Inspecting drainage systems

Encyclopedia Article

Drainage systems are installed to lower the water table and thereby improve growing conditions. Approximately half of Iowa's cropland uses subsurface drainage. To achieve the agronomic potential of a soil, adequate tile drainage is necessary. Now is a good time to check drainage systems in fields and identify potential drainage problems before crops reach full size.

Common problems

The most common problems are misaligned, collapsed, or broken tile. Such problems may be indicated by wet spots on the field that used to drain properly. There also may be a blowout, or a place where water is bubbling up out of the ground or where soil has been pulled into the line. Examine your drainage outlet, and if you see sediment, you should start looking for unusual wet spots on your field.

Drainage Strategies:

Controlled Drainage

For some applications, producers are looking at installing controlled drainage systems, which consist of a series of “dams” in the line that help maintain the water table close to optimum levels. The idea is to stairstep the water table up and down the main and allow producers to store a little extra water in dry weather. Another benefit is that in the spring, producers can achieve some denitrification from 2 to 4 feet below the surface, allowing them to get rid of some of the deep nitrate that would otherwise go down the tile.

Tile can break down from old age and cumulative loading, especially older tile that was not designed for today's heavy equipment and axle loads. Improper installation also can lead to drainage failure. If tile is not installed below the frost line, freeze-thaw cycles can fracture tile. The original quality of tile itself is important, too. If the tile materials absorb too much water, the tile is more likely to disintegrate over time.

There are other potential in-line tile problems, including in-line siltation. Occasionally, uniform soil particle size creates problems. Unstable soils (those with weak soil structure) erode into the tile line. If the cracks or slots in perforated tile are too wide, installing tile with a fabric or a filter sock on it may be the only way to prevent sediment entry into the line. (This problem is most common in silty alluvial soils). Another cause is nearly level grades, where the flow velocity in the line is so low that it cannot suspend or carry sediment, so it settles out and fills the line.

For drainage systems that include terraces, make sure every terrace drains completely in approximately 2 days after a normal rainfall. If the water stands too long, check for a plugged inlet or outlet. If you don't see an obstruction or cannot clear it, hire a contractor to resolve the problem (it can usually wait until after harvest).

Leaving a small buffer strip around tile standpipes and inlets can settle out sediment before it enters the tile. If tile inlets are plugged with crop residue, clear them. Make sure that any vegetation around the standpipes is free of weeds, trees, and brush because thick weeds and brush can plug inlets, and tree roots can plug the tile. It's also a good idea to check intakes for machinery or livestock damage. Repair or replace broken and bent intakes and paint inlets or use warning flags to make them visible to machinery operators. Erect a temporary fence around the intake to keep livestock away.

Leave a small buffer strip around the tile standpipe.

This drainage system outlet is clear with an animal guard in place.

Unmapped tile lines

Drainage Strategies: Tile Plow

Tile plows are going like hotcakes; more than 3,000 of them have been sold in the Midwest alone. If you want to do it yourself, successfully, here are a few important considerations:

  • get training
  • understand how the machine operates
  • understand the limitations of the machine
  • the tractor has to go very slow, less than 1 mph
  • opt for the professionals in very flat grades; tile plows aren’t nearly as sensitive as the bigger professional machines

Remember that a number of old, unmapped tile lines exist, but there's no reliable technique for finding them. If you hit an old line when installing a new system and it's clean inside and carrying some water, hook it up to the new line to ensure existing drainage remains functional. Also make sure that any outlets are free and clear with an animal guard is in place. Try to keep trees at least 50 feet away from tile. If you tile through an area with trees, install non-perforated tile.

Water quality issues

Constructing and using wetlands at tile outlets for water quality is a great idea. Even with best management practices in place, most tile lines contain water at or above the nitrate drinking water standard. It is conceivable that producers may have to install water treatment systems for drainage system outlets in the future. Wetlands are a promising option: 1 acre of Iowa wetland can treat drainage from 100 acres of cropland. But if you drain a wetland in violation of Swampbuster, you could lose all of your federal farm benefit payments--a potentially devastating penalty.

If you have questions about drainage systems, check with a trusted, local contractor or staff at the nearest soil and water conservation district office. If you hire a contractor, the best advice is to use one who is familiar with the local soils and landscapes.

This article originally appeared on pages 104-105 of the IC-488(13) -- June 17, 2002 issue.

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