Challenges in Variable-Rate Irrigation

December 5, 2015 02:16 AM

Fixing infiltration problems can make a good tool even better

Without a doubt, variable-rate irrigation (VRI) is a giant step to higher yields, wiser use of resources and lower input costs. But there’s more to efficient water management than applying more here and less there.

You have to get the water into the soil, says Farm Journal Field Agronomist Ken Ferrie. If it runs off, crops go thirsty. If it accumulates in lower spots, it waterlogs the soil and stifles roots. 

Efficient irrigation requires a three-pronged approach: 

  • Determine where water is needed based on soil type
  • Apply the correct amount of water using VRI technology
  • Make sure water infiltrates rather than runs off 

“The basic concept of irrigation is simple: meet the crop’s season-long demand for water and avoid stress,” Ferrie says. “Inventory the soil’s water-supplying ability, which is tied to soil type and texture. Consider the amount of seasonal rainfall. 

Water infiltration tests revealed poor water percolation on sandy soil was preventing variable-rate irrigation from realizing its full potential on this Farm Journal test field. A soil’s water infiltration rate might be improved by creating a vertical environment, including no-till, strip-till and cover crops.

“Compare that total to the crop’s requirement, and supply the balance with irrigation. It’s an ongoing process that irrigators monitor all season long, revising their irrigation program at least weekly, if not daily,” he adds.

Because soils vary in their water-supplying ability, farmers know applying a standard rate across an entire field overwaters some areas and underwaters others. That’s what led to variable-rate irrigation technology. But Farm Journal’s VRI study shows it’s not simply a matter of applying more water to this soil and less water to that one.

Ferrie used yield maps and thermal imaging to pinpoint areas of crop stress along with soil sensors that revealed when water fronts were separating and the crop needed more water. (When applied water meets the moisture front in the soil, the water “pulls” itself up through the plant and transpires from the leaves into the atmosphere.) 

“Using VRI technology, we applied more water to the zones that needed it,” Ferrie says. “To our surprise, a number of the areas where we added water showed no reduction in stress. Moisture sensors showed we were not changing the soil moisture content. That told us the additional water we applied was running off, rather than infiltrating.”

Using a rainfall simulator developed at Cornell University, Ferrie’s staff studied the infiltration rate, which is affected by soil texture, surface crusting, residue cover and soil conductivity. Conductivity is a characteristic of soil texture: The number of electrical charges on soil particles determine the soil’s ability to break the surface tension of water.

“In soil with poor conductivity, if you fill a small hole with water, it’s surprising how long it takes it to seep away,” Ferrie says. “The areas with coarse sands and low conductivity suffered the most water stress. Slope was also a factor. Poor soil conductivity combined with slope resulted in rapid runoff.” 

VRI became not just a matter of applying more water to certain areas, but applying it at a rate the soil could absorb. That required applying less water with each pass of the center-pivot irrigation and turning the pivot faster. “Instead of applying 0.8" of water at every turn of the pivot, we needed to apply 0.2" or 0.3" in multiple applications,” Ferrie explains.

Yield results from studies comparing a standard rate to frequent, smaller waterings suggest Ferrie is on the right track. He compared a uniform rate of 14.8" of water over 16 applications to using VRI technology to apply 14.8" of water on sand knobs and 9.2" on sandy loam soil over 24 waterings.

“With more frequent, smaller waterings, the sand knobs yielded 30.8 bu. more per acre because we did a better job of matching the application rate to the infiltration capacity of the soil,” Ferrie says. “The sandy loam soil also showed a yield increase of 16.9 bu. per acre, with 4.7" less water applied.  That indicated we had been overwatering those areas in the past.”  

“We have learned we have to match application rate to infiltration capacity to avoid wasting water and reducing yield,” Ferrie summarizes. “That’s almost impossible to do without variable-rate technology.” 

Infiltration rate—a component of soil health, texture, conductivity, slope and surface residue cover—is a concern even if you solely rely on natural rainfall. For irrigators, even VRI technology might not maximize water usage if soil has poor infiltration. 

“Most farmers understand infiltration issues on heavy clay, where the surface tends to seal, but our research confirms infiltration might also be a problem in sandy soil with some slope and poor conductivity,” Ferrie says. “It also indicates there are steps you can take to improve the infiltration rate.”  

Previous experience shows converting soil from a horizontal to a vertical environment—no-till, strip-till or vertical primary tillage in the fall followed by vertical leveling in the spring—can improve infiltration. In the irrigation study field, converting from tillage to no-till on sandy soil at the top of an elevation rise increased the infiltration rate from 6.7" per hour to 8"—and about half as much on a sidehill. 

