Effect of Free and Controlled Drainage on Water Balance and Soil and Drainage Water Salinity under Rainfed Canola in Paddy Fields

Subsurface drainage, as a structural modification to improve the productivity of paddy fields, changes the hydrology of the fields. In this research, the response of water balance and drainage water salinity to free and controlled subsurface drainage systems during winter cropping in paddy fields was investigated by using the DRAINMOD-S model. The data were collected during two canola growing seasons (2015-2017) from a paddy field with a subsurface drainage system (drain depth of 0.65 m and drain spacing of 30 m). The calibrated and validated model was applied to explore effects of conventional and controlled (by controlling water table depth at 40 cm)- subsurface drainage on water balance and soil and drainage water salinity. The model showed acceptable capability for simulating water table depth and drainage water salinity during calibration and validation processes. Based on the simulations, for drain depth of 1.5 m, increase in drain spacing from 30 to 70 m resulted in 805.3 and 741.6 kg ha-1 decrease in total salt load under free and controlled drainage systems, respectively. For drain spacing of 30 m, increase in drain depth from 0.5 to 1.8 m, caused 2.7- 9 mm and 5.1- 14.3 mm increase in drainage water in controlled and free drainage systems, respectively. Simulation results indicated that, to decrease salt load, a drainage system with 30 m drain spacing and 0.5 m drain depth is suitable for both free and controlled drainage conditions. Based on the results, controlled drainage can be used as a management tool to diminish environmental problems in heavy paddy soils from the viewpoint of salt load and drainage water volume.