Water Table Management Strategies
This section will discuss how managing tile water flow and discharge can be used to decrease nutrient loading to surface waters.
Golden Rule of Drainage
Only release the amount water necessary to insure trafficable conditions for field operations and to provide an aerated crop root zone – any drainage in excess of this rule likely carries away nitrate and water that is no longer available for crop uptake.
All across the Midwest, research is being conducted on management practices that improve tile drain water quality without hurting crop yield. This is called conservation drainage. Source: Illinois Agronomy Handbook.
Conservation Drainage Practices are meant to reduce nutrient transport from drained land and not reduce drainage performance. Source: Illinois Agronomy Handbook.
One of the conservation drainage practices used is controlled drainage. This practice makes use of a control structure that can be used to manage the level of water above the tile line. This structure is placed close to the outlet. Source: Illinois Agronomy Handbook.
Water levels are controlled by adding or removing "stop logs" or by the use of flow valves. There are many variations in the shapes and sizes of structures. Source: Illinois Agronomy Handbook.
The practice is only suitable on fields that need drainage, and is most appropriate where a pattern drainage system (as opposed to a random system) is installed or is feasible.
The field should be flat (generally less than 0.5 percent slope) so that one control structure can manage the water table within 1 to 2 feet for as many acres as possible. If drainage laterals are installed on the contour, the practice could be used with greater slopes. From Questions and Answers About Drainage Water Management for the Midwest (Purdue Extension).
Flatter fields require fewer overall structures and allow each structure to manage a larger area. A field is typically divided into "drainage management zones," each managed by one control structure. From Questions and Answers About Drainage Water Management for the Midwest (Purdue Extension).
Drainage Water Management (Controlled Drainage)
Outlet at drain level
Outlet near soil surface
A watergate valve provides an increase in hydraulic head in tile lines.
This reduces the number of water control structures in the field, so there are less obstacles to crop around.
Here are a few pictures showing a water control structure with lift gates.
Structure costs range from $500 to $2,000, depending on height, size of tile, structure design, manufacturer, and whether it is automated. Some producers fabricate their own structures. Installation costs may be about $200 for a basic structure in a new drainage system installation, but may increase depending on the size of the structure, level of automation of the structure, and for retrofit situations.
Assuming grades are flat enough for one structure to control 20 acres, initial costs would be in the range of $20 to $110 per acre. From Questions and Answers About Drainage Water Management for the Midwest (Purdue Extension).
Drainage water management system operated for both water quality and yield benefits
Controlled drainage can be used to raise or lower the water table, depending on the time of year and water needs. This practice can be used to raise the water level after harvest, reducing nitrate loading into surface waters. It can also allow higher water levels to be present if needed during dry growing seasons. Source: Illinois Agronomy Handbook.
A normal management operation would be to set the water table control height to within 6 inches of the soil surface from November 1 and lower the water table to the level of the tile on March 15. Water is held back from November to mid March.
After planting, the water table is managed to provide optimum moisture conditions for crop growth during the growing season, with the water table again lowered to tile level during harvest. Source: Illinois Agronomy Handbook.
Controlled Drainage Structures and water table management can target agronomic goals, water quality goals, or both. Source: Illinois Agronomy Handbook.