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In cold regions, freeze-thaw cycles play a critical role in many engineering and agricultural applications and cause soil water flow and heat transport studies to be much more complicated due to phase changes involved. A fully coupled numerical module for simulating the simultaneous movement of water, vapor, and heat during freezing-thawing periods was developed and incorporated in the Hydrus-1D software in this study. To avoid numerical instabilities caused by a sudden increase in the apparent heat capacity during a phase change, a new approach based on the available energy concept was adopted to adjust soil temperature when the freezing temperature is reached. The proposed freezing module's performance was then validated using experimental data collected at three field sites with typical seasonal freezing/thawing processes. Results showed that the model could efficiently obtain a convergent solution and that simulated soil moisture and temperature variations captured the observed data well. Driven by soil matric potential and temperature gradients, both liquid water and water vapor flowed towards the freezing front. The isothermal liquid flux was the most significant component of overall flow in most soil depths except in the frozen layer, where it decreased by 1–5 orders of magnitude from values before freezing. Instead, the thermal vapor flux was the dominant moisture transfer mechanism in the frozen layer and contributed about 10% to the ice formation. These results indicate that the model, which considers the coupled movement of water, vapor, and heat, can better describe the physical mechanisms of the hydrological cycle in the vadose zone during the freezing-thawing periods. © 2021 Elsevier B.V.

In arid areas, vapor flow has been recognized to contribute significantly to the mass and energy transfers and to play a critical role in maintaining surface vegetation and ecosystems. To better understand the continuous spatial and temporal variations in liquid water and water vapor contents under different climatic conditions, soil water contents, temperatures, and micrometeorological variables were observed in-situ in the Mu Us Desert of northwestern China. The collected data were then used to calibrate and validate the Hydrus-1D model simulating either the coupled movement of water, vapor, and energy or only isothermal water flow in soils (the latter as a reference). The results of the coupled model were not only in better agreement with observed data, but also advanced our understanding of underlying mechanisms of soil water flow. For the coupled model, the isothermal liquid flux was the most significant component of the total water flux. Three diurnal stages were identified for thermal liquid and vapor fluxes on dry days, while the vapor flux became almost negligible during rainfall events. The results indicate that isothermal liquid, thermal liquid, and thermal vapor fluxes should be considered simultaneously when evaluating soil water flow in arid regions, while the isothermal vapor flux can be neglected. Vapor flow contributed, on average, about 13% of the total water flux in the uppermost soil layer during the analyzed period at the study area, and this ratio often exceeded 20%.