Experimental study and numerical simulation of soil water and salt transport under dry drainage conditions

In this research, the effect of dry drainage on the distribution of soil water content and salinity were experimentally and numerically investigated for 84 days. In the experimental study, soil water content and salinity were measured at different soil depths and four times after planting. Numerical simulation was carried out with HYDRUS-2D which was calibrated and validated using measured data from the experimental study based on different assumptions. Results showed that dry drainage could not only control the soil water content of the irrigated area but also transfers large amounts of salts from this area to the fallow area. So that at the end of the experiment, soil surface salinity of the irrigated and fallow area increased up to 145% and 270% more than initial values, respectively. Based on statistical indices, the accuracy of HYDRUS-2D simulation was improved by considering differences between parameters of the irrigated and the fallow areas including soil hydraulic and solute transport parameters. Furthermore, this accuracy was increased due to the application of the multiplicativity approach for combining water and salinity stresses. In addition, two indices of variations, soil surface salinity percent and soil salinity ratio, are recommended as suitable criteria for determining salt scratching time from soil surface of the fallow area and ensuring the sustainability of the dry drainage system.