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.
When to lower the outlet elevation in spring
As planting time approaches, the outlet level should be lowered to drain depth to allow continuous free drainage. Although we have had good experience with leaving one board in the bottom of the structure, we suggest that the grower gain some experience with the lowest setting first. Don’t be in a hurry to lower the outlet level in the spring. Seven to 10 days prior to your target planting date should be soon enough to lower the drainage system outlet. The drainage system and the drainable pore space in the soil profile will empty quickly after which trafficable field conditions are determined by atmospheric conditions. Trafficable field conditions require that the soil be dried beyond what the drainage system can remove.
This drying occurs by evaporation from the soil surface or transpiration by plants. Any rain refills the emptied pore space and prolongs the process of surface drying. Warm and windy days accelerate the process.
Managing the boards during the growing season
The recommended outlet level should be set at 16 to 20 inches below the soil surface. In the first years while gaining experience, some growers may decide to take on a little less risk and use a setting at 24 inches. It is typically recommended to make this setting change after mid-June. If a producer is willing to spend more time managing the drainage system, they are encouraged to set the boards at 28 to 30 inches following seeding of the crop, then adding boards up to 16 to 20 inches when the crop reaches V3-V4 stage. This could retain some water that would have drained away in late May and early June. Otherwise, wait until mid-June when the crop roots are well developed. With large rain events, it may be necessary (rarely) to lower the boards for a while (12 hours to two days) to lower the water table in a timely manner, especially early in the growing season before the crop’s root system is well established. Single rainfall events are not problematic unless the total rainfall exceeds an inch. Consecutive day rainfall events of one half inch or more present a greater concern for causing a lack of aeration in the root zone. It is a good idea to have a couple of water table observation wells installed to help guide the decision whether to lower the outlet or not.
Dr. Norman R. Fausey, Retired Supervisory Soil Scientist, USDA-ARS Soil Drainage Research Unit, Columbus,OH; and Adjunct Professor, OSU Department of Food, Agricultural and Biological Engineering. Dr. Fausey can be reached at email@example.com.
Professor Larry C. Brown can be reached at 614-292-3826, or firstname.lastname@example.org. This column is provided by the OSU Department of Food, Agricultural and Biological Engineering, OSU Extension, Ohio Agricultural Research and Development Center, and the College of Food, Agricultural, and Environmental Sciences.