Water for agricultural irrigation is the most precious natural resource in western agriculture. Due to ever tightening water supplies, more attention is being focused on shifting from flood-irrigated cotton to more efficient systems which could include sprinkler and buried drip systems.

This could happen sooner rather than later.

Helping Arizona cotton farmers look ahead to potential irrigation system shifts are agricultural researchers at the U.S. Arid Land Agricultural Research Center (ALARC) located at Maricopa, Ariz.

ALARC is part of the U.S. Department of Agriculture’s Agricultural Research Service, and is based at the Maricopa Agricultural Center farm.

Working to steer this effort closer to reality are scientists Kevin Bronson and Kelly Thorp of ALARC’s Water Management and Conservation Research Unit.

Unit leader Bronson says, “ALARC’s mandate is field-scale water management research in terms of improving water use efficiency. We want to maintain high productivity but reduce the inches or acre feet of water used to grow crops, especially cotton.”

Each year, ALARC researchers tap technology to study water use efficiency in crops grown in the Grand Canyon State, including experimental crops. For cotton, 2016 is the third year of sprinkler field trials and the first year of buried drip field trials conducted by Bronson and Thorp.

Variable rate irrigation

The research duo is using variable rate irrigation (VRI) with sprinklers in a 16-acre field and drip line buried eight inches deep in a nearby four-acre field.

The overhead sprinkler system includes a Lindsay Zimmatic linear move sprinkler system with six spans and an overhang, and retrofitted with a variable rate irrigation system.

Thorp explains, “With an overhead sprinkler system we don’t need bed shaping and related tillage.”

The overhead sprinkler system is managed using precision agriculture tools to provide timely, variable applications across the field, including remote sensing, computer modeling, and other tools from their technology bag.

Thorp says, “VRI allows us to manage the field so we can explicitly show how cotton responds to different water rates and timings from June through August.”

Overhead sprinkler system

When operating the overhead sprinkler system at 50 percent of the full system speed, one-third inch of water is delivered in each pass of the system. It uses about 850 gallons of water per minute when irrigating the entire field at the full rate. During the summer months, most irrigations include several passes per week.

The system includes Senniger nozzles, plus bubblers to dribble the water between the rows.

Thorp says, “The key is you want the water to infiltrate where it’s applied. Otherwise it negates the technology.”

The local water supply is mostly Colorado River water delivered by the Central Arizona Project, plus some groundwater.

Field details

The run length in the sprinkler field is about 600 feet, including the borders, a short run required to retrofit the machine on existing fields on the Maricopa Agricultural Center farm.

The cotton field includes traditional 40-inch rows planted April 25 with Deltapine 1549 B2XF seed. Prescription maps help guide the sprinklers on how much water to apply and where.

Reduced percolation

“With sprinkler irrigation, we have found that deep percolation losses are greatly reduced compared to flood irrigation and almost no water is leached below the root zone,” Bronson says. “In buried drip, you get the added benefit of reduced evaporation from the soil.”

Thorp is a research agricultural engineer. He says less water is required by the cotton plant earlier in the season and more water later in the season. The critical water months for cotton are June to advance vegetative growth, July for flowering and early boll growth, and August for boll fill.

“If you have a given amount of water, I believe August would probably be the most critical time to apply water, with July probably the second most important,” says Thorp.

His comments are based on advanced computer modeling.

“The end result of the overhead sprinkler system should be water savings and higher cotton yields,” he says.

Reduced evaporation

“By shorting the water in the early season, we are reducing evaporation loss since we don’t have a closed canopy. We may also affect the root growth since the roots will have to search deeper for water. This can result in a better root system to support improved plant growth later in the season.

Bronson says it’s important to monitor water applications early in the season for sprinkler and drip systems.

“In our hot, arid environment, you can’t cutback water too early or else you can get smaller plants, possibly resulting in earlier flowering and fewer fruits. This could result in reduced yield potential.”

Water strategy
Overall, the ARS team is applying about 40-44 inches of water season long with the sprinklers.

Thorp said, “We put on about 10 inches of pre-plant water to fill the soil profile, let it dry, and then planted.” This was followed by three-inches of water to germinate the crop.

June irrigations will total about 1.5 to 2 inches of water per week, irrigated over 6-8 hours per day, two-to-three days a week. July and August irrigations will total about 2.5 to 3 inches per week (10-12 inches per month).

Total water use from June through August could total about 30 inches.

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Rye cover crop

To help conserve water in the soil profile and reduce soil erosion prior to planting, the researchers planted rye in mid-December as a cover crop and terminated the crop with Roundup before planting cotton.

Rye was planted as the seed cost less than wheat, barley, and other small grains. Bronson says rye is winter hearty, requires less water, and grows tall – taller than barley or wheat over a short period of time.

Bronson and Thorp have discussed water use in cotton over the years with cotton growers who estimated their annual water use to grow the crop in the 5-6 acre feet range with flood irrigation.

27 inch water savings

The researchers believe the sprinkler system could save about 27 inches of water with total water use around 44 inches, or close to 3.5 - 4 acre feet total – a two acre feet-plus savings.

The actual amount of water needed to grow cotton depends on various factors with soil type high on the list. In Central Arizona, sandy clay loam soil is common.

The ground in the sprinkler trial is more of a Trix clay loam soil commonly found on alluvial fans and flood plains with slopes of zero to 5 percent, an annual precipitation of about 7 inches, and an average temperature of about 70 degrees Fahrenheit.

These are the conditions in many areas in Arizona’s low desert.

Maximized fertigation

Another benefit of sprinkler and drip irrigation is the ability to maximize fertigation.

Bronson states, “Fertigation into a sprinkler or drip system is the most efficient way to apply fertilizer in irrigated agriculture. Fertigation in a flood system means dribbling the liquid into a canal where the delivery is less uniform and harder to mix and monitor.”

The ARS sprinkler and drip projects are mostly tax payer funded through USDA with additional dollars from Cotton Incorporated.

Automation benefits

Another benefit of an overhead sprinkler system is the machine is automated. It eliminates the need for someone to work with siphon tubes and monitor the water in the canal.

And if an equipment problem occurs, software is available to alert the user via cell phone.

In summary, Bronson says the sprinkler and drip systems are more efficient, and open the door to the benefits of conservation tillage through reduced soil and water losses.

He says, “The cover crop also reduces soil evaporation, and planting cotton into it reduces tillage time and expenses so it’s a win-win situation.”

This summer, the cotton is expected to grow well in the sprinkler and drip fields. Bronson and Thorp will analyze their findings in the months to follow.

The findings should provide cotton growers with important information which could help them gain a clearer view of irrigation strategies beyond flood irrigation.