Migration Characteristics And Numerical Simulation Of Water And Solute In Layered Soil

In recent decades,social and economic development,agricultural production and people's life have been seriously affected by problems such as the degradation of water and the inefficient use of water resources due to groundwater mining and irrational irrigation.Additionally,the disorder of “three industrial wastes” and the massive application of chemical fertilizers and pesticides have caused soil and groundwater pollution problems that endanger the quality of crops and even human health.In nature,not all soils exist as homogeneous soils,layered soils are very common,and there are many different soil configurations.In the layered soil,the permeability of each layer of soil is different,which directly affects the distribution of water and solute in space and time.Therefore,it is necessary to deeply explore migration characteristics of water and solute in different soil configurations in order to better grasp water movement and pollutant transport behavior in natural soil,and then provide theoretical basis for water resources optimization and soil and groundwater pollution prevention.The work was divided into two parts,theoretical research and laboratory experiment.In theoretical part,Hydrus-1D software was used to design five soil configurations by setting a 2 m thick layered soil profile,configuration one(upper sand and powder),configuration two(upper sand and sticky),configuration three(clamp type),configuration four(sand type),configuration five(bottom sand leakage type).The laboratory experiments were divided into two groups: soil column water infiltration experiment and soil column solute transport experiment.It mainly included three aspects.First,based on five soil configurations,with the help of Hydrus-1D software,the effect of time and space dispersion on the numerical simulation of layered soil water flow and solute transport under different boundary conditions was studied.The upper boundary of water flow and solute transport were selected as constant pressure head,variable pressure head/flux and constant concentration pulse input,and the lower boundary was free drainage boundary and zero concentration gradient.The second was to analyze the influence of different factors on water migration characteristics of layered soil under unsaturated conditions and verify the validity of the hydraulic parameters.Third,taking non-reactive solute bromide as research object,the transport characteristics of solutes under different unsaturated and saturated soil configurations were discussed.Concluded as follow:(1)The influence of time and space dispersion on the simulation of layered soil water flow was different,which was specifically manifested in the different time and space steps of different configurations of soil when the numerical simulation was stable under different boundaries.The initial time step under three boundaries of configuration one had no effect on the simulation basically.The minimum time step was stable at 10-5-10-4 d,while the space step was different under different boundaries.Under two boundary conditions of configuration 2,the initial time step was stable within 2 × 10-5-10-2 d,the minimum time step should be 10-5-10-4 d.The space step should be 1-10 cm under constant head boundary,and it should be 1-5 cm under 0-0.5 d constant head boundary(h0=-800 cm).For configuration 3,the spatial step size under the constant head boundary was convergent and stable within 0.5-1 cm.Under 0-0.5 d constant head boundary,the minimum time step should be 10-5-10-3 d,and the space step should be 0.5-5 cm.Under the two boundary conditions of configuration 4,the minimum time step was 10-5-0.01 d,and the space step should be 0.5-5 cm.Under two boundary conditions of configuration 5,the minimum time step should be 10-5-10-3 d.The spatial step under different boundary conditions had different simulation stability.The spatial step was 0.5-10 cm under constant head boundary;the spatial step was 0.5-5 cm under 0-1 d constant head boundary.Additionally,when there is a relatively clay layer with very low hydraulic conductivity in layered soil,the boundary conditions also had an important influence on numerical simulation of the soil water flow.When the water supply capacity of the upper boundary was too large,the soil layer would be "oversaturated" and the numerical solution will be unstable.(2)The effect of time and space dispersion on numerical simulation of solute transport in saturated layered soil saturation was obviously different.Comparing the results of solute transport simulation under configuration 1 under unsaturated and saturated conditions,it was found that the numerical simulation under unsaturated conditions has certain requirements for the time step.Only when the minimum time step is within 10-5-10-4 d can the numerical solution be calculated.But the time step has little or no effect on the stability of the numerical simulation in the solute transport simulation under saturated conditions.However,the spatial step size had a greater impact.Among them,the spatial steps of configuration one,two and three under constant concentration input and pulse input should be 0.5-2 cm,and the spatial steps of configuration four and configuration five should be 0.5-5 cm.Furthermore,comparing the results of solute transport simulation under configuration 1 under unsaturated and saturated conditions,it was found that the initial time step under unsaturated conditions had no effect on the solute simulation,and the minimum time step should be 10-5-10-4 d.It had the same convergence range as the simulation of water flow migration alone.Additionally,in the simulation of solute transport,the dispersion value should be properly estimated and selected according to the soil scale.(3)Due to the rapid change of hydraulic conductivity at the layered interface,it also had a certain influence on the simulation of water flow and solute transport.Especially when there was more viscous soil,water flow and solute would stay at the clay-soil interface,making the pressure head and solute concentration of the nearby soil higher than other observation points.Additionally,for the problem of numerical solution instability generated at the layered interface and the infiltration interface due to improper value of spatial step,it can be improved and solved by encrypting nodes at the interface and performing non-uniform dispersion.(4)In the ponding infiltration experiment of layered soil,the wet front changed from non-linear propulsion to linear propulsion after reaching the soil layer interface,and the infiltration rate gradually decreased.The effect mechanism of ponding depth,soil configuration and initial water content on water migration is different.Ponding depth mainly affects the water migration process by affecting the pressure potential at the infiltration interface,which has a direct effect on infiltration flux.The effect of soil configuration on water transport is mainly due to differences in pores and clay content in different texture soils in the profile,resulting in different permeability and water holding capacity,and it causes hydraulic blockage at the abrupt soil interface.The effect of initial water content on infiltration was through two aspects.The effect of water potential gradient directly related to the soil water content and the disintegration effect of the soil aggregates indirectly related to the water.Therefore,these three factors can be appropriately adjusted to reduce the amount of soil leakage to optimize field water management.Additionally,using the Hydrus-1D software to invert and optimize the parameters was more suitable for the simulation and prediction of indoor layered soil water infiltration characteristics than the measured parameters of RETC fitting.(5)The indoor ponding infiltration and outflow experiments were used to compare the solute transport characteristics under different soil configurations under saturated and unsaturated conditions.Under saturated conditions,using the outflow Brconcentration under the two configuration experiments,the saturated hydraulic conductivity and dispersion inverted by Hydrus-1D were different.And due to the difference in saturated hydraulic conductivity and dispersion,the infiltration rate and Brpenetration curve were different obviously.Under unsaturated conditions,the two configurations are significantly different due to the water conductivity and infiltration rate,which in turn leads to differences in the change trend of the conductivity value of each layer of soil and the solute transport and distribution.