Cover crops might also help. “This past spring, on a small scale, we tried to improve the sandy soil’s infiltration by planting a radish cover crop,” Ferrie says. “Then we planted directly into the cover. The result was a fourfold increase in the infiltration rate. 

“Now we want to see if the effect of the radishes continues into the next growing season. Elsewhere, we’ve observed, in sandy soil, radish holes tend to fill in when the plants die (unlike clay soil, in which the holes remain as late as July). We also want to find out if we can get the same effect on infiltration by planting radishes in the fall.”

Poor infiltration might also be caused by a dense soil layer beneath the surface, placed by horizontal tillage. (See “A Vertical Environment Maximizes Water Infiltration” below.)

The nature of center-pivot irrigation systems might create yet another situation in which you’ll want to improve infiltration. “It’s a situation in which VRI alone isn’t enough,” Ferrie says. 

A center-pivot system has nozzles at fixed intervals, typically 8' or 10' apart. As the pivot turns in a circle, 
the end travels faster and covers more acres. The system might travel 1½' per minute at the pivot point, but 5½' per minute at the outside span. Using larger nozzles increases the flow rate, at constant pressure, the farther from the pivot point.

This table shows how coverage and flow rate vary across a 1,300' center-pivot system, applying 6 gal. per minute (gpm) per acre.

For example, to apply a constant rate of water through a 1,300' center pivot, the flow rate might be 7.2 gal. per minute (gpm) for the first 130' of boom but 138.6 gpm for the last 130', Ferrie explains. (See table to the right.)

To apply 1" of water per hour on an 80-acre field, covering half a circle, in 12 hours, the application rate might be 3" per hour for the first boom section, but 11" per hour for the last section.

“What if there’s a sand hill, a rise in elevation, where the infiltration rate is only 4" to 6" per hour—half that of the other soil in the field?” Ferrie says. “If the knob is close to the pivot point, you’re OK, but if it’s at the end of the boom, even with VRI, you’re applying water faster than the soil can take it in. The excess will run off. The only option is to improve the infiltration rate.”

Stay tuned. We’re still learning tricks to make variable-irrigation technology, a great tool, even better. 

As a limited yet vital input, water demands a high level of diligence. The Water Management series details how farmers can manage earth’s most valuable resource to boost yields and profit. water_management

A Vertical Environment Maximizes Water Infiltration

When poor soil conductivity or subsurface layers prevent water infiltration, converting fields to a vertical environment can help, says Farm Journal Field Agronomist Ken Ferrie. Vertical farming might include no-till, strip-till or vertical primary tillage in the fall followed by vertical leveling in the spring.

If you switch to no-till or strip-till to improve water infiltration, the last pass before converting should not be horizontal tillage, Ferrie emphasizes. “If you use a horizontal tillage tool, it will create a layer—a change in soil density—at the depth of tillage, which water will have trouble penetrating,” he says. 

To transition from horizontal tillage in a corn/soybean rotation, run an in-line ripper in soybean stubble, then make a vertical leveling pass in the spring before planting corn. Or, run a conservation chisel in cornstalks to remove subsurface layers, level vertically in the spring before planting soybeans, and then no-till the following crops. 

Farm Journal studies show the importance of switching from horizontal to vertical tillage and removing subsurface layers before instituting no-till or strip-till.

In one study, on Ipava silt loam soil, Ferrie compared three management regimes. One treatment consisted of horizontal tillage—a soil finishing tool over soybean stubble in the spring. The second plot had been no-tilled for four years, but it contained a 4”-deep tillage layer, placed there during the last tillage pass with a soil finisher. The third treatment had been no-tilled for four years, but subsurface layers had been removed before converting from horizontal tillage to no-till. 

If you opt to no-till or strip-till, be sure to remove subsurface density layers before converting. The last pass before no-till or strip-till must not be horizontal tillage.

When water was applied with a rainfall simulator, the soil-finisher plot took in 1.9" per hour. The no-till plot with the layer absorbed 2" per hour. The no-till plot with no subsurface restriction—a true vertical environment—infiltrated 2.6" per hour.   

“Looking at the top few inches of soil, we saw no visible difference in the no-till plots in terms of residue cover or night crawler activity,” Ferrie says. “Where the subsurface layer was present, four years of no-till had not been able to remove it, and it was still restricting water infiltration.” 

To document the effect of the layer, Ferrie drove metal rings into the soil. He drove one ring 2" deep and the second one 4" deep to penetrate into the layer. He poured 3" of water into the rings. In the shallow ring, the water infiltrated into the soil in 4½ minutes. In the deep ring, it took 20 minutes and 42 seconds. 

“In the shallow ring, water percolated down to the layer, then spread across it,” Ferrie says. “In the deeper ring, the water stood until it was able to soak through. Both situations can cause ponding, runoff and wasted water.” 



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