Improved water quality through water table management in eastern Canada

A two-year field study was conducted in eastern Ontario to evaluate the effects of water table management on the quality of subsurface drain flows. Drainage discharge volumes and NO3--N concentrations in drainage effluent were monitored during the growing seasons. The 3.5 ha experimental field was strip-cropped with corn (Zea mays L.) and soybean (Glycine max Merr.). Controlled-drainage subirrigation was used to manage water from precipitation and subirrigation. Time two controlled water table treatments were 50 cm and 75 cm from the surface. There was also a conventional free-drainage treatment (FD). In 1995, mean NO3--N concentrations in drainage water were reduced by 84% and 75% by the 50 and 75 cm controlled water table depths, respectively. In 1996, drain flow volumes and NO3--N concentrations were significantly reduced relative to free drainage. Total drain flow was reduced by 42% by the 50 cm controlled water table, while NO3--N concentrations were reduced by 61% and 52% by the 50 cm and 75 cm controlled water tables, respectively. Consequently, NO3--N loadings were also reduced by 94%, and 30% by the 50 cm and 75 cm controlled water tables, respectively. Improvements in drainage water quality were attributed to both reduced drainage outflow and enhanced denitrification in the controlled water table plots, In addition to significantly reducing NO3--N pollution, economic benefits were achieved through fertilizer equivalent savings, and enhanced crop yields were achieved through subirrigation. Therefore, the practice of water table management in the region was demonstrated to be both environmentally and economically sustainable.