Coupled Water-Vapor-Heat Transport In The Unsaturated Soil And Its Numerical Simulation

In arid and semi-arid areas, the soil water content in the top shallow unsaturated zone originates not only from precipitation (i.e. atmospheric water) but also from vapor condensation (i.e. condensation water). In such areas, annual precipitation is low, while solar radiation and evaporation are strong. The vapor condensation-induced increment of soil water content in the top shallow soil layer plays a critical role in maintaining the desert-plant, the vegetation and the ecosystem in these areas. In unsaturated soil, soil water transports in both liquid and vapor phase. The transport implies the frequent exchanges of mass and energy between the liquid and the vapor. Such kind of coupled soil moisture and heat transport mechanism is actually the formation mechanism of the condensation water in the top shallow soil layer.The thesis aims to understand the coupled soil moisture and heat transport mechanism and explain the formation mechanism of the condensation water. To achieve this aim, experiments need to be designed to observe soil moisture content, soil temperature, soil matric potential and micro-meteorological elements (e.g. air temperature, relative humidity, solar radiations, wind speed, precipitation and evaporation and so on). There were two experiments designed to do so: the sand bunker experiment and the field experiment. The sand bunker experiment shows that the variation of soil moisture content was influenced by soil temperature. The soil temperature gradient directed the soil water transport. For the top shallow soil layer, when the temperature gradient was downward, the soil moisture content increased, vice versa. To further verify the observed temperature gradient-directed soil water transport, the modified HYDRUS-1D, which was referred to the coupled water, water vapor and heat flow in the soil, was used to simulate the soil moisture and temperature measurement. The numerical analyses provided insight into the diurnal movement of liquid water and vapor driven by the gradients of temperatures and pressure heads in the subsurface zone. The simulated spatial-temporal distribution of liquid water flux, vapor flux and soil temperature showed a detailed diurnal pattern of soil water dynamics in relatively dry coarse sand. To examine whether the diurnal pattern can be reproduced under the real arid/semi-arid condition, the field experiment was conducted in the Badain Jaran Desert.Above-mentioned model (HYDRUS-1D) is based on traditional PdV model, which considers only the vapor diffusion, but vapor advection and dispersion. The water vapor transport mechanism includes diffusion, advection and dispersion. To consider them fully, soil air have to be treated as an independent state variable. On the basis of PdV model, the thesis developed a coupled water-vapor -dry air-heat transport model. With the Badain Jaran Desert experiment, both the new developed model and the PdV model were used to calculate surface evaporation. The result showed that the new developed model simulation was closer to the measured evaporation than the PdV model, especially when the soil was relatively wet. Based on the result, to understand the difference in calculating evaporative flux by the new model and the PdV model, we discuss the distribution of gradient driving forces and conductivities. The results show that isothermal soil water flux is the main reason for the difference. The new developed model establishes a stepping-stone for understanding fully the complicated coupled transport mechanism in soil and the formation mechanism of condensation water.