corn field
COMMON DENOMINATOR: Although it's a primary driver in crop water use, one of the challenges in adopting ET to schedule irrigation is finding a common denominator for irrigators.

Growers use ET to respond to plant demands

Some TAPS competitors used evapotranspiration info to get more crop per drop.

Sociology is an interesting thing — particularly when it comes to modern farming practices. Nothing drives the adoption of modern ag tech and practices quite like realizing a consistent return on investment.

That's been the focus of the TAPS (Testing Ag Performance Solutions) competition at the University of Nebraska-Lincoln's West Central Research and Extension Center.

And while marketing was ultimately the deciding factor for this year's winner of the overall profitability category, it's not hard to see those that were profitable in the competition also made efficient use of inputs like water and fertilizer.

What was especially telling was the use of monitoring tools, including an eddy covariance tower from Li-Cor to measure evapotranspiration. While not all involved in the competition made use of the ET data, those that did used it to create a complete picture of the water balance on their contest plot, and ultimately used it to improve irrigation scheduling.

Because ET is also correlated with yield, it played a big role in the irrigation efficiency component of the competition, notes Daran Rudnick, Nebraska Extension crops and water management specialist, and one of the organizers of the program.

ET is a function of how much water evaporates off the field and is transpired by the crop. Throughout the 2017 growing season, the TAPS site saw about 18 inches of ET on the rain-fed control acres and up to 25 to 26 inches on participants’ irrigated plots. ET for every plot was calculated, and ET of the control treatment that received no irrigation was subtracted. That number was then divided by the amount of applied irrigation for each individual plot to calculate irrigation efficiency.

The goal was to use as much water through ET as was applied through irrigation to optimize efficiency. Those that didn't use all of the water applied through irrigation were considered less efficient.

So, the more efficiently growers used available water, the closer their irrigation efficiency component was to the No. 1 — although a few who saw more ET than they applied through irrigation saw even higher efficiency ratings.

"One farm applied 2.5 inches. They timed their irrigation events well to avoid stressing the crop at critical growth periods at tasseling and silking, and were able to keep the plants healthy longer, which allowed the plants to use more stored soil water and rainfall," Rudnick says. "They put on 2.5 inches, but the ET was about 4.5 inches above the control treatment. So instead of an irrigation efficiency of 1, they had an irrigation efficiency of 1.8."

Using ET works as a tool for irrigation scheduling and responding to crop demands; yet, not everyone uses it to make irrigation scheduling decisions and not all participants used ET data in the TAPS competition. Roric Paulman, who farms near Sutherland and won the profitability category of the competition, says access to and understanding what the data is saying are big roadblocks. For example, there are a handful of ways to calculate ET or measure it directly.

"The first challenge is in farmer language. With eddy covariance, it was in millimeters and expressed in a way I didn't know how to sort out. That common denominator is important. The same is true for reference ET with the crop coefficient method. What were they using for the coefficient? Is it alfalfa or grass? They're all expressed in different examples," Paulman says. "It's also about making the connection between irrigation rate, soil water-holding capacity and plant water use. Because you have to know what your sprinkler is putting on and how much is in the soil. You can't assume; you have to go out and verify."

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