Experimental and Numerical Study on the Coupled Processes of Salt Migration and Moisture Movement Under Evaporation in the Vadose Zone

In arid and semi-arid regions, soil salinization has emerged as an escalating environmental challenge. Soil salinity not only alters the soil structure but also influences water movement and distribution. The coupled processes of water movement, heat transfer, and solute transport in the vadose zone interact dynamically, warranting an in-depth investigation into coupled processes of matter and energy. This study developed a numerical model of coupled water-vapor–heat–salt transport in the vadose zone, validated through evaporation experiments and compared with a conventional model excluding osmotic potential. It is found that salt presence reduces evaporation rates while enhancing soil moisture movement. Liquid water movement is primarily governed by matric and osmotic potential gradient, whereas water vapor movement is dominated by temperature gradients. Matric potential influences water vapor movement only at the soil surface, and the impact of salt on water vapor movement diminishes with increasing water content. Notably, matric potential significantly affects water vapor movement only when soil water vapor relative humidity is below unity. The proposed model effectively describes multi-field coupling transport and clarifies the role of osmotic potential in regulating liquid and vapor water dynamics.