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Drainage water management PDF Print E-mail
Thursday, January 03, 2013 1:26 PM

Subsurface tile drainage is an essential water management practice on many highly productive fields in the Midwest. However, nitrate and phosphorus carried in drainage water can lead to local water quality problems and contribute to eutrophication in Lake Erie and hypoxia in the Gulf of Mexico.

Drainage water management is a new strategy that reduces the nitrate and phosphorus loads while maintaining adequate drainage for crop production. The following six paragraphs with slight revisions and illustrations are taken directly from a fact sheet entitled “Drainage Water Management for the Midwest WQ-44”.

Drainage water management is the practice of using a water control structure in a main, sub-main, or lateral drain to vary the depth of the drainage outlet. The water table must rise above the outlet depth for drainage to occur. The outlet depth, as determined by the control structure, is:

• Raised after harvest to limit drainage outflow and reduce the delivery of nitrate to ditches and streams during the off-season. The outlet is lowered in early spring and again in the fall so the drain can flow freely before field operations such as planting or harvest. (Figure 1)

• Raised again after planting and spring field operations to create a potential to store water for the crop to use in midsummer. (Figure 2)
The practice is only suitable on fields that need drainage, and is most appropriate where a pattern drainage system (as opposed to a random system) is installed. The field should be flat (generally less than 0.5 percent slope) so that one control structure can manage the water table within 1 to 2 feet. The producer must be able to manage the drainage system without affecting adjacent landowners.  Narrower drain spacing reduces the risk of yield loss due to excess wetness during the growing season.

The number of control structures depends on field topography and the desired uniformity of water table management. Flatter fields require fewer overall structures and allow each structure to economically manage a larger area. One structure can typically control at least 10 or 20 acres and each control structure costs about $1,000.   Yield benefits vary but for Midwest conditions, long-term crop yield benefits of up to 5 percent with controlled drainage could be expected. There may be a risk of excessive moisture in some years, but the risk can be minimized with proper management.

Studies have found reductions in annual nitrate load in drain flow ranging from about 15 percent to 75 percent, depending on location, climate, soil type, and cropping practice. Nitrate load is reduced by about the same percentage as drain flow is reduced, since most studies have found that drainage water management does not change the nitrate concentration in the drain flow.  In regions where much of the drainage takes place during the winter (such as Illinois, Indiana, and Ohio), the reduction is likely to be greater than where most of the drainage takes place later in the year.

Farmers should plan to use the same amount of nitrogen fertilizer on their crops because the soluble nitrate stored in the root zone will may be lost when the water is released before planting.  The phosphorus research is still being conducted, but in North Carolina, as much as a 35-50 percent reduction in soluble phosphorus was found in outlet water (Evans, Gilliam, and Skaggs, 1996).

Contact the Putnam County Soil & Water Conservation District for information on cost share and EQIP grants for installing control structures on your farm. For more information on management or to see the actual factsheet, visit extension.purdue.edu/extmedia/WQ/WQ-44.pdf


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