Experimental Investigation And Mathematical Simulation Of Soil Secondary Salinisation And Its Prevention

Soil Secondary Salinization (SSS) is a hazadous phenomenon widely happening around the world, especially in arid and semi-arid regions. It has been studied all the time. In the present thesis, the theory of the flow and heat and mass transfer in porous media is applied to the traditional agricultural field. It may produce new solution to the old problem. In the study of SSS, combination of both mathematical simulation and experimental research is an effective way. In the present thesis, experimental researches were conducted to study the influence of environmental factors such as ground water, evaporation rate. The experiments with plants were also conducted. The mathematical model was established based on the porous media model of "seven field-phase change" and some simulation results were got and analyzed.Based on theoretical analysis of soil solute transfer, the mathematical model of describing simultaneous heat, moisture and gas migration in porous media, and mathematical model of solute transfer in soil was developed using the volume-averaging method. Mathematical simulation was conducted to investigate soil secondary salinization owing to different watertable at environmental conditions, which might be applicable for agricultural practice. In this paper, the dynamic characteristics of soil salt migration and its influence factors are analyzed. Basing on the theories of solute transport and heat and mass transfer in porous media and taking the temperature effect into account, the model of moisture, heat and salt transport, which involved root water extraction, absorption-desorption, is established. By use of numerical method, the transient distributions of salt, temperature and moisture in soil, which might be useful to provide necessary information for preventing and curing soil salinization, are obtained and the influences of soil properties on salt migration are also analyzed.A series of experiments were designed and conducted in a closed-loop wind tunnel to study solute transport in porous media of soil. In such situation, Critical Depth of Ground watertable (CDG) acts an important role. Both mathematical simulation and experimental results indicate that moisture/solute transfer in the process of soil secondary salinization under different conditions of ground watertable exhibits different patterns as well. The influence of plant and its roots were also studied. Moreover, the comparisons of solute transfer between in cropped soil-bed and in bare soil-bed are also conducted.Based on the theoretical analysis and experimental result and mathematical simulation results, the quite typical phenomenon of "salt crusting" was simulated and observed in laboratory, which have not been reported in literature before.Based on present research work, the mechanism of soil secondary salinization was analysed to be the combination effect of temperature, humidity, wind speed, ground watertable , plant and its roots, etc. A new general principle of preventing soil secondary salinization was proposed as to "cut" the connecting channel between the saline groundwaer and the soil surface or root zone under vegetated condition. A new method of soil secondary salinization prevention was proposed as to be "slanted land" plantation pattern.The present work is financially supported by the National Natural Science Foundation of China (No.50376015).