Predicting the impact of drainage depth on water quality in a cold climate

The impact of drainage depth on hydrology and water quality in southern Minnesota was investigated through a field experiment and computer modeling. Subsurface drainage systems were installed on 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 NO ₃-N (NO ₃-N) 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 NO ₃-N were reduced for the shallow drains by up to 47 percent. Similar effects were not observed, however, for the narrowly spaced drainage systems. Reductions in NO ₃-N loss were attributed primarily to reductions in annual drainage runoff volume because relatively little difference in NO ₃-N concentrations were observed among watersheds. We theorize that the reduced annual drainage volume in the shallow systems was accompanied by an increase in deep seepage below the drainage systems. Modeling was conducted to complement the field observations. Eighty-five year DRAINMOD simulations were conducted with a previously calibrated dataset, for drainage depths of 60, 90, and 120 cm. The objective of the simulations was to investigate the sensitivity of DRAINMOD to drainage depth for the soil of interest and assess the potential long-term reductions in drainage volume from shallow drains. The simulations are deemed "preliminary" because although they were based on calibrated inputs for a similar soil, no calibration was performed with observations from the drainage depth experiment. Results of the simulation showed decreased annual drainage volume with decreasing drain depth, but decreases were not the same magnitude as those observed in the field experiment. Moreover, the simulated annual drainage volumes were quite variable in drier years that had less annual drainage; increases in annual drainage were predicted in many of these years. Relative crop yield was also evaluated with the DRAINMOD simulations. Decreases in relative yield were apparent in the shallower systems, but not significantly so at the 90-cm depth. The 60-cm depth simulations, however, predicted crop failure in 75 percent of the simulation years - a result that is probably not realistic. The drainage depth field experiment will continue and future modeling work will focus on using the data from that study for model calibration and long-term simulation.