Drainage Water Management (Controlled Drainage) Update

By: Norman Fausey and Larry C. Brown

In 2015 we estimated more than 200 Drainage Water Management (DWM) structures had been installed in Ohio. At this time we estimate as many as 500 have been installed or are to be installed this spring. Many of these are have been installed in Northwest Ohio, in the Lake Erie Basin. A substantial number of structures have been installed to reduce liquid manure discharges from the application of liquid manure on subsurface drained cropland all across Ohio.

The primary purpose of DWM is the reduction of soluble nutrients discharged from subsurface (tile) drainage systems to ditches and streams. Soluble nutrients move with the water, so to reduce nutrient discharges, it is necessary to reduce the discharge of water from the subsurface drainage system. Extensive research from Ohio and across the Midwest indicates that DWM can substantially reduce the discharge of drainage water during the non-growing season compared to free drainage at drain depth. DWM structures allow the drainage system outlet elevation to be raised or lowered rather than being unmanaged.

Drainage water management is first and foremost an environmental protection practice intended to reduce nutrient delivery to streams. DWM may have some production benefits, but it is not primarily a production practice. Non-growing season (winter) management is essential to realize the environmental benefits. A much greater level of management is needed for crop production benefits than for environmental benefits.

Figure 1 illustrates average monthly values (millimeters) of precipitation, subsurface drainage, and evapotranspiration (ET) for Ohio conditions without DWM. The graph clearly illustrates that most of the drainage occurs during the non-growing season (November through March), so the goal of DWM is to raise the outlet elevation during this time period to reduce the discharge. Raising the outlet elevation causes water to move to streams by other pathways including runoff and lateral seepage. The preferred pathway is lateral seepage, not runoff. Also, the graph illustrates the very small amount of drainage that occurs, on average, during June through August or September that might be captured by DWM for crop use.

Maximum target outlet water levels settings are illustrated in Figure 2.  These settings may serve as an example of the plan a grower may follow when implementing drainage water management. As you read the following paragraphs you may want to refer back to this illustration.

Holding back water during the winter

The outlet level (top of the boards) should never be set higher than 12 inches below the soil surface during the winter (fallow period in Fig 2). This setting will help assure aeration in the upper foot of the soil. A winter setting at 12 inches below the soil surface will also help minimize runoff because precipitation and/or snow melt water can still infiltrate and leave the field through the subsurface drains. Microbes are dormant below about 50 degrees Fahrenheit, so short periods (one or two days) of anaerobic conditions will not kill off the existing population of microbes. Loss of soil structure in very wet soils is primarily due to slacking of aggregates caused by prolonged wetting. As clays swell, the mineral and organic bonds that build structure are weakened and structure degrades. So, if the boards are set at least a foot below the surface during the winter, the upper foot will not be unduly subjected to impacts on soil structure.

It is important to remember that proper management of the water control structure and its settings does not include fully closing off the outlet. Management is simply raising or lowering the outlet elevation using the boards. When the system is in operation and heavy rainfall occurs, possibly raising the water table level above that top board, the excess water will flow over the upper board and discharge to the stream or ditch. This is the suggested way to manage the system for winter operation.

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