In the northern fringes of the Missouri Bootheel, an old irrigation system is new again — one that provides both field drainage and sub-irrigation to a crop.
“We're getting back to a system we moved away from in the early 1980s,” says Mark Nussbaum, USDA-NRCS engineer, who spoke at a soil and water conference at the Delta Center, Portageville, Mo.
“It involves the application of tile on a lateral manifold system to drain wet soil in the spring and fall. What we found with the laser installation, was that it didn't take very much to add sub-irrigation capabilities — an outlet structure and adjustments in our thinking about manufacturing a dual-purpose system.
“We've been able to take the same technology and systems that control laser leveling and apply them to tile installation. That means we can get away from the old open trench installation methods.”
Being able to do so “tripped the trigger; the accuracy and quality of the new system went way up, while the cost plummeted.”
The average cost of open trench installation for anything the USDA's Natural Resources Conservation Service is currently doing is $1.01 per foot — and that's only to dig and bury; it doesn't count the cost of putting the product in the ground.
With the new tiling system, the average cost of installation runs about 24 cents per foot. That 75 percent reduction in cost “has made all the difference,” Nussbaum says. “Tile installation is a high-footage-per-acre project. The tile spacing we're currently using is from 30 feet to 45 feet, measured between tile centers.”
There are several tile installation plows available. The ones used in the Bootheel have the ability to install to about 4.8 feet of depth, maybe up to 5 feet.
The drainage/sub-irrigation system improves agronomic conditions by providing drainage during wet periods. A secondary goal — fast becoming a primary goal — is to provide high efficiency irrigation capability.
Nussbaum says there are two reasons NRCS is interested in it.
Producers are requesting technical assistance.
Water quality benefits. “This is a tough thing to quantify, but we've found if we can provide a controlled drainage water table environment, a better predictability of nitrogen usage is possible. That can lead to reduced nitrate losses from the field.
“Water table control during the winter results in nitrate losses being further reduced. These systems also lessen surface runoff by about 30 percent, which can reduce phosphorus losses.”
To date, project locations have centered around Cape Girardeau, Scott, and St. Genevieve counties, with the most intensive area, by far, in Cape Girardeau County.
The systems are typically placed in creek bottoms, smaller floodplains that average 10 acres to 40 acres. Soils are very fertile, productive for at least 35 inches in profile depth.
“That's conservative; most probably are fertile to 48 inches or more,” Nussbaum says. “These fertile soils are over a restrictive layer that's similar to a Sharkey clay. Fields are very wet in the spring and fall, and many contain spring, winter, and fall seeps. The sticky subsoil, which once caused the wetness problems, now acts as a floor to hold water during sub-irrigation.”
Several things go into designing one of the systems.
“First, we do a certified wetlands determination. We're working with soils that if they were a bit wetter couldn't be worked from a drainage standpoint. If much wetter, they would be classified as wetlands and would qualify for the Wetlands Reserve Program.”
NRCS soil scientists use an electromagnetic machine that measures the differences in soil properties throughout the field. They then probe where the electromagnetic values are different, find soil trends, and identify correlations.
“I think this is one of the most important things we do,” Nussbaum says. “The soil scientists determine the depth to the restrictive layer. If we install the lateral tile in a restrictive layer, that type of soil can cover it. If that happens, the tile is isolated and might as well have not been installed.”
If the restrictive layer is shallower than about 40 inches, the field may not be a good candidate for the dual system.
An anecdotal observation is proving extremely applicable, say those involved in the installations. Look for fields with a large number of crawfish stacks in the spring and fall — these have typically proven well-suited to a drainage/sub-irrigation system.
“They are an indicator of the water table and what's going on, so look for them,” Nussbaum says.
Land-graders usually survey candidate fields and NRCS coordinated the grading design on the surface with the drainage/sub-irrigation design.
