Experiments And Numerical Simulation On Testing Parameters Of Soil Water Characteristics With Evaporation Method

The matrix potential(or matrix suction)of soil water changes with soil water content.The relationship curve between matrix potential and soil water content is called soil water characteristic curve.The soil water characteristic curve which belongs to the fundamental physical properties of the soil indicates the relationship between energy and quantity of soil water,and plays an important role in the study of soil water movement and solute transport.The methods for measuring soil water characteristic curve include tensiometer method,sand funnel method,pressure membrane method,centrifuge method and so on.Soil permeability coefficient also showed a curvilinear relationship with the change of water content.The theoretical model which express nonlinear relationship between soil water characteristic curve and permeability coefficient contains Van Genuchten model(V-G model),BrooksCorey model and so on.The parameters of these theoretical models need to be determined by observing the soil water characteristic curve.The laboratory evaporation is a new method for measuring soil water characteristic curve.The evaporation tester--Hyprop is used to carry out this research.The basic principle of Hyprop system is Schindler linear assumption.The soil moisture characteristic curve and unsaturated permeability coefficient curve are calculated by measuring the change of the suction in two different depths and the change of soil sample quality during the evaporation process.The samples conclude the fine sand of Badain Jaran Desert,the clay of China University of Geosciences(Beijing)grassland,and the sandy loam of Erdos.In the evaporation test,there is a great difference in the curve of suction increasing for different soil samples.According to the experimental results,the soil moisture characteristic curve obtained by Schindler linear assumption presents a good type of "S".The method can give the air-entry value for fine sand sample.However,the consistency of unsaturated permeability coefficient data is poor.The characteristic parameters can be obtained preliminarily by fitting the experimental data with the V-G model.In order to better understand and prove the applicability of the linear assumption,the Hydrus-1D model is established to develop the numerical simulation study of the evaporation process based on the V-G model.The results of forward modeling show that the relationship curve based on the linear assumption of water content and suction in the middle of the sample is consistent with the actual curve,but the suction value is larger than that of the actual data.The results of the inversion show that the characteristic parameters obtained by the evaporation method are different from the parameters obtained by the Hydrus-1D model,which means that the results of the evaporation method may need to be corrected.The correction method needs to be further studied.