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Another Window on Irrigation

Wednesday, February 1, 2012
filed under: Irrigation/Water Use

In these times when maximizing crop water-use efficiency is more important than ever, a team of Colorado-based USDA Agricultural Research Service scientists believes there’s a better way to measure how irrigation water affects a crop’s yield.

Traditionally, that impact is gauged on the basis of “increased yield per unit of irrigation water applied.” But the ARS Water Management Research Unit (WMRU) at Fort Collins is measuring it in terms of “yield per unit of water actually taken in (consumed) by the crop.” That type of measurement, called a “crop water productivity function,” eliminates all water that does not enter a plant’s roots.

It’s an important distinction in this era of growing pressure to “make more with less” when it comes to irrigation water supplies. That’s especially true in farming areas where nearby municipal and industrial users are buying up farmland in order to obtain that land’s water rights — one example being along the Front Range of Colorado.

This different approach reflects “the actual strain on groundwater supplies, because water used by crops is effectively ‘lost,’ while most unused rain and irrigation water returns to groundwater or flows into streams for use downstream,” points out Tom Trout, research leader of the WMRU team.

“We’re trying to figure out a way we can sustain irrigated agriculture in the best condition possible — and still free up the water that needs to go to the cities,” Trout explains. It’s also important for farmers to know, as well as possible, how valuable their water is to crop growth if and when the day comes when those water rights are being sold, he adds.

The 2011 season marked the fourth year of a study to determine how much water is actually consumed by four crops common to the High Plains region: corn, wheat, sunflower and pinto beans. Trout and his colleagues are growing these crops in rotation on a 50-acre limited irrigation research farm near Greeley, Colo. ARS operates the farm collaboratively with Colorado State University at Fort Collins, which is about 30 miles northwest of Greeley. Trout’s WMRU colleagues on this study include agricultural engineer Walter Bausch and plant physiologists Dale Shaner and Louise Comas.

The data from this study will be used by the Fort Collins ARS Agricultural Systems Research Unit to develop a computer “decision support” model to provide farmers with documentation of water savings and information on the economic viability of limited irrigation, crop by crop, to help in their decision making.

Will Limited Irrigation Save Water?

Farmers using limited irrigation do not give crops the full irrigation amounts needed for maximum yields. Instead, they use partial irrigations timed to critical growth stages.

The WMRU team designed the original study to see whether limited irrigation is best economically for each of the four crops, and to help farmers with irrigation timing and amounts and other options. The four crops are being grown with six levels of irrigation: 100% (full irrigation); 85%; two at 70%; 55%, and finally 40% of full irrigation. The 40% level equates to what is essentially a dryland farming situation in this part of Colorado.

In terms of irrigation efficiencies, furrow (surface) irrigation has relatively low efficiency, compared to overhead applications. “As you go up the ladder, sprinkler irrigation would rank fairly high in terms of efficiency,” Bausch points out. “You can get up into the 70, 80 and 90% efficiency range, depending on the type of nozzle package that’s on the sprinkler.” Drip irrigation is even more efficient.

The WMRU team is using drip irrigation in its research project, with the tapes placed on the soil surface rather than being buried (as is typical in commercial applications). While drip is rarely used commercially on large-acreage crops like the four in this study, the team went with drip irrigation “because we have very good control of the amount of water we’re putting on — and there’s very little waste,” Bausch explains. “Even though it’s a surface drip system, the soil surface doesn’t get all that wet. It’s a band, 12 to 16 inches wide. And after the plant develops a canopy, the soil surface is pretty shaded. So evaporation is minimal.”

The research farm was set up to enable precision water control and accurate field measurements of water consumed. Use of drip irrigation eliminates the many variables found in furrow and overhead sprinkler irrigation. A series of meters and valves measures irrigation amounts.

A field weather station helps the scientists predict the rate at which water is consumed — both transpired through plant leaves and evaporated from the soil surface. Actual soil water depletion is measured by moisture sensors down to six feet. Irrigation timing is based on both the predicted rates of crop water use and the soil water depletion measurements.

A “high boy” platform with digital cameras, infrared detectors and an infrared thermometer is driven through the plots weekly to monitor crop growth and leaf temperature, an indicator of crop water deprivation.

Sell One Bushel of Corn or 2,000 Gallons of Water?

