Influence of different subsurface drainage systems on nitrate losses from rainfed-canola croplands

In this research, which was carried out in the 4.5-ha consolidated paddy fields of Sari Agricultural Sciences and Natural Resources University during two cropping cycles of 2015-2016 and 2016-2017, the influence of different drainage systems on total nitrate loss into local surface water resources was investigated. These systems included three regular subsurface drainage systems with different drain depths and spacings of, respectively, 0.9 m and 30 m (D0.90L30), 0.65 m and 30 m (D0.65L30), and 0.65 m and 15 m (D0.65L15), and a bilevel drainage system consisting of four drain lines with 15 m spacing and 0.65 and 0.9 m alternative depth (Bilevel). In addition to daily measuring drainage fluxes, nitrate concentrations in the collected drainage water were also measured every other weeks during the cropping cycles. Daily average drainage discharges under Bilevel, D0.90L30, D0.65L30, and D0.65L15 varied in the ranges of 0-231 cm3 s-1, 0-220 cm3 s-1, 0-227 cm3 s-1 and 0-250 cm3 s-1, respectively. Analyzing precipitation-drainage discharge correlations reveals that the precipitation intensity of 10 mm d-1 is the threshold of drainage capacity reduction, and precipitation intensities beyond this threshold may result in water logging challenges in the study area. Consolidating the selected drainage systems may result in annual nitrate losses of 2.2-34.7 kg ha-1 into the surface water resources; however, the D0.65L30 systems may have less environmental consequences in this view of point. Therefore, environmentally sustainable operations of these systems for expanding rainfed-cropping requires precious investigations when selecting drain depths and spacings.