Subsurface (tile) drainage has greatly expanded recently with increases in precipitation, commodity prices, and land values along with technological advances that make installation easier and less costly. Improved drainage benefits agricultural production by lowering high water tables, which reduces crop stress from poor aeration and allows for timely field operations. This results in increased yields with less variability. While subsurface drainage benefits agricultural production, there are concerns about negative environmental impacts. Nitrate carried in drainage water is of particular concern from subsurface drainage. Nitrate delivered to surface water via subsurface drainage system can lead to local water quality problems. Furthermore, much of the nitrate transported by streams and rivers in the Mississippi River Basin eventually reaches the Gulf of Mexico where it contributes to the hypoxic, or "dead" zone.

Conservation drainage is an emerging set of designs, structures, and practices meant to maintain the benefits of drainage while minimizing negative environmental impacts. Conservation drainage considers both environmental and production goals in drainage system design and management to address water quality and flow concerns. Conservation drainage practices include: nutrient and crop practices such as nutrient best management practices and alternative cropping practices, which incorporate cover crops or perennials in the rotation; subsurface drainage practices such as improved system design, drainage water management, and shallow drainage; and impoundment and treatment practices such as bioreactors, treatment wetlands, saturated buffers, two-stage ditches, and culvert sizing. Some of these practices are being researched and demonstrated in South Dakota and other Upper Midwest states. Two of these conservation drainage practices that have been gaining particular attention are drainage water management and bioreactors.

Drainage water management (controlled drainage) uses water control structures (see photo) to raise or lower the outlet elevation to manage the water table depth. This controls the flow of water from the field, since water must rise above the outlet elevation for drainage to occur. The control structures are managed to restrict drainage at times when full drainage is not needed (the non-growing season). The outlet elevation can also be raised following spring field operations to provide an opportunity to store some additional water for crop use later in the growing season, which may provide a yield benefit in some years. By restricting drainage at times when it is not needed, the overall volume of drain flow is reduced and, consequently, nitrate losses are reduced. Studies indicate that reductions in nitrate losses of 15 to 75 percent are possible depending on local conditions. Drainage water management is best suited to flat fields (less than 0.5% slope) and becomes impractical as slopes begin to exceed 1%. "Drainage Water Management for the Midwest: Questions and Answers About Drainage Water Management for the Midwest", a regional Extension publication, provides more information on this practice.

Bioreactors are an edge-of-field practice designed to remove nitrate from the drain water before it is released through the outlet. A bioreactor consists of a subsurface trench filled with a carbon source, typically wood chips, through which the drainage water is passed. Control structures are used to control the flow of water through the bioreactor and to allow excess flows to bypass the system so that drainage isn't restricted. The wood chips are colonized by bacteria that convert the nitrate in the drainage water to nitrogen gas that is then released harmlessly into the atmosphere. Research results indicate that bioreactors can reduce nitrate levels in drainage water by 30 to 70%. Bioreactors are typically located near the outlet along the field edge, so little if any land needs to be taken out of production. Bioreactors can be retrofitted to existing drainage systems, and may last 10 to 20 years with relatively little maintenance. SDSU is beginning a 3-year project to demonstrate and evaluate the effectiveness of bioreactors in South Dakota. An SDSU Extension handout, "Bioreactors for Drainage Water Treatment", is available for more information on bioreactors.