Submitted by: Janice Janiac, Golden Horseshoe Regional Communication Coordinator
Controlled drainage is the regulation of the water table by means of pumps, control dams, check drains, water level control devices or combination to maintain the water table at a depth favorable for crop growth, according to Land Information Ontario.
In conventional tile drainage, the water table lowers to the level of the tile. Also known as drainage management, controlled drainage is the practice of using a water control structure to raise the depth of the drainage outlet, holding water in the field during periods when drainage is not needed. By adding a control structure, essentially an underground gate system, the outlet elevation can be raised or lowered by adding or subtracting gates. Generally speaking, controlled drainage removes water in times of excess but keeps water for times of low moisture.
Why it matters
Approximately 45% of Southern Ontario cropland has been tile drained. As a general rule of thumb, 70% of moisture is extracted from the upper half of a crop’s root zone.
|Crop||Depth50 (cm)||Depth95 (cm)|
source: Fan et al., 2016
Based on this work by Fan et al., the maximum acceptable water table height is approximately 30-50cm below surface. The addition of controlled drainage allows for timely water management within the root zone, particularly in times of low moisture.
Research also shows that controlled drainage is more effective in limiting nitrate losses than phosphate losses through tile water. Some studies show that where soil P levels are high, controlled drainage can actually result in the release of more P. Generally, controlled drainage results in a reduction of the total amount of water leaving a site through the tiles, therefore the annual load of N and P leaving the site through tile runoff is reduced with controlled drainage.
A joint study by OMAFA, LICO and AAFC in 2008 evaluated soil suitability (soil type and drainage) and topographic suitability in Southern Ontario landscapes for controlled drainage potential. Results of the work indicated that less than 2% of the Southern Ontario landscape received a ‘good potential’ score, with most occurring in Essex, Chatham-Kent and Lambton counties.
View your farm’s suitability ranking at OMAFRA’s AgMaps portal: http://www.omafra.gov.on.ca/english/landuse/gis/ portal.htm
Since much of Southern Ontario is already tile drained, retrofitting for controlled drainage is more likely, and cost effective, than new installations. Retrofitting can be simple or challenging, depending on site conditions. See image examples from Kevin McKague, OMAFRA’s Water Quality Engineer, presented at OSCIA’s 2020 Annual Conference.
Ultimately, the simpler the retrofit, the lower the cost and the quicker the return on investment.
It is generally accepted that 65% to 95% of the annual runoff leaves a field through tile drainage. The majority of this runoff takes place during the non-growing season, and therefore this water is lost and not available to growing crops. Timing of these losses outside of the growing season make controlled drainage less effective as a means of supplying water for crop growth during the growing season.
Controlled drainage can be an effective BMP for water quantity and quality control, but application is site-specific based on suitability and cost-effectiveness.