Assessing water and nitrate‐N losses from subsurface‐drained paddy lands by DRAINMOD‐N II

Abstract In this study, the effects of various drainage systems on water and nitrate‐N losses were investigated using DRAINMOD‐N II. Required field data were obtained during three growing seasons of canola in a subsurface drainage pilot in Mazandaran Province, northern Iran. The calibrated model was used to assess the effects of different drain depths ( D = 0.10–0.90 m with 0.10 m intervals) and spacing ( L = 10–90 m with 10 m intervals) on seasonal drainage water and NO 3 − ‐N concentration in drainage effluents. DRAINMOD‐N II performance was assessed using different criteria including absolute deviation (AD), root mean square error (RMSE) and determination coefficient ( R 2 ). The simulated and observed drainage discharges (0.97 vs 0.96 mm day −1 ) and NO 3 ‐N concentrations (9.1 vs 14.1 mg l −1 ) were in good agreement in the calibration process. The model performance was also acceptable in the validation process (AD = 0.59–0.79 mm day −1 ; RMSE = 1.01–1.28 mm day −1 ; R 2 = 0.59–0.79 for drainage discharge and AD = 8.3–16.3 mg l −1 ; RMSE = 12.4–27.6 mg l −1 ; R 2 = 0.4 for NO 3 ‐N). Based on the scenario analyses, the D0.40L50 drainage system was the best one, resulting in fewer environmental effects from the nitrate‐N and water loss viewpoints. © 2020 John Wiley & Sons, Ltd.