Protecting Water Quality Using Controlled Drainage as an Agricultural BMP for Potato Production

Abstract. Subsurface drainage is used for removing excess water from agricultural lands to improve productivity, although it can cause water quality problems downstream. Agricultural drainage is identified as one of the contributing factors to Lake Winnipeg’s nutrient enrichment and subsequent water quality deterioration. It has been suggested that controlled drainage in the Lake Winnipeg watershed could be a potential best management practice (BMP). The objective of the study was to evaluate the flow and nutrient export through subsurface drainage by comparing free drainage with overhead irrigation (FDIR) and controlled drainage with subirrigation (CDSI) under potato crop. The experiment was carried out in southern Manitoba in a potato field with sandy loam soil using the same irrigation water source. Subsurface drainage was installed in 2009 at an average design depth of 0.9 m. Controlled drainage was accomplished by using drainage control structures at drainage outlets. Subirrigation was carried out by pumping water back through the subsurface drainage network. Drainage flow, nutrient export, and weather parameters were monitored during the growing seasons of 2010 and 2011. On average, controlled drainage reduced the flow volume by 91% in 2010 and 54% in 2011 compared to free drainage. The drainage volume from FDIR plots ranged from 13% to 53% of the rainfall received, while for CDSI plots it varied from 0% to 8%. In both years, average nitrate concentrations were lower in CDSI compared to FDIR plots. However, average phosphate concentrations were lower in 2010 and higher in 2011 under CDSI compared to FDIR. Compared to FDIR plots, average nitrate-N export from CDSI plots was reduced by 98% in 2010 and 67% in 2011. Average phosphate export was reduced by 94% in 2010; however, in 2011, it was 15% higher than FDIR. The majority of the export happened during the period dominated by heavy drainage outflow, which coincided with spring snowmelt. The nutrient export reduction by CDSI is attributed mainly to the drainage volume reduction by controlled drainage.