Controlled drainage is the process of adjusting the elevation of a drainage system outlet. This is done to control the volume of water leaving the field and reduce nutrient losses. Controlled drainage is considered an in-field practice.
Controlled drainage structures are best suited to fields with very flat topography (less than 1 percent slope). They are placed on the edge of fields that have systematic drainage, consisting of a main drain and laterals that empty into the main. These structures can be implemented on fields with greater slope, but more control structures or alternative in line structure and management time are required.
Controlled drainage is installed to reduce nutrient export through tile drains. Controlled drainage allows adjustment of the field water table to allow freeflowing drainage to accommodate field trafficking with equipment or water storage for crop use.
Figure 48a. Field-installed drainage water management structure. Credit: Greg LaBarge, OSU Extension.
Nutrient load reductions vary based on soil properties, topography, and management practices. Studies in Ohio indicate that elevating the outlet during the nongrowing season reduced the:
- annual discharge by 20–40 percent (Fausey, 2005).
- annual nitrogen loads by as much as 22 lbs/acre (Fausey, 2005).
- dissolved phosphorus loads by 0.4 lb/acre (Williams et al., 2015).
- A minimum of 20 feet of nonperforated tubing is needed to adjust the drainage system outlet elevation.
- An antiseep collar on the field side of the structure is needed to avoid flow that bypasses the structure.
- A number of structures of various design and materials are commercially available.
- The field area controlled by each structure will vary depending of the topography and the layout of the subsurface drainage system. Several structures are often needed, and recently, in-line devices have been designed to extend effectiveness over more area.
- Costs are site-specific, but they typically range from $600–$3,000 ($30–$100 per acre). Annualized costs versus installation costs should be compared.
Drainage water management happens throughout the year through an automated system, or it happens manually by inserting/removing stoplogs. The drainage system outlet is elevated during the nongrowing season by inserting stoplogs into the control structure after crop harvest until the top of the stack is within 1 foot of the ground surface (Figure 48a).
Two to three weeks prior to planting, the stoplogs are removed and the drainage water flows freely to accommodate farming activity and early root development (Figure 48b). Four to six weeks after planting, some stoplogs can be inserted to elevate the outlet and retain water that reaches the drains during the growing season (Figure 48c).
For best practice, ensure that there is free drainage (as shown in Figure 48b) during harvest.
Figures 48b and 48c. Inspecting and adjusting boards to manage water table level. Credit for both images: USDA-ARS Soil Drainage Unit.
Extensive design details are provided in NRCS Conservation Practice Standard 554.