Soil Water Infiltration Characteristics And Hydraulic Parameters Of Gravel-sand Mulched Fields With Different Planting Years

Gravel-sand mulched fields is a unique dry-farming and water-saving agricultural model formed by farmers in the arid and semi-arid regions of northwest China to cope with the natural conditions of scarce precipitation and water scarcity.However,with the increase of planting years,the structure of soil and gravel media and the ratio of gravel in the gravel-sand mulched fields have been changed.The gravel-sand mulched fields generally showed a decrease in moisture content,the deterioration of soil tillage,and the gradual degradation of soil infiltration and moisture retention function,and seriously restricts the sustainable development of the gravel-sand mulched fields.In recent years,the ecological restoration,classification withdrawal,and restoration and reconstruction projects have been gradually carried out in the concentrated distribution areas of gravel-sand mulched fieldss.Therefore,the study of the soil water infiltration process and the characteristics of hydraulic parameters could be crucial to understanding the soil water distribution,formulating sound irrigation systems,and estimating surface hydrological flow.In order to clarify the characteristics of soil water infiltration and hydraulics parameters of gravel-sand mulched fields with different planting years,a typical gravel-sand mulched fields concentration area in Zhongwei,Ningxia was selected,and the characteristics of soil water infiltration and hydraulics parameters of gravel-sand mulched fields with different planting years(2,5,10,20,30 a and 40 a)were investigated by field survey,field sampling,in situ test and indoor simulation test.The main conclusions were outline as follows.(1)The spatial coefficients of variation of gravels of different grain sizes in the gravel-sand mulched fields were all greater than 10%,which were of medium variability.The spatial heterogeneity ratios of gravels of different grain sizes varied widely(0.26%～82.48%),and their spatial heterogeneity was influenced by both random and structural factors.The spatial autocorrelation analysis showed that the global Moran’s I index of each grain size gravel was higher than 0.674 and the z-values were all bigger than 1.96,indicating that there were highly significant spatial dependence characteristics of different grain size gravels with aggregated distribution.The geodetector model identification showed that the explanatory power of the spatial variation of different grain size gravels in gravel-sand mulched fields was in the following order:planting year>slope aspect>surface roughness>slope>profile curvature>plane curvature>surface relief,and the single factor explanatory power of planting year was significantly higher than other factors.The spatial distribution of gravels of different grain sizes in the gravel-sand mulched fields soil under different environmental conditions varied significantly and showed an aggregated distribution.The planting year,slope aspect and surface roughness were the main factors influencing the spatial distribution of gravels of different grain sizes in the gravel-sand mulched fields.(2)According to the analysis of vertical one-dimensional soil column infiltration tests for different planting years,it could be seen that the initial infiltration rate,stable infiltration rate and average infiltration rate all tend to decrease as the planting years increased,and the wetting front and cumulative infiltration amount gradually decreased at the same time.Combining the R2 and its physical significance of four infiltration models,namely Philip model,Kostiakov model,Horton model and general empirical model,the Horton model was the best fit for the water infiltration process of gravel-sand mulched fields at different planting years.The MAE between the values of water content of soil profiles with different planting years and the measured values fitted by the one-dimensional algebraic model ranged from 0.48%to 2.09%,the RMSE ranged from 0.52%to 2.13%,and the D,were all greater than 0.782,indicated that the one-dimensional algebraic model could better describe the distribution of water content of soil profiles at different depths in gravel-sand mulched fields under different planting years.(3)According to the analysis of the horizontal one-dimensional soil column suction tests for different planting years of the gravel-sand mulched fields,it was found that the depth of the wetting front and the cumulative infiltration volume tended to decrease with increasing planting years at the same suction time,and the suction rate in the Philip model and the average diffusion rate in the Green-Ampt model both tended to decrease gradually.In the Brooks-Corey model,the water uptake and unsaturated diffusivity tended to decrease with the increase of planting years at the same soil water content.This indicated that the water infiltration capacity and water holding capacity of the gravel-sand mulched fields soil decreased significantly with the increase of planting years.(4)With the increase of planting years,the soil capacity increased significantly(P<0.05)and the saturated hydraulic conductivity decreased significantly(P<0.05),and the staining pattern of the profile of the gravel-sand mulched fields showed that the soil and stone mixing increased with the increase of planting years,and the boundary between the gravel cover layer and the lower soil layer was gradually blurred,and the overall water flow pattern of the profile staining changed from uniform piston flow to finger flow with the increase of soil depth,and the substrate flow area decreased and the priority flow area increased.The area of substrate flow decreased and the area of priority flow increased.As the planting period increased,the ratio of profile staining area,the proportion of matrix flow and the depth of matrix flow decreased,and the proportion of priority flow,the coefficient of variation of profile staining and the irregularity of wetting front traces tended to increase.The combination of staining characteristics and fractal dimension showed that the longer the planting period,the higher the inhomogeneity of the staining pattern of the gravel-sand mulched fields,the proportion of matrix flow area gradually decreased,and the proportion of priority flow area gradually increased and the development degree increased.