Layered Soil Infiltration Experiment And Numerical Simulation

Precipitation,rivers,farmland irrigation,will infiltrate in the soil and shallow groundwater.The soil in nature generally presents a layered structure due to the influence of geology,biology and human activities.This layered structure considerably affects the distribution and movement of moisture in the soil profile.It is of theoretical and practical significance to investigate the infiltration process of moisture in layered soils for the study of shallow groundwater recharge and regional water balance calculation.The simulated soil profile was obtained from the experimental area of Shangbo Village,Yifengdian Town,Jimo District,Qingdao City.The physical and chemical properties of soil and the soil water retention curve were measured in the laboratory.The infiltration experiment of layered soil column was used to study the water transport law under different initial water content and soil configuration.The tensiometers were installed in each layer of the soil column to monitor the change of the pressure head at that location over time.Simultaneously,the change of the location of the wet front over time and the cumulative infiltration volume were recorded,and the infiltration characteristics of the layered soil were analyzed.Hydrus-1D software was used to simulate the one-dimensional water movement under experimental conditions,and the different combinations of measured variables in the infiltration process were used as objective functions to optimize the layered soil hydraulic parameters by numerical inversion,which provided a reference for the selection of measurement variables on estimating the unknown parameter of soil-hydraulic properties during the transient water flow.The simulation of water flow has achieved with good results.One set of well-fitted parameters was selected to simulate the flow movement under certain initial and boundary conditions to obtain the leakage of soil water under different conditions.One set of well-fitting parameters was selected to simulate soil flow under given initial and boundary conditions,and soil water leakage under different preset scenarios were obtained.The main conclusions of this study are as follows:(1)In this paper,infiltration experiment on layered soil columns under ponding condition was conducted in laboratory.It was found that different soil configurations and initial water content have a greater impact on water movement,and that water needs to flow when the initial water content is low.When the initial moisture content of the soil is lower,and the more viscous layer is in the middle,the time required for the water to pass from infiltration to outflow is longer,indicating that when the more viscous layer is located on the upper layer,the blocking effect on the water flow is stronger.When the initial moisture content is high,the soil’s water conductivity is stronger.Different soil configurations,whether coarse or fine is in the upper layer,have a retarding effect on the flow of water.The reduction of seepage is more obvious when the difference between two adjacent soil layers is greater.And the resistance is more obvious to seepage when the initial moisture content is lower.The infiltration rate of air-dried soil in the initial period of infiltration was relatively large,and the cumulative infiltration volume increased rapidly.The infiltration rate gradually decreased and stabilized with time.However,in the layered soil that is nearly saturated,the cumulative infiltration volume increases rapidly only at the initial stage of infiltration,the infiltration rate remains almost constant after the thickness of the accumulated water is stable,and the cumulative infiltration volume curve tends to be linear.(2)Using Hydrus-1D software to simulate the infiltration test of layered soil column,the cumulative infiltration volume and the measured data of pressure head were taken as the objective function to perform inversion for soil hydraulic parameters.A comprehensive analysis of the inversion results under three different objective functions was performed from the determination coefficients,the coefficient of variation,the correlation matrix of optimization parameters,and the standard error of the optimization parameters.When the objective function only contains cumulative infiltration,although the inversion of the simulated and measured values was highly consistent,R~2 reached 0.999,the correlation of the optimized parameters was higher and the standard error was larger,and the accuracy of the parameters verified by the pressure head was less effective.R~2 was only 0.338.The pressure head was used as the objective function to perform parameter optimization and the cumulative infiltration measured value was used to verify the effect.It can achieve 0.99.Having a combination of the objective functions for pressur head and the comulative infiltration rate can reduce the non-uniqueness of the solution and improve the accuracy of the solution in a certain extent.(3)Simulating the water movement under different conditions with different initial boundary conditions,and comparing their soil water leakage in the depth of 2 m in the field,and found that different soil configurations,initial water content and rainfall quantity will have an impact on soil water leakage.Soils with a high initial moisture content,a large amount of rainfall,and poor permeability are calculated to have the highest leakage.When the soil with poor permeability is located in the middle layer,it can effectively reduce the leakage of soil water.