Hit the Spot

March 11, 2011 11:53 PM
Hit the Spot

Irrigated corn growers spend big dollars to ensure their crops get enough water at the right time. But a recent study led by Farm Journal Associate Field Agronomist Missy Bauer indicates that many producers may not be applying the amount of water they think they are. That can affect yield and efficiency of fertigation and chemigation. Not to mention, it can waste energy.

The study was conducted in six Michigan and Indiana fields. Using a technique devised by Lyndon Kelley, an irrigation educator for Michigan State University and Purdue University, Bauer placed collection cups (32-oz. cups from Taco Bell) every 10', in a line from the pivot point to the far end of the irrigated area. She then determined the amount of water that was under- or overapplied in 10' increments down the length of the pivot.

Too little or too much. The study had two goals, Bauer explains: to determine whether the systems were applying a uniform amount of water from end to end and to determine whether they were applying the amount of water for which they were calibrated.

Not one of the six systems was applying as much water as the operator thought. The desired rate in each case was 1⁄2" of water. The average actual rate was 0.41"—82% of what the operator intended.

The measure of uniformity from nozzle to nozzle—the application of water evenly across a field—is called the system uniformity coefficient, expressed as a percentage.

In Bauer’s study, the average system uniformity coefficient for the six systems was 81%.

"You will never get 100% efficiency," Kelley says. "But most equipment is capable of 90% to 92%."
The Natural Resources Conservation Service offers an assistance program to help irrigators improve uniformity to at least 85%. "Almost all systems will benefit from some corrections," Bauer says.

"If the uniformity coefficient is from 80% to 85%, you may need to analyze your sprinkler package, and individual sections will benefit from corrections," Bauer continues. "Below 80% calls for adjusting or replacing sprinklers and making corrections to individual sections."

An inefficient system means some areas get too much or too little water at every irrigation. "A 30% deviation in an 8" application year means some areas receive as little as 5.6" while others receive 10.4"," Kelley says.

irrigation calibration cornnav

The graphs above show deviation from the target application rate in individual fields. The sawtooth pattern indicates the amount of variability. If a system was 100% effective, there would be no variability and the graph would look like a straight line.

Such graphs show you where to look for problems. In field No. 1, the system uniformity coefficient was only 76%, meaning the system was not applying water uniformly. The high points show that the sprinklers near towers 2 and 6 were applying too much water.

The low points in several areas show where sprinklers were applying too little. All sprinklers need to be checked. "This is an older center-pivot system, so some sprinklers may be showing signs of age and need to be replaced," Bauer says.

In field No. 2, "the uniformity coefficient was 84%—not too bad, but there were a few issues," Bauer says.

A few areas were being over- or underwatered. "We found some problems with nozzles that accounted for that," says Kevyn Van Wert, who manages the irrigation for North Concord Farms in Concord, Mich.

Bauer also found that the system was applying only 0.38" of water, although the operator had the equipment set to apply ½".

"We discovered that the system was moving at a speed different from that of the sprinkler chart," Van Wert explains. "We learned that by placing stakes at the desired distance and timing how far the system actually moved in one minute."

Finally, the downward slope of the orange (average) line shows the system was applying more water near the pivot and less at the outer extreme.

"The problem could turn out to be an inadequate water supply," Bauer says. "If that is the problem, possible solutions are to speed up the pump or see if the well is capable of providing more water."

No visible symptoms. One thing is certain, Van Wert says: "We will do more calibrating of our center-pivot systems. There were no symptoms visible in the crop to suggest its performance was not what we anticipated."

Calibration is well worth the time and effort, Van Wert adds. "Over-applying is wasteful, and underapplying shorts the crop," he says. "If you apply fertilizer or pesticides through the irrigation system, they also get over- or underapplied."

In his own studies, Kelley found that the first 25 systems he examined could be tweaked to gain 15% more water efficiency. "That means 15% of the water was not getting where it was supposed to," he says. "From an energy standpoint, if you can reduce water use by 15%, that’s the biggest energy savings you can get without investing in new equipment."

If weather is dry and your crop really needs the additional water, getting it the right amount at the right time can have a huge impact on yield, Bauer adds.

You can calibrate systems using information, procedures and spreadsheets available at Kelley’s website, www.msue.msu.edu/stjoseph. Click on Irrigation on the left side of the page.

