Experimental Study On The Effect Of Soil Macropore Characteristics On The Transport Of Water And Solute Preferential Flow

Preferential flow is an important mechanism affecting the non-equilibrium water and solute transport in soil,and it is also a key way for soil and groundwater recharge,runoff and pollutant transport.Macropore flow is the most common type of preferential flow.The characteristics of macropore structure are the most direct factors that affect the preferential flow.Therefore,based on the selection of representative structural characteristics,this paper studies the effects of macropore diameter,number and location on soil water and solute transport in saturated clay soil by indoor artificial macropore soil column simulation experiment and numerical simulation experiment,Finally,the double permeability model in Hydrus is used to study the influence of the parameter change caused by the change of macropore structure on solute transport.The main contents and conclusions are as follows（1）The influence of diameter on water and solute transport in single through macroporous soil column is analyzed quantitatively and qualitatively.It is found that there is a positive correlation between the macropore diameter and the saturated hydraulic conductivity,and the growth rate of the saturated hydraulic conductivity of the macropore soil column less than 1 mm is greater than that of the macropore soil column more than 1mm;The contribution rate of macropore domain to water flux is between 10%and 30%,and the low proportion is mainly related to the non compaction uniformity of matrix and macropore plugging;The results show that the penetration curves of single pore soil column show double peaks in different degrees,and the significant difference of penetration characteristics occurs between 4mm and 7mm macropores,indicating that the key diameter affecting the preferential flow is between 4-7mm;The contribution rate of macropore domain to solute flux is about 25%,which is related to the non-equilibrium flow generated by matrix domain;There is a negative linear correlation between 5%arrival time of solute and macropore diameter（R²=0.937）.The key diameter of solute preferential flow migration is between 4-7mm.（2）The effects of quantity and combination mode on water and solute transport in the same diameter through macroporous soil column are analyzed quantitatively and qualitatively.It is found that there is a linear correlation between the number of macropores and the saturated hydraulic conductivity（R²=0.999）,and the increase of the number of macropores significantly affects the saturated hydraulic conductivity;The breakthrough curves of 4 and 16 1 mm macroporous soil columns show similar characteristics of early penetration,bimodal and tailing,and the preferential flow rate obtained by normalized 5%arrival time is close,which may be related to the interaction between macropores leading to the transverse dispersion of solute in porous media;The contribution rate of the macropore domain of the porous soil column to the total solute flux is also about 25%,and the rest mainly flows out in the form of matrix through transverse dispersion around the macropores;Compared with different diameter and number of soil columns with the same porosity,it is found that the preferential flow degree of small pore is much larger than that of single large pore,which indicates that the interaction of large pore has a great influence on the preferential flow.（3）The migration of water and solute in macropores from complete separation type to complete accumulation type was analyzed qualitatively.It is found that the saturated hydraulic conductivity and preferential flow rate from high to low are:aggregation>separation>complete aggregation>complete separation,which indicates that the influence of macropore location on preferential flow may be related to different degrees of lateral dispersion of solute,and the key factor of location lies in the influence radius of single macropore and the threshold value of pore density.（4）The problems in the experiment are summarized.The experimental problems are that the position of soil side wall is not compacted evenly,which leads to excessive water flow from the side wall of soil column,resulting in low solution concentration;The advection of water and solute occurred in the bottom sand layer,resulting in the failure to collect the solution at the corresponding position;In the strict sense,it should be the preferential channel of high and deep permeability pores with different permeability coefficients,which is not consistent with the actual significance.The assumption that the preferential channel is the void is slightly different,and needs to be improved.（5）The influence of macropore structure on the model parameters and simulation results was studied by using the double permeability model in Hydrus.It is found that the change of macropore related parameters mainly affects the peak value and position of the bimodal curve,and the sensitivity of the first peak value to the parameters is from large to small:K_sf>n_f>（?）_f>others;The sensitivity of the second peak to the parameters is from large to small:K_sf>n_f>（?）_f>others;The sensitivity of the second peak position to the parameters is from large to small:K_sf>（?）_f>a>n_f>others.The sensitivity of normalized 5%arrival time to parameters was from large to small:K_sf>n_f>a>（?）_f>Disp_f>others.Combined with the analysis of experimental data,the change trend of model results caused by the change of macropore structure is close to the measured data.When using Hydrus software to simulate the dominant water fraction and solute transport,the parameter setting should be based on the actual characteristics of macropores,pay attention to the corresponding key parameters,and make reasonable adjustment and setting.