Two-dimensional Simulation Of Water-heat-salt Transport In Vadose Zone Of The Coastal Plain

The coastal plain of Tianjin is a land of marine degradation.The landform type is plain and the groundwater level is buried shallow.The seawater impregnation makes the groundwater have high salt content.Therefore,under the semi-arid climate,the salt accumulates along the surface layer of soil with strong evaporation,resulting in the widespread distribution of secondary salinization.The phenomenon of soil salinization not only leads to the decline of biodiversity in coastal areas,but also the degradation of ecosystems,which also restricts the economic development of coastal areas.In this paper,the interaction between water,heat and salt in the soil profile of the aeration zone is considered at the same time.Based on the monitoring results of water,temperature and salt content in the soil profile of the aeration zone under two different rainfall conditions obtained from the indoor soil tank test,the characteristic parameters of water,heat and salt transport in the soil profile are calibrated by the software HYDRUS-2D,and then the simulation is carried out on this basis.The purpose of the study is to investigate the effects of different rainfall on the redistribution of water and salt in the soil profile,and to clarify the law of water,heat and salt change during the infiltration-evaporation process,and to provide scientific basis for the restoration of saline alkali land and the regulation of soil water resources and groundwater resources in coastal areas.The main conclusions of this paper are as follows:(1)Combining field investigation and soil water sample analysis experiment,the vadose zone of the study area was divided into three layers and the salinization characteristics of the profile were analyzed.0-10 cm soil layer is the soil water belt,10-40 cm is the intermediate belt,and 40-80 cm is the capillary water belt.The 0-10 cm soil layer is silty clay loam,and the 10-80 cm soil layer is silty(sandy)clay.The type of soil salinization is sulphate-chloride type,and the total salt content of groundwater is more than 3g/L,which belongs to salt water.According to the classification standard of soil salinization in China,the soil in the study area belongs to coastal mild Salinization Soil.(2)Carry out indoor soil tank simulation experiment to study the transport process of water,heat and salt in the soil profile of the aeration zone under different rainfall intensities.The results show that the rainfall infiltration-evaporation process causes cyclical fluctuations in soil water potential,conductivity and temperature,and the lower the horizon is,the later the periodic fluctuation is.Larger rainfall will have a more pronounced effect on the water,heat and salt redistribution in the vadose zone.(3)Based on the laboratory experiments,the HYDRUS-2D model was established to simulate the water,heat and salt in the vadose zone profile,and the model parameters of water,solute and heat transfer were obtained.The simulation results were evaluated and verified.It was found that the water potential,conductivity and temperature of the rain simulation experiment fit well,and the model parameters are reliable.The fitting effect was evaluated by calculating the root mean square error,the relative error,and the Nash efficiency coefficient.The relative error values of each parameter are small,which reflects a good simulation accuracy.The sensitivity analysis of water and salt transport parameters showed that the sensitivity of water transport parameters is?>n>K_s>?_s>?_r,in which n and K_s are negatively correlated with the model output dependent variable and other parameters are positively correlated.The Diffus.W.sensitivity in the salt transport parameters is higher than Disp.L.