahmood said.

Past modeling by Husker scientists compared the level of evapotranspiration for natural grass to that of irrigated farmland and of non-irrigated farmland. The analysis, Mahmood said, found that irrigated fields can generate up to 36% more evapotranspiration than natural grass covering the same area. Non-irrigated farmland had 2% greater evapotranspiration.

“Irrigation and increased humidity can really impact hot days, and that can definitely impact human health,” Mahmood said.

The GRAINEX project involves a partnership of six institutions: Nebraska, Western Kentucky University, the University of Alabama in Huntsville, the University of Colorado Boulder, the National Center for Atmospheric Research and the Center for Severe Weather Research. The researchers’ data collection covered two periods in the 2018 growing season: May 29-June 16 and July 17-30.

GRAINEX researchers drew on the resources of their partner organizations to assemble an extensive variety of weather sensors, on the ground and in the air, to monitor areas of southeast Nebraska from Merrick County on the northwest to Johnson County on the southeast. Weather balloons were a significant component of the research. Every two daytime hours during the July study period, the researchers sent the devices aloft to collect data on the atmospheric pressure, temperature, humidity and wind speed. Over the course of the project, the scientists deployed 1,200 balloons.