Investigation of surface drainage soil desalination in paddy fields using field experiments and numerical simulations

The performance of the surface drainage method, which has been widely implemented for paddy field desalination and reclamation following the Great East Japan Earthquake, has not been quantitatively evaluated. Moreover, owing to the complex field conditions of paddy soils, few existing studies have established coupled models that simultaneously account for variations in water, heat, and solutes in air-ponded water-soil interactions. In this study, a surface drainage desalination experiment was conducted in an actual paddy field in Natori City, Miyagi Prefecture, Japan, and a multiphysics numerical model was developed and validated using COMSOL Multiphysics. Simple trench and mole drains were incorporated into the model to further evaluate the performance of the combined desalination system. The results showed that water convective velocity near the soil surface was significantly higher, with ponding water velocity approximately 1,000,000 times that of soil water. Variations in meteorological factors (air temperature, solar radiation, and wind speed) drastically influenced desalination efficiency, leading to an increase in rates by up to 6.8 times in the rice root zone (0-30 cm) compared to the constant conditions. Repeated drainage and re-flooding remarkably improved desalination but required optimal water management. Drainage systems, including simple trenches and mole drains, effectively increased the desalination rate by up to 2.1 times. Overall, this study provides a critical technical reference for salt removal from rice paddy fields by integrating field experiments with numerical simulations.