Water and salt transport simulation in the wheat growing area of the North China Plain based on HYDRUS model

Soil moisture and salinity are key factors that affect crop growth. Thus, it is important to investigate the mechanisms of water and salt transport to further clarify the process of soil water utilization in crops. The HYDRUS-1D model was applied to examine the spatial distribution and vertical variation in soil water and salinity and to explore the factors influencing water and salt transport. The model incorporated long-term soil moisture observation data and the results of indoor soil column experiments at the Yucheng Comprehensive Experimental Station, Chinese Academy of Sciences, a typical farmland in the North China Plain. Moreover, the applicability of the HYDRUS-1D model to the study area was evaluated. The results showed that the simulation of water transport in shallow soil had greater errors than that in deep soil, owing to the influence of external factors. The root mean square errors (RMSEs) of the water transport simulation results were 0.0348 cm³·cm^-3, 0.0179 cm³·cm^-3, 0.0179 cm³·cm^-3, 0.0122 cm³·cm^-3, and 0.0053 cm³·cm^-3 at 10 cm, 20 cm, 30 cm, 40 cm, and 60 cm, respectively. The mean value of the Nash-Sutcliffe efficiency (NSE) coefficient was 0.826, and the coefficient of variation was 0.0560, indicating that the simulations of water transport were stable and consistent with the measured values. The soil column experiment results showed that after irrigation with 8 L water, salt salinity in the vertical direction first increased and then decreased; the RMSEs of the simulation results of salt transport were 0.181 g·kg^-1, 0.131 g·kg^-1, 0.120 g·kg^-1, 0.034 g·kg^-1, and 0.027 g·kg^-1 after 12 h, 24 h, 40 h, 45 h, and 48 h, respectively. The mean error was 0.174 g·kg^-1, which was in good agreement with the measured values, indicating that the model was suitable for the simulation of water and salt transport in the study area. However, owing to the influence of evaporation, tillage, and crop root system, large variations in physical and chemical properties resulted in large deviations between the simulated and measured values of salt transport in shallow soil, and the NSE coefficient reached 9.71. After 48 h of infiltration, the soil salinity peaked at 23 cm, 26 cm, and 29 cm depth with 8 L, 16 L, and 24 L irrigation quotas, respectively. These results showed that increased irrigation quotas can enhance salt leaching. This study confirmed that the HYDRUS-1D model could be used for theoretical studies of water and salt transport in the study area. This study also provides a theoretical basis for further exploration of water and salt transport in winter wheat, optimizing farmland water resource management, and improving the water resource utilization efficiency in the North China Plain.