UNL study confirms irrigation's impact on humidity, yields possible link to rain

UNL study confirms irrigation's impact on humidity, yields possible link to rain

Monday, October 31, 2022

By: Nancy Gaarder

Increased rain in Illinois and Indiana. Less rain in some Nebraska communities.

Muggier weather. Changes in winds.

Intense irrigation in Nebraska is having a complex effect on the weather, researchers at the University of Nebraska-Lincoln say

“Some of the impacts are very discernible, you can feel it on a day-to-day basis. Other impacts may not be noticeable immediately,” said Rezaul Mahmood, a professor at the UNL School of Natural Resources and director of the High Plains Regional Climate Center. “Irrigation impacts our weather, climate and well-being in many different ways.”

Scientists have been studying the impact of irrigation on weather for years because of irrigation’s importance to global food security and the need to understand how its increasing use might change the weather. Irrigated fields produce about 40% of the world’s food, and its use is growing.

What have scientists found?

The most noticeable, localized effect that irrigation has on the weather is to make an area more humid, Mahmood said. People sense that through muggier and more uncomfortable weather. That humidity also suppresses temperatures, so it’s harder for hot weather to generate records.

Other changes aren’t discernible by people but can be detected through large-scale scientific studies, analysis of decades of data and sophisticated computer modeling. It’s this type of research that has led scientists to conclude that irrigation is changing wind patterns locally and affecting rainfall patterns over a large area.

Irrigation in Nebraska appears to have led to slight decreases in rainfall within the most heavily irrigated areas, while potentially causing slight increases in rainfall many hundreds of miles away, Mahmood said.

States as far away as Indiana may be seeing slight increases in rain due to irrigation in Nebraska, Mahmood said.

While the amount of rain involved is minor, a fraction of an inch over decades, “the science is very solid,” Mahmood said.

Nebraska, it turns out, is the perfect laboratory to study irrigation, Mahmood said. The state leads the U.S. in irrigated land, and the U.S. ranks third globally for amount of irrigated land.

In Nebraska, researchers have access to adjacent swaths of cropland that are very different in terms of rainfall patterns and irrigation potential. The water-rich Ogallala Aquifer provides a foundation for widespread irrigation over semi-arid land in central Nebraska. The aquifer doesn’t extend farther east to areas where rainfall is more plentiful, so crops in those areas depend upon rain.

“We can observe the transition from rainfed to irrigated agriculture,” he said. “Rainfall changes as you go west; one or two counties can make a difference.”

By its very nature, irrigation leads to artificial conditions. Irrigated soil is wetter than rainfed soil because it’s watered more regularly. And it’s watered under hotter weather — when the sun is out and temperatures are up. Rain, by its nature, falls on cloudy, cooler days.

Using a $2 million grant from the National Science Foundation, researchers from six institutions undertook a massive study of irrigation in Nebraska in 2018, and that study is still bearing fruit. Partnering with UNL were 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.

Dubbed GRAINEX for the Great Plains Irrigation Experiment, the study placed weather sensing equipment across 3,600 square miles of southeast Nebraska — from Merrick County on the northwest to Johnson County on the southeast. Mahmood is the principal investigator for the study.

Results continue to be studied. Earlier this year, Mahmood and his colleagues published research explaining how irrigation affects wind patterns. That study was published in the March edition of Geophysical Research Letters.

Mahmood said irrigation weakens a type of afternoon wind that is important to cloud formation. It’s not clear, he said, how much impact the change has on clouds and storms.

But the findings point to the importance of Nebraska as field laboratory for the study of irrigation.

“For the first time, we have been able to collect this kind of extensive data, and that allowed us to investigate (local) wind circulation,” he said. “That has not been done before.”

Studies such as these have wide-ranging implications. Just as water managers understand that aquifers aren’t limitless supplies of water, they now understand that irrigation is affecting weather. As climate change heats up the Earth’s atmosphere and more irrigation is undertaken, it’s possible that the effects of irrigation will become more pronounced.

“The bottom line is that when you change land use, that impacts the weather and climate of an area,” Mahmood said. “Planning and adaption, those are things we need to work on.”

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