Four seasons of corn data (2008-2011) showed yields varying from 210 bu/ac for full irrigation down to 80 bushels for the lowest irrigation level. The team found that the corn plants on one acre of land need to consume about 600,000 gallons of water — from irrigation and rain — to produce 200 bushels of corn. “After an initial amount of water to get the corn growing, the consumption rate stayed about the same through all six levels of irrigation—about 2,000 gallons per bushel of corn,” Trout says.

This flies in the face of the traditional belief that crops use water less efficiently as they get more of it. But in this experiment, Trout found that while that’s true in terms of drops of irrigation water applied or rainfall, it is not necessarily true in terms of drops of water consumed. In other words, there is little reduction in the amount of water corn takes in to produce each bushel, despite the reduction in the amount of irrigation water applied. This may make limited irrigation less attractive financially, at least for corn, in this region.

“Corn farmers might do better financially to use full irrigation on a portion of their irrigated acres, rather than limited irrigation spread over all the acres,” Trout says. “They could then sell or lease the water rights on the nonirrigated acres. Another option would be to grow a different crop that requires less water, if the economics of limited irrigation work for that crop.”

These results are preliminary, he emphasizes, and may vary with changes in the timing of water applications, type of crop or variety of corn.

Sunflower Results

Sunflower data are available for three growing seasons at the Greeley location — 2008, 2010 and 2011. Rainfall during those three growing seasons amounted to 7.5, 6.0 and 4.0 inches, respectively. After a fall or spring strip-till pass, the sunflower was planted during the first week of June each year into heavy corn residue.

As would be expected, sunflower plant height was highest with the 100% and 85% irrigation levels, and lowest at the 40% level. The increased canopy cover with even the 55% irrigation versus 40% contributed to better yields in 2010 and 2011. Temperature measurements indicated that the sunflower canopy is cooler than the air temperature, which in turn suggested that the sunflower was not experiencing crop water stress. “The plant growth period between R3 (latter bud stage) and R6 (ray flowers wilting) is very sensitive to crop water stress; consequently, adequate irrigation during this period is critical to minimize plant water stress and boost crop yield,” Bausch points out.

In terms of the yield:water relationship in sunflower at the Greeley site, “the 2008 data indicate smaller responses to the deficit irrigation treatments due to fairly high soil moisture to start the year and ‘untimely’ precipitation,” Bausch explains. Overall, across the three years, “water production function based on applied irrigation curves downward as the water application decreases,” he observes, meaning that “the marginal productivity of irrigation water (i.e., additional yield per unit of additional water) is relatively low near full irrigation.”

On the other hand, the water production function for yield based on evapo-transpiration is relatively straight. “This implies that sunflower is equally efficient in its use of every additional unit of water consumed (about 180 lbs per inch of water applied) — and the marginal value of the consumptively used water is fairly constant over a wide range of water applications,” Bausch reports.

The water use numbers for sunflower in 2010 and 2011 — the best data years — showed that it required about 17.5 inches of water consumed to produce a little over 3,000 lbs/ac of sunflower seed. This is equivalent to about 180 lbs/inch or 150 gal/lb [compared to corn at 22.5 inches to produce 210 bu (12,000 lbs) or 2,500 gal/bu or 45 gal/lb]. “Thus, at full irrigation, corn produces about three times the seed yield per unit water,” Trout says.

“For sunflower, this rate of water use stayed pretty constant as we reduced irrigation. We got about 2,000 lbs/ac of seed with 12 inches of water consumed; and we were still getting decent yield (1,650 lbs) with 10 inches of water. However, with corn, 10 inches will not produce yield in this area; it takes about 13 inches of water to get any consistent yield. Corn has much higher potential water productivity, but sunflower is more drought tolerant.”

So what is the bottom line for sunflower, based on these three years of limited irrigation research at Greeley? Although corn is more productive with good water supply, sunflower is more drought tolerant and will produce more dependably with a poor water supply, according to Trout and Bausch. For both crops, if the cost of your water is based on irrigation amount, deficit irrigation can be beneficial because it uses rainfall and stored water more efficiently. However, if the value of your water is based on consumption, you will likely be better off to reduce your acres and irrigate at near full water requirements, state the ARS scientists.

— Don Lilleboe

Note: Part of this article consists of information provided in “Growing Crops and Saving Water in the West,” written by Don Comis, from the August 2011 issue of Agricultural Research, a publication of the USDA Agricultural Research Service.
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