Drainage Water Management Effects on Tile Discharge and Water Quality

Drainage water management (DWM) has received considerable attention as a potential best management practice (BMP) for improving water quality in tile-drained landscapes. The objective of this study was to evaluate the effects of DWM on subsurface drain discharge as well as on nitrogen (N) and phosphorus (P) loads in drainage water. Tile discharge and nutrient concentrations were measured from two adjacent tile drainage outlets in an Ohio, USA headwater watershed for 7 years (2006–2012). A control structure was installed in 2009 to allow DWM at one of the outlets from 2009 to 2012. A before-after control–impact (BACI) study design was used to assess the impact of DWM on tile discharge and nutrient loads. Results showed that DWM significantly decreased annual tile discharge between 11 and 178 mm, which was equivalent to an 8 to 34% reduction inflow. DWM significantly decreased annual NO3 N loads by −1.3 to 26.8 kg ha−1 (−8 to 44%) and annual dissolved P loads by 0.04 to 0.51 kg ha−1 (40 to 68%). Nutrient concentrations were not significantly affected by DWM indicating that decreases in nutrient loads were primarily due to reductions in tile discharge rather than changes in concentration. Results from the current study support the use of DWM as a BMP to decrease N and P loads in subsurface drain discharge throughout the U.S. Midwest. Future research should focus on quantifying the effect of DWM on nutrient transport in other flow paths (e.g., lateral seepage, surface runoff) to further evaluate its use as a BMP in tile-drained landscapes.

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