Drainage Water Management: Use of a control structure to manage drainage of water from fields throughout the year. The practice reduces the loss of nitrate and can increase crop yields in some years. Also called controlled drainage.
The Iowa Nutrient Reduction Strategy assigns an average 33 percent reduction in nitrate leaving the field for this practice. Drainage water management has little impact on the concentration of nitrate in the drainage water, but by reducing the volume of drainage water leaving the field, the overall load or amount of nitrate is reduced.
The water level is managed by installing control structures that allow for the outlet to be raised when drainage isn’t critical – such as outside the growing season – and during summer for potential water storage in the field. At times when drainage is needed, the outlet can be lowered and the system operates as a conventional drainage system.
The practice is best adapted to flat fields (up to 1 percent slope) because steeper, more variable land requires more control structures to manage water levels within a desirable range. Existing tile drainage systems can be retrofitted for drainage water management, but the utility may be limited depending on how the system was designed. New systems designed with drainage water management in mind can optimize the effect. The practice is better adapted to pattern-tiled fields as compared to random layouts.
The advantages of drainage water management, include reductions in nitrate loading to surface water and the potential for increased crop yields in some years. Yield advantages tend to be modest (up to 5 percent) and do not occur every year. The disadvantages of the practice include the cost of the water control structures, increased management required to adjust outlet levels and increased potential for runoff and soil erosion when water levels are high.
The overall fate of the water and nitrate not lost through the drainage system is an open research question.