Review of coupled water, vapor, and heat transport of the vadose zone in the seasonal frozen soil region

Abstract: Seasonally frozen soil regions refer to those areas where soil is frozen for 15 days or more per year. More than half of land surface is occupied with the seasonal snow cover in China. The freeze-thaw process can significantly change the soil properties, as well as the water and heat transferring in the vadose zone. Among them, temperature and water vapor have posed a significant impact on the soil moisture, due to the soil subjected to the dry condition in most of the seasonally frozen regions (belonging to the arid and semi-arid areas). It is gradually recognized as the significant effect of vapor flow on the soil water movement for both freezing and non-freezing periods over the past several decades. It is a high demand to couple the water, vapor, and heat transport suitable for the actual conditions of seasonally frozen soil regions, in order to reveal the influencing mechanism of soil hydrological cycle. The coupled theory of water, vapor, and heat transport was firstly proposed by Philip and de Vries. The total soil water flux was then divided into four components, including the liquid water flux and water vapor flux driven by water potential and temperature gradients, respectively. Since then, extensive researches were also carried out to continuously improve on the coupled transport. Once the soil was frozen, the liquid water, vapor, and ice were coexisted in the unsaturated zone. Two aspects were also observed in the influence of phase changes between liquid water and ice on the coupled water, vapor, and heat transport. Several hydraulic parameters were calculated to determine the hydrological cycle, such as the soil freezing curve, and hydraulic conductivity for the liquid water. The spatial and temporal distributions of soil moisture in the vadose zone were dominated by the seasonally freeze-thaw process as well. The contents of unfrozen water and ice also changed significantly with the variations of soil temperature. Numerical simulation was gradually utilized in this research field with the ever-increasing computational capacity and simulation accuracy. Great challenges were still remained on the coupled numerical model, due to the influence from the ice-water phase change. An appropriate coupling model was crucial to the numerical simulation via the reasonable simplification. The underlying mechanism of coupled water and vapor flow was gradually revealed from the simulation using different models. Specifically, the vapor flux was one of the most important components in the soil water movement, usually accounting for 10%-30% of the total water flux. Furthermore, the vapor flux was depended mainly on the relatively low soil moisture and large temperature gradient in the shallow layer. The vapor flow was much more significant during the freezing period, due mainly to the impeded flow of liquid water in the presence of ice. Consequently, some research directions that needed to be strengthened in this field were proposed to deepen the theoretical fundamentals and the practical tasks in the seasonal frozen soil areas. Firstly, the condensation and accumulation of water vapor can greatly contribute to the vegetation under soil drought and freezing stress. It is of great significance to maintain the desert vegetation ecosystem, where the soil water is critical to the vegetation growth in the fragile ecological areas. The vegetation module can be combined with the coupled model water, vapor, and heat transferring. Further studies can be implemented to explore the specific impact of liquid water and vapor on the surface vegetation in seasonal frozen region. Secondly, the coupled transport of liquid water and vapor can also impact many engineering construction activities as well, such as the frost heave in the railway embankments that caused by the continuous liquid water and vapor transport from the deep soil layer. Finally, in-situ monitoring and simulation can be strengthened to reveal the detailed process of liquid and vapor transport below the surface impermeable layer. The finding can also provide the scientific basis for the disaster prevention and control during freeze-thaw process.