When to use subsurface drainage
Subsurface drainage of wet and saturated soils helps to remove excess water, thus reducing plant stress by allowing aeration of the root zone. This excess water removal from the subsurface allows soils to enhance their productivity. The excess water can carry nitrate-N, one of the forms of nitrogen the plant takes up when it absorbs water through its roots. Nitrate-N is essential for plant growth; however, its loss with subsurface drainage water can potentially lead to water quality problems.
Practices that reduce the amount of nitrate-N lost from farmland need to be considered to reduce the water quality impacts. Several practices under consideration include wetlands, bioreactors, controlled drainage, saturated buffers, shallow drainage, reduced drainage intensity, and winter forage or cover crops. Controlled drainage is a practice where the water table in the field is managed such that the amount of drainage water leaving the field is controlled. This article discusses information on controlled drainage in terms of the agricultural fields under corn and soybean cropping systems.
In a typical subsurface drainage system, the water table in the field is lowered by the uncontrolled gravitational flow of excess water through tile drains placed at a certain depth and spacing. These systems can be managed to control the lowering and subsequently raising of the water table by use of a control structure.
A control structure has an inlet and outlet for the tile drains, with several stop logs in between. As the water enters, it has to rise above the stop logs height to access the outlet. Height of the stop logs can be changed when the water table needs to be lowered or when it needs to be raised.
Farming operations, such as planting, spraying, harvesting, etc., require optimum soil moisture conditions, such that these operations can be completed without hassle. The control structure can be used to lower the water table in the field in fall and early spring prior to these operations. Height of the stop logs over which the water has to flow can be reduced, allowing the system to become free-flowing.
After harvest, the stop logs’ height can be increased so water doesn't drain from the field, holding back nitrate-N with it. During midsummer and early fall after field operations are done, the stop logs’ height can be increased, creating storage of water, and thus nitrate-N, within the field.
A control structure changes a typical subsurface drainage system into a system where the water table can be controlled. New systems can have controlled drainage structures installed along with new tile. For retrofitted or new systems, caution should be used as controlled drainage structures are not suitable for all fields and require appropriate management.
Flat fields, with a 0.5% slope or less, can have only one structure to control the water table within 1 to 2 feet on as many acres as possible. On rolling topography with greater slopes, control structures can potentially be used if the laterals have been installed on the contour. The number of structures needed in a field with significant elevation changes can be relatively large and can potentially have little effectiveness in managing the water table.
Management is required on the part of the producer to operate the control structures such that the adjoining farmland is not affected. During the growing season, the control structures need to be carefully operated such that the plant roots, especially for soybeans, are not subject to excess water stress resulting in yield loss. During the sustained periods of heavy rains in the growing season, the height of the stop logs may need to be temporarily lowered to allow excess water to drain out.
Control structures, fitted with mechanisms that can change the stop log heights remotely, are available to help with water table management. During the fallow season between fall harvest and spring planting, the management is relatively easy as it only requires raising the stop logs after harvest and lowering them two to three weeks prior to planting.
Nitrate-N mass reduction
A control structure does not change the concentration of nitrate-N in the drainage water that passes through it. Instead, the use of a control structure allows for the drainage water to be managed within the field, thus aiding in reduction of the nitrate-N mass or nitrate-N load leaving the field.
These reductions are achieved due to direct reduction in water volume leaving the farmland and the nitrate-N mass associated with it. Research in several Midwest locations has shown appropriate use of controlled drainage structures can help reduce annual nitrate-N mass or load in drainage water between 15% to 75%. The wide range in reductions is indicative of variability depending upon climate, soil type, cropping system and location.
The second potential reduction is achieved due to deep seepage of the water. As water stands in the soil profile and is not allowed to drain, potential exists for this water, and the nitrate-N associated with it, to seep deeper into the profile beyond the tile drain. The extent of how much deep seepage occurs and how much of the nitrate-N mass is retained by deep seepage is not fully known. As the water seeps deeper, the extent of denitrification that can potentially occur along the deep seepage pathways is not fully known.
Questions on controlled drainage including yield impacts, number of acres, fertilization changes, tile freezing, plugging, blowouts and others are covered in WQ-44, “Questions and Answers About Drainage Water Management for the Midwest,” available at .
Arora is the ISU Extension field agricultural engineer at Nevada in central Iowa.
Photo: Matt Helmers
This article published in the February, 2016 edition of WALLACES FARMER.
All rights reserved. Copyright Farm Progress Cos. 2016.
Field Conservation Maintenance/Practices