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NMSU studies subsurface drip irrigation

As the lingering effects of drought stretch across New Mexico's farm belt, a New Mexico State University scientist says it's time for more farmers to consider the potential water saving of subsurface drip irrigation, especially for alfalfa, a thirsty crop that just happens to be the state's leading cash crop.

In drip irrigation, water is applied directly to plants' roots through a series of black plastic lines or drip tape buried more than a foot deep.

Drip irrigation has been around for more than four decades, but has yet to take hold among agricultural producers, even those in traditionally dry states.

“The biggest hindrance is cost,” said Robert Flynn, an agronomist with NMSU's Agricultural Science Center at Artesia who has been experimenting with a drip system for almost five years.

“It's not a cheap investment, and there's a learning curve associated with drip irrigation,” he said.

“Still, most of the growers in this area who have recently tried it are quite pleased with the experience. Sure, there's some grumbling when the system is being installed, but by mid-season they're happy not having to stand out in 100 degree weather, trying to irrigate.”

For many producers, shifting to drip irrigation means a change in mind set, said John White, Dona Ana County horticulture agent with NMSU's Cooperative Extension Service. Today, the most likely candidates to shift to a drip system are those who are already pumping irrigation water from wells and need drip irrigation's higher efficiency, improved yield benefits and better weed management, he said.

It was at the behest of some of those sweltering southeastern New Mexico farmers back in the mid-1990s that Flynn was asked to examine water-conserving technologies. He began with several small trial plots, studying spacing and drip tape depth.

In 1999, NMSU received a grant from the U.S. Department of Interior's Bureau of Reclamation to conduct a larger five-acre study.

At the time, most previous research studies had focused on traditional row crops, but Flynn's choice was New Mexico's $161 million annual alfalfa crop.

“Alfalfa is a large acreage crop that also is a high water user,” he said. “It takes at least 36 inches of water a season to get a marginal crop. More typically we're putting 45 to 55 inches a year on these crops to achieve yields of six to eight tons of alfalfa per year.”

Flynn found that by using drip irrigation he could cut the amount of water used per season by 10 inches, a big savings.

“We also looked at 15 commercial alfalfa varieties under drip irrigation and found no significant differences in production,” he said. According to the New Mexico Agricultural Statistics Service, the state's growers produced 1.45 million tons of alfalfa last year.

Today, using a sophisticated weather station at the science center, Flynn and his research team can accurately match the water needs of alfalfa and other crops to the actual transpiration rate of the plants on any given day.

The five-eighths-inch black plastic drip tape used at the Artesia science center is placed at intervals 30 inches apart and has an emitter spacing of 24 inches. The tape itself is 14 inches below the soil surface.

At that depth, traditional planting or harvesting equipment can be used without problems, but more research is needed on use of heavy equipment used to haul the crop out of the field, Flynn said.

For the average producer, installing a new drip system can be a large-scale undertaking, and it must be done correctly, Flynn said. Costs can vary widely — $500 to $2,000 an acre — depending on the quality of the materials, acreage and the size of the pump and filters needed.

The longevity of a drip system depends largely on the quality of the tape, but many producers expect their tape to stay in the ground at least 15 years.

One question often asked of Flynn concerns irrigation water with high saline content because the water goes directly to the roots, bypassing the normal filtering process as the water moves downward through the soil.

“In this case, you're distributing water in a completely different way than in traditional furrow or sprinkler systems,” he said. “Salinity buildup can be a problem under poor water or soil conditions, which is why you always have to do a soil test and water assessment before investing in drip irrigation systems.”

In addition, a subsurface drip system has to be periodically flushed to clear lines of silt and contaminants, which increases the amount of water used, Flynn said.

And while there have been numerous improvements to drip irrigation systems over the past decade, he pointed out there were some shortcomings, including root intrusion into the tape emmiters. The root problem has largely been addressed with chemically-coated drip tape emitters that halt root intrusion.

Flynn's only other problem has been the occasional gopher that burrows along and cuts into the plastic water lines.

“We're getting a handle on that, but it continues as a nuisance,” he said.

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