In his drainage water management project, Minnesota farmer Darwin Roberts placed his water-level control structure in the water main along the edge of his field.
As a longtime water conservation advocate, Darwin Roberts saw not one, not two, but three reasons for installing a drainage water management (DWM) system on his Granada, Minn., farm.
"First, for nutrient retention," he says. "It will retain nitrogen and phosphorus loads from runoff, which means nutrients stay in the field where the crop can use them and out of waters. The second reason is yield. We’re able to better store water in the field where the crop can use it during the heat of summer. Third, it builds organic matter in the soil."
With DWM, also known as controlled drainage, water-level control units containing gates are installed at tile mains, submains and lateral tiles to vary the depth of drainage water outlet. Initially operated manually, there are now several units available, some of which can be installed underground in the tile line, that avoid obstructing field traffic. That is important to consider, since fields with varying grades may require more strategically placed control units than flat fields require.
How control units work. Drainage occurs when the water table rises above the outlet depth. In controlled drainage, the outlet depth is determined by the raising or lowering of gates in the water-level control structure. Typically, a farmer will:
- raise the outlet depth by adjusting the gates inside the control unit after harvest in order to limit drainage flow, thereby reducing the nitrate/phosphorus loads to drainage ditches and downstream water.
- lower the outlet depth in the spring, two weeks or so before planting, and again in the fall prior to harvest, so drainage can flow freely out of the field.
- raise the outlet depth again after spring field operations so essential water may be stored in the field for drier midsummer crop use.
Controlled drainage is best installed in a pattern tile drainage system. Fields must have 1% or less slope, allowing a single control structure to manage the water table to within 1' to 2' for as many acres as possible.
Fields are typically divided into drainage management zones according to grade and slope, with one control structure managing each zone. Gary Ekstrom, a Truman, Minn., tiling contractor who
installed Roberts’ system, advises locating control structures in as unobtrusive a place as possible.
"Fewer control structures will be less expensive, and that also means there will be fewer things in the field to farm around," Ekstrom says. "My customers, for the most part, are looking to have a nice, bare, square quarter section. They want to put on the auto-pilot and go."
Going underground. For more convenient field operations, the newest in-field control units,
such as the Water Gate by Agri Drain Corporation, are installed underground in the tile line.
Water Gate is a float-operated unit fully automated by drainage flow and pressure. It maintains a 1' increase in water elevation between the downstream side of the float-activated head pressure valve and the upstream side of the valve. A "master-controlling" water-level control structure is installed in the tile main at the lowest point of the drainage system being controlled.
The Water Gate control unit is located 1' in flow-line elevation above the water-level control structure to better keep water stored where it needs to be in the field. Depending on a field’s grade and slope, several Water Gates may be used in series by locating additional units at 1' elevation level increments.
- Mid-February 2011