Damage Control

February 8, 2011 03:38 AM
Damage Control

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.

Another technological advancement is electronically controlled water-level control units, in which the control gates can be remotely operated—even from a home computer. This means a farmer does not need to slog through a muddy field to manually raise and lower the gates, which is when gates typically must be raised or lowered to maintain the proper drainage flow.

Yield boost. Roberts installed his system on a corner of his farm in late fall of 2006, in part using funds from an Environmental Protection Agency grant. The system tiles 12 acres but drains about 20 acres. This section of his farm meets the parameters for controlled tile systems: less than 1% slope and not consisting of excessively sandy or excessively blue-clay soils.

Roberts installed new mains running alongside the field parallel to a county road, so he was able to install the one water-level control unit in the main tile line at the edge of the field, out of the way of field traffic. Ekstrom, going by the design of an engineering firm, installed 4,000' of laterals at 75' spacings. No other in-field control units were installed. The total cost of the project was $6,397 for design, materials, control units and installation.

Corn yields on the newly tiled ground after two of three full growing seasons (using a corn-soybean rotation) have met Roberts’ expectations, with a 3 bu. to 5 bu. per acre increase.

This is a typical range, according to controlled-drainage studies across the Midwest. A recent Ohio State study determines that for each dollar spent on drainage, farmers can expect $3 to $4 back in corn and soybean profit.

"It’s even getting better now," Roberts says. "However, I haven’t seen quite the yield advantage on soybeans. Water retention on beans doesn’t seem to be as crucial because soybeans don’t like wet feet. Overall, we have seen a 2 bu. to 3 bu. advantage on beans."

While he knows he’s retaining nitrogen and phosphorus loads from drainage water, Roberts hopes to embark on a testing program with the Water Resource Center at Minnesota State University for conclusive results. Many studies have indicated reductions in annual nitrate load in drain flow from 15% to 75%. The variables in that range include location, climate, soil type and cropping practice.

For Roberts, a longtime supervisor on the county soil and water conservation board, chief concerns are nutrient retention, water quality and yield. He believes tiling with controlled technology can also help boost land values.

"Yes, tiling is expensive, but the long-term payback is tremendous because of the value it adds to the land," he says.

The ROI on DWM

The Agricultural Drainage Management Coalition, a group of Midwestern tiling contractors/designers, tiling equipment manufacturers and commodity groups, have drawn up a cost-versus-income comparison for a controlled drainage project installed on 160 acres with five water-level control structures. Income projections are based on $4 per bushel corn.

Cost Analysis

  • Added cost per acre for system design and tile: $50
  • DWM gate structure price and installation: $1,100 each
  • Total cost: $15,700, or $98.13 per acre
  • 15-year amortized cost (without tax credits or funding from cost-share programs now available, such as the Environmental Quality Incentives Program): $1,608.28 annually, or $10.05 per acre

Annual Corn Income Projections (after DWM installation)
(Projections based on 3% and 5% yield increase scenarios, which many experts believe is the range in yield potential for DWM)

  • 3% yield increase of 4.8 bu. per acre x $4 = $19.20, minus amortized annual cost ($10.05). Net: $9.15 per acre, or $1,464 on 160 acres.
  • 5% yield increase of 8 bu. per acre x $4 = $32, minus amortized annual cost ($10.05). Net: $21.95 per acre, or $3,512 on 160 acres.
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