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A solution for tile drainage

A new conservation practice called Drainage Water Management, or DWM for short, is gaining a lot of attention among farmers, tiling contractors and others. The practice helps reduce the amount of nutrients, such as nitrates and phosphorus, from negatively impacting the environment, and it is now available for USDA cost-share funding in Iowa.

Subsurface drainage tile makes profitable crop production possible on Iowa’s flatter landscapes. DWM allows farmers with subsurface drainage tile to hold water in root zones when crops need it and drains the water away when there is too much. The practice manages the timing and amount of water discharged from agricultural drainage systems.

With appropriate management, DWM systems may also retain water needed for late-season crop production.

Key Points

Installing a drainage water management system is a good solution for tile drainage.

The system uses existing tile lines in a way that helps solve pollution problems.

DWM offers valuable options to those who have very flat ground to drain.


USDA’s Natural Resources Conservation Service is promoting DWM to improve water quality in the state’s rivers and streams. Allen Gehring, state engineer for NRCS in Iowa, says his agency can assist farmers by implementing DWM plans. “This system uses existing tile lines in a way that makes them part of the solution and not part of the problem,” he says. “Drainage Water Management offers valuable options to Iowa landowners with very flat ground.”

How this system works

The DWM system includes retrofitting existing tile with a water-control structure. Each structure controls an elevation-defined area, based on the lay of the land and the tile system layout already in place.

Bruce Atherton, agricultural engineer for NRCS in Iowa, says the structures are small and reasonably priced, and operating instructions are fairly simple. For example, a farmer would need to make about six trips per year to adjust control structure elevation in a 75-acre field with five control structures.

Water level elevations are adjusted using riser boards. Following is what a farmer might expect for annual operation and maintenance:

Remove riser boards to drop the water table levels about 10 days prior to planting and before any spring tillage.

During the growing season, stack riser boards to potentially raise the water table high enough to provide capillary water to the crop root zone.

Before fall harvest, if needed, remove boards to lower the water table 10 days before fall fieldwork.

After harvest, install riser boards to potentially raise the water table up even further — near the ground surface — to hold water and nutrients in the field and soil over winter.

Water quality issues

The Gulf of Mexico’s “Dead Zone” has been largely attributed to agricultural runoff from the Midwest. Agriculture has been similarly targeted closer to home, where amounts of nitrogen in Iowa’s drinking water supplies have increased dramatically throughout the past century.

For example, in a 2000 U.S. Geological Survey study, average annual nitrate concentrations in the Des Moines and Cedar rivers have increased nearly seven times over the last 100 years, from about 0.6 milligrams per liter to as much as 4.6 mg/l.

And data provided by Des Moines Water Works dating back to the 1930s shows nitrate levels in the Raccoon River stayed consistently below 1 mg/l until the
mid-1960s. Since then, nitrate levels have increased to as high as 10.2 mg/l in 2002. In the last 20 years, average nitrate levels in the Raccoon River averaged 7.3 mg/l.

Linda Kinman, public policy analyst with Des Moines Water Works, says she and other watershed advocates agree that elevated nitrate levels in many of Iowa’s major watersheds coincide with changes that have taken place in agriculture. “Nitrate levels began to increase about the same time as a major increase in the number of corn and soybean acres, as well as the broader use of fertilizers,” she says.

Along with helping to reduce the amount of nitrates and phosphorus from existing drainage tile, using the DWM practice can also help crop production during dry years by holding water in the soil profile. In a 20-year study conducted by North Carolina State University, controlled drainage plots experienced significant corn yield increases (10.4%) compared to the free drainage plots. Soybean yields increased in all years, as well.

Further assistance is available. Iowa farmers may be eligible for financial and technical assistance to implement DWM through the NRCS-administered Environmental Quality Incentives Program, or EQIP. Iowa farmers may receive assistance to create a DWM plan, install control structures, or manage the control structures. For more information about DWM, visit your local NRCS field office.

Johnson is with the USDA NRCS in Iowa.

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SMART DRAINAGE: Drainage water management uses a series of in-line water gates with a water level control structure to control the water table in a tile-drained field. This system uses existing tile lines in a way that makes them part of the pollution solution and not part of the problem.

This article published in the May, 2012 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.