Mechanism Of Coupled Water,vapor,and Heat Transport In Seasonal Frozen Soil Region Of The Mu Us Sandy Land

The seasonal frozen soil region occupies 53.5%of the Chinese territory,and the abundant resources in this area are of great significance to human living environment and activities.Due to the influence of seasonal freeze-thaw cycles,liquid water,water vapor,and ice coexist in the vadose zone,and the migration and transformation processes among them are complicated.Related studies about the mechanism and numerical modeling of the coupled water-vapor-heat movement are relatively few and contain some shortcomings,which limits the understanding of the mechanisms of the hydrological cycle in the vadose zone and many environmental and engineering problems.This thesis takes the seasonal frozen region of the Mu Us Sandy Land as the research area.Focusing on the mechanism of the coupled water,vapor,and heat movement during both freezing and non-freezing periods,this study is carried out by means of in-situ observation,laboratory and field tests,and numerical simulation.The distribution and variation in soil moisture,temperature and water flux under different spatial and temporal scales are revealed.The main achievements are as follows.(1)Through long-term monitoring,the characteristics of soil moisture and temperature in the unsaturated zone were analyzed.Affected by frequent precipitations and intense evaporation,soil water content at the shallow 10 cm depth fluctuated significantly during the non-freezing period.The variations in soil moisture and temperature decreased with soil depth,and there was a significant lag.During freezing period,the unfrozen water content decreased obviously when soil temperature reached the freezing point,and the ice content increased accordingly.The maximum freezing depth exceeded 110 cm.Besides,vapor density in soil was closely related to the variations in temperature,and its value became small when the soil was frozen.(2)By collecting data from in-situ observation and laboratory and field tests,a coupled water,vapor,and heat model was established using the Hydrus-1D software to analyze soil water migration.Meanwhile,a model that only considered liquid flow due to isothermal forces was also established as a reference.Comparatively,the simulated results of the coupled model were in better agreement with observed data.The average RMSE values were 0.007 cm~3/cm~3and 0.7?for soil water content and temperature,respectively,indicating that the coupled model should be considered in evaluating soil water movement in arid and semi-arid areas.Due to neglecting the liquid and vapor fluxes driven by the temperature gradient,the fitting errors of the isothermal model were relatively larger,and the discrepancy increased gradually during the simulation period.(3)The diurnal variations of soil water flux under different meteorological conditions were analyzed.Affected by evaporation,the isothermal liquid flux was upward throughout the soil profile in dry days.The flux become extremely large during rainy days,with the convergent and divergent zero-flux plane occurring in the surface layer.The isothermal vapor flux could be negligible during both dry and wet conditions.Driven by temperature gradient,there existed apparent diurnal changes in the thermal liquid and vapor fluxes.The maximum value of vapor and liquid flux appeared in the surface layer and the 20 cm depth,respectively.During rainfall process,the liquid flux increased significantly,while the vapor flux decreased.(4)According to the theory of soil water migration under freeze-thaw conditions,a fully coupled numerical model for simulating the simultaneous transport of water,vapor,and heat was developed and incorporated in the Hydrus-1D software.The existing numerical problems were solved in the calculation procedure.For example,the calculated soil temperature was adjusted when the freezing temperature was reached based on the available energy theory.The performance of proposed model was then evaluated using the measured data,with average RMSE values of 0.006 cm~3/cm~3 and 0.6?for soil water content and temperature,respectively,indicating a better simulating accuracy.The model was further assessed using the measured data from other freezing sites and showed good applicability.(5)The coupled movement of water,vapor,and heat in the vadose zone during the freeze-thaw period was simulated and analyzed using the proposed model.During the typical freezing period,soil water content in the shallow layer exhibited diurnal variations and the freezing front dropped continuously.On the other hand,the thawing process were bi-directional starting from both the surface and the deep layer during the typical thawing period.Driven by soil matric potential and temperature gradients,both liquid water and water vapor flowed towards the freezing front,resulting in the decrease in the soil water content of the deep layer.In the frozen layer,the liquid flow was impeded due to the presence of ice,decreased by about 1?5 orders of magnitude from values before freezing.Instead,the vapor flux became relatively larger.(6)Typical conditions were selected to quantitatively analyze the driving forces of soil water flow.The isothermal liquid flux was the most significant component of overall flow in most soil depths,while the thermal vapor and liquid fluxes had effect on water migration in the shallow and deep layers,respectively.During the non-freezing period,the thermal vapor flux occupied about 20%of the total water flux,and this ratio increased with decreasing rainfall amount.When soil is frozen,vapor flow played a dominant role in moisture transfer,and the isothermal vapor flux should not be neglected.Above research results provide scientific basis for the rational use of water resources and ecological environment protection in Mu Us Sandy Land.In addition,the proposed numerical model can be also applied to other freezing regions,which plays critical role in revealing the mechanisms of hydrological cycle of the vadose zone in seasonal frozen soil area.