Diagram contrasting conventional drainage with two types of conservation drainage -shallow drainage (typically one foot shallower than conventional drainage) and controlled drainage, which involves a control structure that lets farmers adjust water levels throughout the year.
Woodchip bioreactor diagram
Conservation drainage refers to several emerging technologies and methods that provide the benefits of conventional agricultural drainage (namely, removal of excess water from fields) while reducing nitrates and other potential pollutants carried via drainage water to nearby waterways.
The agricultural drainage systems developed over the past 100 years in Minnesota have made large areas of the state very productive cropland. Typically, drainage ditches were constructed first; then subsurface drainage pipe (commonly called "tile") was installed to remove water from poorly drained soils and convey it to drainage ditches or streams.
Today, many agricultural drainage systems in Minnesota are old and overloaded. As these aging systems are updated or replaced, there is a window of opportunity to incorporate new designs and practices that protect water quality and improve the capacity of the drainage system to manage floodwaters caused by snowmelt and heavy rains. Some of the conservation drainage approaches being researched and demonstrated in Minnesota and other Midwestern states include controlled drainage, shallow drainage, woodchip bioreactors, saturated buffers, rock inlets, alternative ditch design and various kinds of storage basins. Several of these approaches can be readily adapted, singly or in combination, to existing drainage systems.
Controlled or managed drainage involves water control structures that allow farmers to regulate and adjust water levels. In general, these systems work best on flatter fields (with a slope of 1% or less) of at least 20 acres. The degree of management needed depends on whether the water control structure is used to raise the water outlet during the fallow season, the growing season or both; typically more management is needed during the growing season. Some water control structures are satellite-controlled, enabling farmers to monitor and adjust them remotely.
Controlled drainage systems are designed to release only the amount of water needed to provide an aerated crop root zone and ensure trafficable conditions for field operations. Generally, any drainage in excess of that amount is no longer available for crop uptake and is likely to carry away nitrate. Compared to conventional, unmanaged drainage, controlled drainage has been shown to significantly reduce the nitrate that flows to ditches and streams from tile drains.
Installing a controlled drainage system may require replacing old tile lines with a new layout. A controlled drainage plan should include a map showing the area draining to each water control structure (including main and lateral tile lines) and a record of planned and actual water table control elevations.
Guidance from USDA's Natural Resources Conservation Service
See contacts for specific programs that fund this practice in the side-by-side payment comparison or contact your local Soil and Water Conservation District