Blocking tile flow is not the same as true sub-irrigation.
Belcher sums up sub irrigation this way: “For optimum plant development, the soil within the root zone should always have adequate moisture to provide the evapotranspiration needs of the plant and soil regime while still providing the oxygen essential to plant health. In addition the soil profile needs to have pore space available to store infiltrated rainfall and thus not cause increased runoff during and following rain events. If the system operator can maintain a root zone regime that provides these optimum growing conditions without increasing runoff, then it would appear that management of the water table is contributing to achieving most, if not all, beneficial plant and environmental objectives.” This system is managed to maintain a consistent water level below the roots and depends on capillary movement upward, providing a constant and readily available water supply to the plants that can increase yields. In the spring and fall, the system is operated in a drainage mode. Sub irrigation can reduce yields and increase runoff if not managed properly.
Today, we are hearing about water control devices that are used to achieve sub irrigation but managing them for different goals. To put this in context, a water table management system has three modes of operation. The subsurface drainage mode is used to freely allow water to drain. The controlled drainage mode is used to raise or maintain a water table above the elevation of the laterals. This is a concept that is catching on for controlling manure and decreasing the risk of it reaching outlets. It can also be used in the fall to reduce fertilizer nitrogen and phosphorus from leaving the field and reaching surface waters. The subirrigation mode is used to raise or maintain a water table above the elevation of the laterals by providing irrigation water to the soil profile via the underground pipe system. This mode would be used during the dry part of the growing season to add enough water to improve crop yields yet can return to subsurface drainage mode from fall to spring.
Drainage is known to reduce standing water and therefore reduces surface runoff of soil and nutrients. Anytime you begin to manage or block this flow in an attempt to reduce nutrients thought the subsurface system, it is vital to not create unintended consequences of increasing runoff.
There is much more to learn about these systems – from the type of soils and slopes that they will function on to design and management.