Vapor transfer mechanism and its influence factors during the freeze-thaw periods in Mu Us Sandy Land

Due to the phase transition between ice and water, soil water and heat transport show complexity during freeze-thaw periods. To investigate the distribution and transfer characteristics of water vapor can provide key information for better understanding the mechanisms of the hydrological cycle in the vadose zone of the sandy land. By establishing the in-situ observation site in the Mu Us Sandy Land, as well as building a coupled water, vapor, ice, and heat model through the modified software program, an analysis was conducted for the vapor transfer process during the freeze-thaw cycles. Results showed that the simulated water content and temperature are highly consistent with the measured data, suggesting a good accuracy and applicability of the proposed model. By comparing the simulation results of the selected non-freezing, initial freezing, downward freezing, and thawing periods, it can be found out that the distribution of soil water content, ice content, and vapor density in the profile changed, and the variation in vapor density was closely related to soil temperature. When soil was frozen, the thermal vapor flux driven by temperature gradient accounted for more than 90% of the total soil water fluxes, and vapor transfer played a critical role in affecting the soil moisture distribution of the profile.