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Manage Your Water Table

16:57PM Aug 25, 2015

It might be time to take a fresh look at farm drainage systems. Two studies conducted by Farm Journal indicate managing the water table can pay off in higher yields.   

Water table management is a new step in precision farming. The goal is to maintain the water supply across the field, as required by varying soil types and topography.

There are two ways to manage the depth of the water table: through the design of the tile system and by installing gates that regulate the flow of drainage water.

On a 40-acre study site, water is managed through various tile designs to cater to topography, says Farm Journal Field Agronomist Ken Ferrie.

“Tile improved yield everywhere it was installed, but the greatest yield improvement came where we controlled the depth of the water table,” he explains. “Water table management outyielded traditional pattern tiling every year.”

The materials to control the depth of the water table were comparable to pattern tiling at 30' intervals but considerably more than pattern tiling with 120' spacing. Over the course of 12 years, though, the system paid for itself many times, Ferrie notes.

At the study site, the designers laid out a system to maintain the desired depth at all elevations in the field. In higher areas, they laid out a fishbone or spiderweb design to keep all the lateral lines at the desired depth, rather than cutting through the area as with normal pattern tiling. That would have placed tile too deep through the higher elevation. “A rise in elevation typically means you have lighter soil, which is more affected by drought,” Ferrie says.

“With this type of tile system, you might change the width of the tile lines because of soil characteristics,” he adds, “but all tile lines remain at the crucial depth on the elevated areas, so rises in the field no longer suffer during dry weather.”

“In the higher areas, instead of going up the slopes with our tile, we go across them. This lets us catch sidehill seeps and reduce the risk of crops burning up during a drought,” he adds.

To drain the areas behind the rise, the crew ran a tile line through the higher area, similar to normal pattern tiling, but used non-perforated tile.

At the second drainage study site, involving 240 acres, the topography is flatter. Ferrie has devoted 80 acres to water table management, using gates in the tile lines to regulate the outward flow of water.

“Maintaining the water table was not our main goal when we began this project,” Ferrie says. “Our objective was simply to improve yield by providing timely drainage based on tile spacing and depth.
“However, we laid the tile so it could be gated in the future. Two years ago, we installed gates, which let us manipulate the water table above the tile depth when desired. If the depth of the tile line is 4' and we want to raise the water table to 3' or 3½', we close some of the gates. At the end of the season, if we chose to, we can close all the gates, and the effect would be like an untiled field.

“In the spring, we lower the water table just enough to farm the field without creating soil compaction,” Ferrie continues. “As the plants grow, we monitor soil moisture. As the roots reach the water table, we open gates and lower the table ahead of them.”

To maintain an artificially high water table, tile lines might have to be closer together than with standard pattern tiling. “If tile lines are wider apart and heavy rain is forecasted, we open the gates ahead of time and lower the water table to make room for it,” Ferrie says. “As the plants grow, we continue to open more gates until we reach the natural flow of the tile. Managing the depth of the water table this way might gain us several inches of water during the growing season.”

As with every management practice, there are challenges. “The tile system must be designed for use with the tile gates,” Ferrie says. “If there is a lot of fall to the tile line on its way to the gate, water could be almost above ground level at the low end where the gate is—similar to the situation with a plugged tile line. In this case, you might have to cut in additional lateral lines, add more mains and install additional gates to prevent saturated zones. The length and the fall, or slope, of the tile line determines how easy or difficult it is to gate a tile system.”  

Designing and gating a tile system is more difficult if the field is already tiled. “You have to know where the mains and laterals are located and their slope and grade,” Ferrie says. “Unless you have good records, this might require a lot of research.”

It might be possible to address changes in elevation with a gated tile system, but it’s more difficult, Ferrie says. If there are significant elevation changes, it might be more practical to use more laterals across the high spots.

The study on the flatter area using tile gates has only been in effect since 2014, which was a very wet year (and 2015 was even wetter), so the impact on yield can’t yet be documented. “But the results of the 12-year study suggest controlling the depth of the water table might be a new frontier for crop production,” Ferrie says.

Rethink the Concept of Drainage

Adopting water table management requires rethinking the concept of tile drainage, says Farm Journal Field Agronomist Ken Ferrie.

“With traditional farm drainage, our only goal is to remove water from the root zone within 24 hours of a saturating rainfall, so we can get oxygen back in,” he explains. “That objective determines the size, depth and spacing of tile lines.”

Managing the height of the water table is more complicated. “Instead of simply removing excess water from the root zone, we want to keep the water table far enough from the surface so plants never have to endure saturated conditions, but close enough for the soil to wick water back up to the root zone as plants need it,” Ferrie says.

The depth of tile required to do this varies by soil type because soils have different wicking capabilities.

“Water moves upward from the water table by adhesion and cohesion,” Ferrie explains. “The water molecule consists of one oxygen ion and two hydrogen ions, which give the water molecule a positive side and a negative side. The positive side clings to negative sites on the soil surface and on organic matter in the soil—that’s adhesion. In addition, the positive end of a water molecule hooks to the negative end of another water molecule—this cohesion enables water to pull itself through the soil.”

The number of adhesion sites in soil determines how far water can be lifted upward from the water table, defying gravity, which exerts a downward pull. “Adhesion sites consist of clay particles and organic matter content in the soil,” Ferrie says. “The relative number of adhesion sites is sometimes called cation exchange capacity [CEC]. A higher CEC means there are more adhesion sites, so we can lift water higher above the water table.”

The effect of adhesion and cohesion is different for each soil type. That’s why a silty clay loam soil, with more adhesion sites and a higher CEC value, might lift water 3', but a sandier soil may only lift it 6" to 8".
To manage the water table, tile must be placed at the depth from which the soil can lift water. “Tile will need to be placed shallower and closer than it would be if the only objective were drainage,” Ferrie says. “The drainage system still must rid the root zone of water in 24 hours, so oxygen can get back in. The water table must also be close enough to the surface for the crop to use soil moisture as long as possible.”

Make Sure You Have an Outlet

The first step in designing any drainage system is to make sure you have an adequate outlet for the water, says Farm Journal Field Agronomist Ken Ferrie. “It does no good to place tile if there is no place for the water to go,” he says.

“The outlet becomes even more critical if your goal is to manage the water table,” Ferrie continues. “In a traditional tile system, you have the option of laying lines deeper and pulling the water table lower to make room for more water in the upper soil profile. (Even then, with continued rainfall, if the water has no place to go, you might run out of storage capacity and wind up with waterlogged soil in a wet year like 2015.) But with water table management, tile lines might need to be shallower, at the rooting depth of the crop. Without a good outlet, the system won’t work.

“You might find installing an outlet for water table management is more expensive than for a traditional pattern tile system,” Ferrie adds. “Finally, make sure you have a large enough main to carry the drainage water from the laterals to the outlet. In some situations, even if you have an outlet, the cost of a large enough main and the distance to the outlet may become prohibitive.”   

 

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