Drainage Water Management (DWMgt) is first and foremost an environmental protection practice intended to reduce nutrient delivery to streams, ditches, streams and lakes. However, DWMgt 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. Our suggested maximum target outlet water levels settings are illustrated in Figure 1. These may serve as an example plan a grower might follow when implementing DWMgt.
Most of the drainage occurs during the non-growing season (November through March), so the goal of DWMgt is to reduce the discharge by artificially raising the outlet elevation during this time period. Raising the outlet elevation using DWMgt structures allows for more storage of water in the soil profile, part of the water may move to streams by other pathways including runoff and lateral seepage. The preferred pathway is lateral seepage, not runoff. A small amount of drainage occurs, on average, during June through August or September that might be captured by DWMgt for crop use. Acceptable management of the DWMgt structure 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. During periods of prolonged precipitation, it may be necessary to check for a higher than normal water table, and adjust the board setting temporarily. Our suggested maximum target outlet water levels settings are illustrated in Figure 1 and summarized below. These may serve as an example plan a grower might follow when implementing DWMgt.
Winter Management: The outlet level (top of the boards) should not be set higher than 12 inches below the soil surface during winter (fallow period in Fig 1). This helps assure aeration in the upper 1 foot of the soil, and will also help minimize runoff because precipitation and/or snow melt water can still infiltrate and leave the field through the subsurface drains.
Spring Management: As planting time approaches, the outlet setting should be at or just above 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 ten 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.
Growing Season Management: 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 (occasionally) to lower the boards for a while (12 hours to 2 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 ½” 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 is Supervisory Soil Scientist, USDA-ARS Soil Drainage Research Unit, Columbus,OH; and Adjunct Professor, OSU Department of Food, Agricultural and Biological Engineering. Dr. Larry C. Brown is Professor and Extension Agricultural Engineer, 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.