Improving soil water, heat and salt transport simulation under freezing conditions considering salt crystallization and its effect on hydrothermal parameters

Moisture and solute transport induced by freezing are critical for assessing salinity outbreaks and runoff during the spring thaw in cold climates. Conventional models often neglect the effects of initial freezing-temperature (IFT) depression and salt crystallization on the coupled water-heat-salt transport and hydrothermal parameters. This study developed a novel coupled water–heat–salt transport model for seasonally frozen saline soils. A dynamic IFT function was proposed, accounting for the interactive effects of water and salt content. A permeability function incorporating the synergistic impedance of ice and salt crystals was also introduced. The model further integrates salt crystallization and latent heat release effects into the coupled transport framework. The model was implemented in COMSOL Multiphysics and validated against laboratory column experiments. Results show that the model accurately simulates the coupling mechanism between IFT depression, phase transitions, and coupled migration processes. The MRE for soil moisture and salinity decreased by 6.56 % and 2.34 %, respectively, and the RRMSE decreased by 8.64 % and 4.58 %, significantly improving simulation accuracy. Key phenomena such as unfrozen water retention, salt accumulation, and crystallization mass (15–19 kg/m³) were captured by the model. It revealed the regulatory effect of IFT depression on water–salt redistribution. Ice and salt crystal reduced the permeability by 3–6 orders of magnitude. Sensitivity analysis demonstrated the model’s adaptability and tunability for various soil types and environmental conditions. The model exhibited strong parameter flexibility, numerical stability, and visualization capacity, offering technical tools for water resource management and sustainable agriculture in cold-region salinized soils.