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Land Improvement Contractors of Ontario


No. 2: Subirrigation

Climatological change, appears to have brought temperature changes in available heat units . Storms and rainfall pattern are seriously affecting sustainable crop production. Spring work periods appear to be shorter and wetter, and while the average rainfall has not changed the distribution of the growing season rainfall has changed. There are fewer rain events but when they do occur they are greater and often produce local flooding, surface runoff and erosion. Recharge of the soil moisture that is needed for plant growth is reduced and plants suffer.

Subirrigation is a practice that has gained acceptance in the northern USA and Quebec. It involves using the tile drainage system to distribute water to maintain a water table near the bottom of the crop root zone. The water moves by capillarity into the root zone and provides a continuous supply of water to plant roots.

In spring and fall the water table control system operates in the usual drainage mode where the water table is lowered to permit tillage and harvesting. During the growing season water is added to the system as required to maintain a design water table elevation.

Water from a ditch, stream, pond, well, or other source is pumped into an access riser (Figure 1) at the upper end of the tile system. Evenly spaced lateral drains distribute the water evenly over the zone. Control stands regulate the water table height. Lateral drains are spaced closer together, compared to tile drainage, for satisfactory subirrigation. They are often a bit deeper. The regular drainage system may be used to test the practice. It may be of value to split the existing drain spacing.



Successful subirrigation has been used on clay, clay loam and sandy soils with success. The field needs to be flat to gently sloping in one direction. The field is surveyed into contour zones having a surface elevation not greater than one foot difference for each zone. A weir-type structure in the line controls the water elevation for one zone.

A denser soil layer must exist below the tile drains to establish a water table (Figure 2). A water table height (W) at the upper end of the zone should be maintained at 12 to 24 inches for corn, and 24 to 30 inches for soybeans. The lateral drains are spaced (L) such that the difference in water table elevation (m) at mid spacing is about 6 inches. Water table fluctuations must be minimized.

The system must be provided with enough water to meet the maximum potential evapo-transpiration of the plants plus any losses due to seepage from the area. In Ontario this is about 5 gallons per minute per acre irrigated, in driest months. More water is needed if the soil is sandy.

Yield increases have averaged 15 to 30 percent in Michigan. The principal crops subirrigated have been corn, soybeans and sugar beets. Irrigation is suspended when corn begins to dent. Subirrigation may have a place on your farm.

For more information and costs ask your local LICO contractor.

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