Water control structures in subsurface tile drainage can help manage water in tile-drained fields to protect water quality and benefit your bottom line. The practice of managing water in the field by changing the outlet level of the tile drainage system is known as drainage water management, or controlled drainage.
Managing water levels can help keep nutrients from leaving fields before they are absorbed by the crop according to “Drainage Water Management,” a publication by the Purdue University Cooperative Extension Service. This publication also notes that some studies have found up to 75 percent reduction in nitrate leaving the field, which can be directly attributed to the amount of water leaving the field through drainage.
Installing water control structures at one or more drainage outlets allows for partial to whole field water management depending on field slope. Farmers with fairly flat fields–less than 0.5 percent slope, according to the Purdue Extension publication–get the most out of their investment because they can manage more acres per structure.
While water control structures are not suitable for all fields, they can provide some great benefits where their use fits.
Recommendation for water control structures is to raise the outlet of the drainage system to about a foot below the ground surface post-harvest. This encourages nitrate transformation to nitrogen gas in the soil as well as reduces the rate of organic matter oxidation, per the USDA’s Natural Resources Conservation Service practice standard on drainage water management.
When the ground thaws in spring, the water control structure should be set to allow for free drainage by lowering the outlet level to the drain depth.
The purpose of lowering of the outlet level to the drain depth is to prepare the field for planting and it is done about seven to 10 days prior to planting. This will allow the water to be drained from the field and create trafficable field conditions.
Once a field is planted and the crop roots are well-established, the outlet level is raised to allow water to remain in the field. It is recommended the water in the field be controlled at a level consistent with the root depth of the crop during the growing season, usually 1.5 to 2 feet below the ground surface.
When there is a stretch of heavy rainfall, lowering the outlet of the water control structure may be necessary to allow excess water to leave the field more quickly. This is particularly important early in the growing season when plant roots are not well-established.
Most control structures are located on field borders that can be easily accessed. Others are automated and can be controlled remotely though a computer or phone app.
No-till fields and fields planted to hay or any other perennial crop have special considerations in drainage water management. They are both especially vulnerable to rapid nutrient loss through tile lines due to well-defined macropores.
Drainage water management can be particularly helpful for keeping more nutrients in the field as well as reduce compaction by breaking up aggregates without disturbing the soil.
Managing drainage water using a water control structure can also keep waterways safe from unintended manure releases through tile lines by having the outlet level in the control structures raised before a manure application. Spreading manure on fields can be risky–a water control structure reduces the risk of manure leaking into surface water.
Finally, properly managing tile drainage water and timely rainfall can increase yields. A long-term study in Ohio showed that acres affected by drainage water management resulted in 6 percent yield bumps in corn and 3.5 percent in soybeans.
Drainage water management by means of water control structures has the largest impact on crop yield in years with drier summers and timely rainfall that would raise the water level in the field when the crop needs it most.
Join the Clinton Conservation District and Michigan State University Extension at Cover Your Assets!, a free workshop and lunch discussing tile water management and other local conservation topics on Thursday, Feb. 23, 2017, from 8:30 a.m.-12:30 p.m. at Clinton County RESA, 1013 Old US 27, St. Johns, MI 48879.
For more information and to register, go to Cover Your Assets! or contact the Clinton Conservation District at 989-224-3720.