Numerical modeling of soil water dynamics in subsurface drained paddies with midseason drainage or alternate wetting and drying management

As a supplemental practice to land consolidation projects, subsurface drainage systems have been installed in paddy fields to allow for crop diversification and to improve the overall productivity of paddy soils. The HYDRUS (2D/3D) model was applied to investigate the combined effects of different subsurface drainage systems and water management strategies on water balance, groundwater table, transpiration efficiency, and water use efficiency in paddy fields. Field experiments were conducted during four rice growing seasons (2011, 2012, 2014, and 2015) at the subsurface-drained paddies of the Sari Agricultural Sciences and Natural Resources University in northern Iran. Midseason drainage (MSD) management was applied in 2011 and 2012, while alternate wetting and drying (AWD) management was adopted in 2014 and 2015. The model performance was evaluated using the model efficiency (EF), root mean square error (RMSE), normalized root mean square error (NRMSE), and mean bias error (MBE) measures. The model had strong predictive capabilities for simulating the mid-drain water table depth for both water managements. Under MSD and AWD, daily evapotranspiration rates varied from 4.9 to 6.2 mm d−1 and 4.9 to 5.9 mm d−1, respectively. Drainage losses were higher under AWD than MSD, while the reverse order occurred for percolation losses. Compared with MSD, AWD improved the transpiration and water use efficiency of rice in the presence of subsurface drainage. Being capable of describing complex effects of AWD and MSD strategies in subsurface-drained paddy fields, the HYDRUS (2D/3D) model can serve as a practical tool for optimizing water productivity in these fields.