Researchers Focus on Water Needs of Alfalfa Fields

A battery of soil moisture and weather instruments on a small alfalfa field on the University of California, Davis, campus is gathering an array of information on how much water the crop needs, and when. Researchers are focused on establishing as precisely as possible crop water needs over a long season that includes numerous cuttings.

“We’re trying to understand how much water this crop really needs,” said Dan Putnam, UC Cooperative Extension alfalfa and forage crop specialist. “Five years ago, I would have said 48 inches, but it looks like it’s a little less than that.”16.07.18c Deficit irrigation

Putnam made his remarks as growers, pest control advisors and researchers looked over numerous alfalfa irrigation and buried drip layout studies on display during the annual UC Davis Alfalfa and Forages Field Day.

In a 2015 trial on the Davis campus, using a lysimeter to precisely measure water use over the course of a year with seven cuttings, the crop needed a total of just under 42 inches of water.

Irrigation management starts before the season, UC specialists said, with soil moisture information to help make informed decisions about when to start irrigating, and how much water it will take to fill the soil profile.

“The Et is not enough,” said Ali Montazar, UC Davis project scientist. “You need to look at the soil moisture data, which can help us answer critical questions like the water status of the soil in the dormant season, when you should start irrigating, and whether your irrigation filled the soil profile.”

One strategy is to fill the soil profile early in the season, and then track the daily Et to decide how much water to apply.

“Monitor the soil to verify the irrigation scheduling,” Montazar advised.

Measurements of soil moisture remain important, in part, because the amount of water available in the root zone varies dramatically, depending on the soil type.

Researchers are fine-tuning their understanding of how much water alfalfa needs over the season, so growers will be able to use weather station information to apply water more precisely.

“We established six acres of alfalfa in 2013 around two lysimeters,” said Daniele Zaccaria, UCCE water management specialist. “We are trying to measure water use with three methods—lysimeters, surface renewal and eddy covariance.”

He said these sophisticated techniques are indicating the alfalfa crop coefficient number to use in conjunction with weather station Et data is slightly lower than previously believed.

“You can get the Et from the Internet, but the real question is what crop coefficient to use,” Zaccaria said. “We get a crop coefficient value of between 0.85 to 0.89, which is different than earlier coefficients.”

Calculating alfalfa water need from weather station data is further complicated because once you know the Et figure, the crop coefficient averages a little under 0.9, but ranges from 0.45 after a cutting up to more than 1.0 from a few days after irrigation until the next cutting.

Once the changing alfalfa irrigation needs are calculated correctly, the cutting schedule can still make it hard to get the water on in time.

“I’m convinced many growers under-irrigate their alfalfa because of the difficulty getting the fields fully watered between cuttings,” Putnam said.

The ability to apply water over the entire field after a cutting is a major advantage attracting more alfalfa growers to subsurface drip irrigation.

In one area of the field on the Davis campus, for example, researchers were only able to flood irrigate eight times over the course of the year, but in nearby plots with subsurface drip irrigation they were able to irrigate 85 times.

“With subsurface drip, we can irrigate more frequently; this is one of the main benefits of subsurface drip irrigation,” Montazar said.

(Article by reporter Bob Johnson, Reprinted from AgAlert, July 6, 2016, California Farm Bureau Federation,

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