The Impact of Drainage Depth on Water Quality in a Cold Climate

Abstract

The impact of drainage depth on hydrology and water quality in southern Minnesota was investigated through a field experiment. Subsurface drainage systems were installed on nine field-sized watersheds ranging in size from 0.8 to 2.5 ha. The nine systems comprised two drainage depths (90 and 120 cm) and conventional (13 mm/day design drainage rate) and narrow (one-half the conventional) drain spacings. Surface and subsurface drainage runoff and nitrate-nitrogen were monitored with automated equipment for 2001 and 2002. Results from the two years show that for the conventional drain spacing, annual drainage runoff and nitrate-nitrogen were reduced for the shallow drains by up to 40 and 47 percent, respectively. The results for the narrowly spaced drainage systems were more ambiguous, however. Reductions in nitratenitrogen were attributed primarily to reductions in drainage runoff volume because only minor differences in nitrate-nitrogen concentrations were observed among watersheds. It is hypothesized that the reduced drainage volume in the shallow systems was accompanied by an increase in deep seepage below the drainage systems. Data from 2003 are still be analyzed and modeling research is underway to predict the impacts of shallow drainage over long climatic records and for other soil types.