Water Quality Benefits of "Shallow" Subsurface Drainage Systems

Abstract

The impact of drainage depth and intensity (design water removal rate) on water quality insouthern Minnesota was investigated with a field measurements from 2001 through 2005.Subsurface drainage systems were installed on sub-field sized plots ranging in size from 0.8 to 2.5ha. The nine systems comprised two drainage depths (90 and 120 cm) and conventional (13mm/day) and narrow (one-half the conventional spacing: 51 mm/day) drainage intensities. Surfaceand subsurface drainage runoff and nitrate-nitrogen were monitored with automated equipment overthe 5-year period. Reductions in annual drainage volume and nitrate-nitrogen loads were observed inevery year of the study, but were not statistically significant due to variability among the nine plots.When ANOVA was performed on the aggregated 5-year dataset, reductions in drainage volume ofand nitrate-nitrogen load were observed for the shallow and conventional drainage systems, andwere found to be statistically significant. The 5-year average annual drainage volume and nitratenitrogenload were lower by 17 and 15% respectively, for the shallow drainage depth and 21 and16%, respectively, for the conventional drainage intensity. Reductions in nitrate loads wereattributed primarily to reductions in annual drainage volume.