“My ideal is a manifold and lateral system 3.25 feet below the surface that's parallel to the surface in both directions,” he says. “If we can dovetail the surface plane with the sub-grade, it works great. We're getting pretty good at that.”
The soils and survey data are used to determine the manifold size and lateral spacing. Currently, the lateral spacing is an estimate.
“In our project area, if you want an irrigation-intensive design, we recommend a 30-foot to 40-foot spacing. If you just want to emphasize drainage, we suggest running 45-foot spacing. Both numbers are probably conservative. NRCS Engineer John Hester is conducting field tests and computer models of various lateral spacings, and we're fine-tuning our designs as the data come in.”
For irrigation, the systems are designed to maintain a water table of about 15 inches to 24 inches below the soil surface during a period of high irrigation demand.
The systems must not be allowed to dry out during irrigation season — hydraulic conductivity plummets in a dry profile.
“If you ever get to a fully dry situation in a sub-grade system, you're in trouble,” Nussbaum says. “The lateral capability of that soil drops by a factor of about a third of its moist capability.”
As for irrigation water sources, many of the fields have previously tiled springs. The springs are wedded to the system and most often provide about 0.5 to 1 gallon per minute, per acre.
“On these sub-irrigation systems, somewhere around June 10 to June 15, producers will put the stop-logs in and try to (maintain) the water level. They then add water 24/7 — somewhere around 1.5 gallons to 1.6 gallons per minute, per acre. The extreme low we've seen is 1.2 gallons, with an extreme high of 1.8 gallons.”
If you've got an 80-acre field, that would be around 120 to 130 gallons per minute, per acre, on a continual basis, and that's including the spring contribution. If you add more than that, water is usually pushed out the tail end.
In the sub-irrigation water movement, water can be seen rising about 16 inches above the table in these soils.
“You'll be able to watch it climb rapidly,” Nussbaum says. “When the crop is drawing water and demanding it, we believe the water can rise around 20 inches to 22 inches. We also believe water can move a maximum of about 20 feet laterally from the pipe during active sub-irrigation period.”
During the drainage system phase, the tiles are drawing water in; during the sub-irrigation season, the flow is reversed.
What does a typical installation look like?
“Fields are graded with a design conducive to the drainage and sub-surface irrigation,” Nussbaum says. “This is the only area I know of that does that. Michigan, Ohio and North Carolina will do opportunistic sub-irrigation. We're not familiar with a dove-tailed design being used anywhere else. The reason that's important is we want to make irrigation predictable and uniform.”
He likes to see a slope and a cross-slope a tad steeper than what's typically land-graded in the Bootheel.
“We've found that the smaller creek bottoms have more grade fall. We can easily go down to a .05 cross-slope grade before incurring increased costs — .05 to a .25 per 100 feet is the sweet spot for either slope or cross-slope. It's to our advantage to have both slope and cross-slope in the field.”
Another tip: “We've learned the hard way to run the manifold line deeper than the laterals. You want laterals to dive off into the manifold at the hook-up points. If there's a lot of sub-grade water, the moment we begin placing pipe, there's through-flow, which is a constant headache.”
Horsepower isn't a factor in installation, but traction is.
“The reason is the (pipe) feed is very slow — around 3 feet per second. Running that slowly allows the hydraulics a chance to keep up. Very often, you'll see double-hitched four-wheel-drive tractors pulling in the manifold due to the heavy draft. The smaller lateral lines pull much easier. The tile is purchased by the 18-wheeler load.”
The stop-log boxes are very simple, usually 4 feet to 5 feet tall. They can be used for through-flow drainage or to back the water all the way to the top of the box. When the water exceeds the desired design level, it cascades over and drains out of the system. During the winter, producers can re-flood the fields to reduce nitrate losses.
“We can put in multiple boxes and stair-step a field. One project we're working on has five boxes that will drain/sub-irrigate 40 acres. The boxes cost about $600 apiece to install. As long as we add water to the system at, or above, the uppermost box, the water cascades down and regulates itself.”