A coupled model for liquid water, water vapor and heat transport of saturated-unsaturated soil in cold regions: model formulation and verification

In cold and arid regions, vapor movement and water flow are crucial to thermal-moisture dynamics of the active layer and control the soil microbial activity, plant growth and engineering applications. Although it is widely recognized that both liquid and water vapor movement are fundamental factors in the quantification of soil mass and energy balance, their computation is still rarely considered in most models or practical applications. Moreover, previous studies on the movement of moisture migration in unsaturated frozen soil are limited. This study was conducted to: (a) implement a fully coupled numerical model that includes water migration in both the vapor and liquid phases and heat transfer by means of conduction, convection, latent heat of vapor diffusion and phase change effects. (b) Verify the numerical model with detailed field monitoring data. (c) Analyze the role of water flow and vapor diffusion in the heat and mass transport. The result showed that the numerical model was able to fully calculate the coupled soil mass and energy budget. Thermal conduction dominated the heat transport in the deeper layer (below 75 cm) in permafrost regions, while the impact of water movement on heat in summer was significant in shallow ground. Soil water was transported by both liquid water and water vapor and water vapor contributed more than 15 % of the water flux at all depths. Vapor and liquid water transport play an important role in soil mass and energy transfer, especially for the shallow surface. It is necessary to consider the coupled liquid water, water vapor, and heat transport in predictions of soil water and heat dynamics in permafrost regions.