The goal of proper water table management is to consistently maintain field ground water levels within an optimum range of depth beneath the ground surface, thereby minimizing plant stress by allowing crops to readily obtain water for growth, while at the same time preventing damage due to root zone flooding. During parts of the growing season, when there is a significant moisture deficit, shallow ground water levels at a WRSIS site are managed by putting water back into the soil through the subsurface drainage pipe network. Subirrigation water table depths are regulated using a weir-type hydraulic control structure placed between the drainage pipe network and its outlet (see section on water table management options). The weir, whose height governs the water table position, is comprised of a number of flashboards inserted within the control structure at its base, one on top of the other. Experience gained by subirrigating corn and soybeans at the three WRSIS sites and at two other research locations (Wooster, Ohio and Hoytville, Ohio), shows that 25 cm (10 in.) is the desired water table depth to be maintained at drain line positions. As a result of the fine-grained silt and clay soils present in northwest Ohio, typical subirrigated drain spacings of 4.6 to 6.1 m (15 to 20 ft), and a ground water level kept at 25 cm (10 in.) beneath the surface along the drain itself, the water table depths found at the midpoint between drain lines are normally between 46 and 51 cm (18 and 20 in.).
Measurement Tools for Subirrigation
Two very important tools are needed for water table management of WRSIS subirrigated fields, a rain gage and observation wells. These tools are crucial for making decisions on adding or removing flashboards from a hydraulic control structure and whether the subirrigation water supply should be continued or shut-off. The rain gage is placed in an open area adjacent to the subirrigated fields and is to be checked after each storm event. Observation wells need to be installed at several locations, including along drain lines, at midpoints between drains, and especially areas within depressions. The water level in the wells should be monitored daily to determine if the water table is at the desired depth. Small diameter observation wells work best because they respond quickly to changing ground water levels in fine-grained soils. The simple observation wells used are comprised of 2.5 cm (1.0 in.) diameter perforated PVC pipe capped at one end and wrapped along its length with a sheet of fiberglass screen taped securely in place.
Typical Procedures in the Early Growing Season
Establishing a base water table early in the growing season is essential. This makes it easier to raise ground water levels later. In northwest Ohio, the base water table is held about 15 cm (6 in.) above the outlet pipe in the hydraulic control structure beginning June 1. After all post emergence field operations are completed and the crops are at the V4 growth stage (third trifoliate leaf stage for soybeans, collar of the fourth leaf visible for corn), the ground water level can be raised for the remainder of the growing season, especially in near average or wetter years, to 25 cm (10 in.) beneath the soil surface along drain lines. This is usually done around June 15 at the Ohio WRSIS sites.
Procedures Used in the Growing Season are Based on Rainfall Amounts.
WRSIS water table management strategies are contingent on whether the growing season climate, with regard to precipitation, is wetter than average, near average, or drier than average. Determination of growing season climate by the farm manager is based on rainfall to date, and to a lesser extent, long range weather forecasts. A simplified schematic of near average, wetter, and drier growing season water table management strategies is provided in the figure below. Rainfall events, especially in near average or wetter growing seasons, often dictate that the subirrigation water supply be tuned off, and for large storms, that the hydraulic control structure flashboards be removed. After the rainfall event is over, normal subirrigation operations are not continued until the water table drops back to the desired level. A water table management strategy for dry growing seasons is still in the process of being developed. WRSIS water table management guidelines are discussed in greater detail by Allred et al. (2003).
Non-Growing Season Procedures
All WRSIS water table management discussion up this point has focused on the growing season. Typically, in the late fall, winter, and spring months (November through May), the subirrigated fields are kept in full drainage mode with flashboards removed from the hydraulic control structures and the water supply from the reservoir discontinued. This is done in large part to help site trafficability during fall harvest and tillage along with spring planting and post emergence field operations. This is not meant to imply that there are no environmental benefits provided by WRSIS between late fall and late spring. On the contrary, offsite discharge remains restricted during this period, and settlement processes in the wetland and reservoir still remove sediment and adsorbed phosphorous from water.
Perhaps a better environmental alternative for the subirrigated fields is to re-insert the hydraulic control structure flashboards to let rainfall alone bring the water table close to the surface during the months of December, January, February, and March when trafficability is not a major concern. This form of water table management is called controlled drainage and would produce anaerobic soil conditions by which excess nitrate is removed through denitrification and more carbon is sequestered due to reduced biodegradation. Winter month controlled drainage is now being tested at the Defiance County site and will require monitoring to assure that salinity build-up in the soil does not become a problem.