"You may also find consultants or soil and water conservation districts that offer calibration as a service," Kelley says. "But the best success stories I’ve seen originated with farmers who did their own calibration. They know when pivots are running and when they are in a good location to perform the test. You might consider making one family member or employee responsible for calibration."

While similar calibration procedures are available for other types of irrigations, they may be more complicated to perform, Kelley notes. "Center-pivot systems offer the most potential for long-term improvement with minimal long-term investment," he says.

Precision Irrigation Is Here

First came yield monitors, then came variable-rate application of fertilizer, pesticides and seed. Now there’s variable-rate irrigation (VRI).

Valley Irrigation has introduced Variable Rate Zone Control for individual sprinklers and spans. The system uses Valley Pro2 control panels, Valley VRI boxes, valves, existing span cable and sprinkler package and prescriptions generated either by the farmer or by CropMetrics, a precision crop management consulting company.

The technology allows for up to 30 different VRI zones along a center-pivot irrigation system and up to 180 sectors around the field. "This creates the potential for up to 5,400 zones around a pivot circle," says Nick Emanuel of CropMetrics.

"Variable-rate irrigation closes a critical gap in the precision agriculture puzzle, tying together all the other pieces," says Jake LaRue of Valley Irrigation. "Variable-rate technology is especially valuable with irrigation because operators make multiple passes with their center-pivot systems throughout the growing season. That lets them adjust their irrigation prescriptions based on changing conditions in their fields."

Research is under way to document the effect of VRI Zone Control on power and water use. "During our limited experience in 2010, we saw water savings of 12% in one field and 13% in another," LaRue says. "We also saw reduced yield variability throughout the field because the farmer was able to minimize areas that were over-watered or underwatered. Reducing variability really is what we are trying to accomplish."

The Valley VRI Zone Control was released in 2010 on a limited basis. It will be available to all farmers for the 2011 growing season.

Another choice for variable rate is the Valley VRI Speed Control, which varies the speed of a center-pivot system around the field, using prescriptions written by the grower or CropMetrics. VRI Speed Control works with Valley Select2 and Pro2 control panels, requiring a software upgrade.

Managing Drainage Water May Boost Yield

Getting surplus water off a crop is just as important as supplying water when needed. That requires drainage.Field drainage isn’t new, but there is a new wrinkle you’ll be hearing a lot about in coming years. It’s called drainage water management (DWM).

"Drainage systems are designed to get excess water off the field quickly and efficiently," says Harold Reetz, executive director of the Agricultural Drainage Management Coalition. "Unfortunately, that continues when water supplies are short. DWM modifies the system to help manage the rate of water loss from the field. It also helps control the loss of nutrients dissolved in the water."

With subsurface drainage systems, DWM involves installing a control structure near the outlet or in the main tile line to regulate the amount and timing of shallow groundwater leaving the field. Typically, operators close the structure after harvest (in effect, raising the water table) and open it again in the spring, so the soil can dry out for planting. After planting, they use the control structure to raise the water table to within 2' of the soil surface to store water during the growing season.

Studies show DWM can reduce the loss of nitrate and phosphorus from fields. The amount of nutrient reduction depends on location, climate and other factors.

"Research to date has led to a generalization that DWM can lower discharges and pollutant loads by roughly 30% to 50%, compared with free drainage systems," says University of Minnesota soil scientist Jeff Strock.

For farmers, the most exciting aspect of DWM is that it may also increase yield. In North Carolina, some of the first studies conducted found that yield increases from DWM averaged 5% or more in some years.

Yield increases would be expected in drier growing seasons, when farmers use the control structure to maintain the water table as the crop grows, Strock says.

Yield results in the study were inconclusive, probably because the majority of the fields received abundant rainfall throughout the growing season. In addition, says the coalition’s Leonard Binstock, "many of the systems were installed on lower areas of fields, close to the tile outlets. Those areas often have heavier soils that hold more water, even in rain-deficient years."

Binstock thinks DWM will be used to set up drainage management zones on higher, doughtier soils, farther away from the outlets where control structures have traditionally been placed.
"We need to stop viewing drainage systems as designed only for stormwater management," he adds. "With drainage, we need to think outside the box."

Control structures can be retrofitted to existing drainage systems or incorporated into new systems. The practice is eligible for cost-sharing through USDA programs such as the Environmental Quality Incentives Program and the Conservation Stewardship Program. To learn what assistance is available in your area, visit your Natural Resources Conservation Service office.

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