Long-term trends in water quality following implementation of controlled drainage

Wet, poorly drained soils throughout the northern Cornbelt are often artificially drained to improve field conditions for timely field operations, decrease crop damage resulting from excess water conditions, and improve crop yields. Drainage has also been identified as a contributing factor to water quality impairments in surface waters. Our objective was to quantify drain flow volume, nitrogen and phosphorus loss, and grain yield from a conventional free-drainage (FD) compared to a controlled drainage (CD) system in Minnesota, USA. A field study was conducted from 2006-2014 on a tile-drained Millington loam soil (fine-loamy, mixed, calcareous, mesic Cumulic Haplaquoll). The field site consisted of two independently drained management zones, 15 and 22ha, respectively. The project used a time-series approach to statistically evaluate treatment effects. Comparison of means with non-parametric method (Wilcoxon signed-rank test for paired data) between FD and CD zones showed that daily flow volume, daily NO3-N load, daily NO3-N concentration were significantly higher by 29, 41, and 58% from the FD zone compared to those of the CD zone (p-value <0.0001). No general trend was found with these parameters using the Mann-Kendall monotonic trend test; however, seasonality was found with flow volume and NO3-N load from the FD zone using the seasonal Mann-Kendall monotonic trend test. Examination of the autocorrelation function (acf) and partial autocorrelation function (pacf) showed that AR and ARIMA are appropriate models for the time series.