Management of agricultural drainage water quality. Chapter 3 - Water table management

There are some agronomic and water management practices which may be implemented at the field and watershed scales to reduce the concentration of chemicals (primarily nutrients and pesticides) in agricultural drainage water. Agronomic practices include tillage, cropping methods, and improved and timely fertilizer and pesticide applications.

One water management practice which has been shown to significantly reduce NO3-N in drainage water is water table control. The technique has been tested in the humid regions of eastern Canada, and the eastern and mid-western United States. It is currently promoted in these regions as a combined drainage/irrigation practice for increasing crop yields and reducing NO3-N pollution from relatively flat, artificially drained cropland. The quality of receiving waters is thus improved. The water quality benefits of water table management are well described in Gilliam (1987) and Gilliam et al., (1979). Water table management systems are currently being tested in some parts of Europe and Asia. The practice is yet to be evaluated in the arid and semi-arid regions, although there is potential for water table management in these regions. However, proper management of the water table is required to ensure that re-salinization of the crop root zone does not occur. Early attempts at sub-irrigation in arid and semi-arid regions failed because of soil salinization. More information is required on the effects of different water table levels on crop yields and salt movement in the root zone in various climatic locations and soil types. For these reasons, the focus of this chapter is on water table management in humid regions.

In the humid regions of North America, water table management has evolved on lands which were first improved with surface and subsurface drainage. Drainage improvements were initially required on these poorly drained soils to increase crop productivity and provide better soil conditions for field machine trafficability. With concerns about the quality and impacts of agricultural drainage water on marine ecosystems, as well as nitrate contamination of drinking water, the drainage systems were modified to reduce the concentration of pollutants in the drainage water. Water table management has been practised for several years on organic and sandy soils in humid regions to reduce drought stress of high value crops.