The Effect Of Shallow Groundwater On Coupled Soil Water Vapor And Heat Transport In Coastal Plain

The shallow groundwater and the high evaporation are common in the coastal plain of China. Groundwater quick recharge affects all kinds of soil physical and chemical processes, especially the coupled water vapor and heat transport in soil, which is crucially important for the local vegetation ecosystems. However, coupled soil water-vapor-heat transport above shallow groundwater tables needs to be further studied at the aspects of theory basis and technical method.Aiming at the soil water and heat transport mechanism above shallow groundwater, the thesis studied the soil water and thermal properties, as well as the water vapor and heat coupling movements based on the laboratory experiments and numerical simulation. The main conclusions are as follows:1. On the basis of the field data and laboratory experiments, the driving force of water vapor and heat transport and soil evaporation and water content varations were studied. The results indicated that experienced cycle fluctuation and increased with the groundwater table. In another word, the shallow groundwater has “cooling” effect on soil layers. Soil matric potential was influenced by evaporation and groundwater recharge. The constant plane should be occurred between the evaporation front and capillary rise zone. The calculated evaporation decreased with the groundwater table depth. Soil columns were divided into saturated, saturated-unsaturated, and absolutely unsaturated types. The surface water content varied irregularly because of the water vapor transition. The second one appeared the obvious fluctuation, and the third one varied rarely.2. To further verify the observed temperature and water content gradients, the 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 results showed that the observation and the simulation fitted well, and the upper soil moisture fitted better than the deeper. It indicated that coupled water vaor and heat transport can be well by the PDV model, especially in the high soil moisture condition.3.The hydraulic parameters were obtained by fitting measurement and VG simulation of moisture and matric potential based on the RETC code. Soil moisture and heat properties were studied by the PDV model. Results indicated that the unsaturated hydraulic conductivity appeaed 24 h fluctuation. The matric potention mainly affected the liquid hydraulic conductivity, while the soil temperature mainly influenced the vapor hydraulic conductivity. Thermal conductivity decreased with the groundwater table increasing becused water content increase the partical water films, which hastened the heat transport.4. Soil liquid water transport was mainly driven by matric potential gradient. The iso-thermal liquid flux was generally two orders magnitude larger than the thermal liquid water flux. Soil vapor migration was mainly influenced by the temerpature gradient. Both the convergent plane and divergent plane exisited in diurnal vapor transport. The contribution of the latent heat flux to total heat flux can not be negelected. The heat transport experienced at least four stages when groundwater table varied from 10 cm to 60 cm.