The rationale for residue in fields with center pivot irrigation systems has already convinced lots of farmers who irrigate to switch to no-till.
In 2001, about 70% of the dryland fields in western Nebraska were no-till. Irrigated fields lagged considerably behind, says Steve Melvin, a University of Nebraska Extension educator working out of Curtis, Neb. Eight years later, about half the area's irrigated fields have some form of conservation tillage.
"We've seen a very significant number of irrigated acres change,” he says.
Look at the water savings resulting from no-till to get an idea why. No-till crops need three to five fewer inches of water per acre, whether from rainfall or irrigation, than crops in conventionally tilled fields, University of Nebraska research indicates.
Pumping costs run $5 to $10 per acre-inch on many farms in the area. That means no-tilled fields get a $15 to $50 per acre water savings.
Residue increases water infiltration in the soil. No-till farmers can decrease pivot system water pressure or even go with a narrower sprinkler pattern. That saves both water and energy. In addition, residue reduces evaporation, adding to the savings.
The idea works well with corn, soybeans and wheat, Melvin says. "Wheat probably makes the best residue for water savings. Soybeans don't produce a huge amount of residue, but if you leave it in place, the soybean residue can provide a benefit.”
Precious drops. Nebraska researchers have spent the past decade trying to save water pumped by the center pivot systems that dot the landscape throughout the western part of the state. "We've been having a situation where the groundwater dropped for a lot of farms. A drought started in the late 1990s, and this year was really the first year in a long time that our farmers have not had to deal with drought conditions. The Republican River
Basin Compact also impacted us in a big way. Allocations came into play and restricted how much water we can pump. All that is why a lot of our research has been focused on water conservation,” Melvin says.
Irrigation farmers producing 200-plus bushels of corn per acre were concerned about planting no-till through the heavy residue that high-yielding crops left on fields. Modern planters that slice through residue helped, however. "There are challenges with no-till, but conventional fields have challenges, too,” Melvin says.
Know moisture levels. No matter the tillage system, he recommends knowing how much water is in the soil profile and making sure space exists to store off-season rainwater. "You should use up to 60% of the plant-available soil water late in the irrigation season to make room for off-season moisture. If fields have a full profile after harvest, there's no storage capacity for off-season moisture,” Melvin says.
"If you leave the field wet in the fall and if you start your first irrigation too soon, you deep-percolate the saved water. It would be in the aquifer for use in future years, but it takes some nitrates and other things with it that we don't like very well,” he says.
A wide array of soil water monitoring equipment gives accurate enough measurements. You can use everything from old-school hand-push probes to electrical resistance blocks and tensiometers, all the way to ultramodern soil moisture data loggers.
In a 2006 report, Mike Morris, energy specialist with the National Center for Appropriate Technology, writes, "You may be surprised to learn how easy and inexpensive it has become to purchase, install and use a state-of-the-art monitoring system. More devices are coming on the market all the time and prices continue to fall.”
Morris urges users to have realistic expectations and not rely too much on exact readings. Instead, pay attention to trends and changes over time.
"Soil moisture measurement, even with ever-more-accurate devices, is still an art as much as a science,” Morris writes. "Soil sensing and measuring devices don't substitute for the judgment, observation and local knowledge that good irrigators acquire over time.”
Proper placement of moisture sensors improves accuracy. Put them in average soil, and stay away from field edges and areas that are normally either wet or dry. Focus on the upper half of the root zone, where plants get most of their water. If you want to measure to 3' or deeper, place sensors at three levels: in the top, middle and deepest third of the total root depth.
In fields with center pivots, put sensors a few sprinkler diameters away from where the pivot normally starts, in the direction the pivot system moves. Monitor a few sprinkler diameters before the spot where the pivot normally stops. Don't put sensors between the pivot point and the first tower since it tends to be wetter there than in the rest of the circle.
Farmers tend to call the drawing down of soil water to make room for offseason rains "mining the soil water,” which Melvin calls "an odd twist of terms.”
Still, he says, it's a good idea for efficiently using water. "We want to mine the soil water as much as we can in irrigated fields and still produce top yields,” he says. Leave about 40% of available soil water in the field.
"Take all the water you can and have a full-producing crop. Then, replenish with off-season moisture. If you leave the field at capacity at harvest, you're not gaining. That water will just percolate to the aquifer,” Melvin says.
Southwestern Nebraska gets enough off-season moisture to refill a silt loam profile after a fully watered crop. Farmers need to use every technique to make sure crops get enough water to reach full potential.
With declining groundwater levels in some areas pulling well capacity down, "interest in water savings has picked up,” Melvin says.
- November 2009