Research On The Test Method Of Water Diffusivity In Unsaturated Soil

Soil water is an important hub connecting surface water and groundwater,and an essential process of hydrological cycle.The unsaturated water movement equation(Richard equation)can simulate the underground water movement process and solute transport process.Research on the test method of water diffusivity of unsaturated soil,improve the test accuracy of water diffusivity of soil,can more accurately describe the process of water movement in the soil,for agricultural irrigation,water conservancy projects and involving water infiltration and other practical engineering problems of great significance.This paper firstly analyzes the advantages and disadvantages of horizontal soil column method and Hydrus-1D method in testing water diffusivity of unsaturated soil.In view of the fact that water content data of horizontal soil column method is easily affected by free boundary,an improved method of horizontal soil column method is proposed to modify the water content near the free boundary.For the Hydrus-1D method,it is necessary to assume upper and lower boundary conditions.The feasibility of the method was verified by Hydrus-1D simulation of one-dimensional horizontal soil column infiltration test and design of indoor one-dimensional horizontal soil column infiltration test.The results show that:(1)When the basic data is ?~x,the calculation results of soil water diffusivity determined by the horizontal soil column method have high accuracy.When the basic data is?~t,the soil water diffusivity is easily affected by the free boundary.The closer the measuring point is to the free boundary,the narrower the water content interval with higher test accuracy will be.When the mathematical model of soil water diffusivity can be accurately determined and the upper and lower boundary can be determined,the Hydrus-1D reverse analysis method has a better suitability for calculating soil water diffusivity.(2)Combining with the constant variation method in advanced mathematics and considering the highly nonlinear Richards equation of one-dimensional horizontal infiltration,the empirical solution of the Richards equation of one-dimensional horizontal infiltration is given,(?),and the improved method of horizontal soil column is proposed.The improved horizontal soil column method is used to analyze the data affected by the boundary effect.The results show that when the boundary conditions are close to the first boundary conditions,the calculated results of soil water diffusivity are in good agreement with the real values.(3)The numerical inverse analysis method was used to analyze the Hydrus-1D simulation data.The results of numerical inverse analysis are not affected by the initial estimates.When there is a certain error in the test result of water content,the calculation result of numerical inverse analysis method also has a high accuracy,with the determination coefficient above 0.9990.And different boundary conditions will not affect the calculation results of the proposed numerical inverse analysis method.