| Layered soil configurations are widespread in nature due to the influences of meteorological,geological and biological processes.Water quality characteristics of rainfall changes in different regions.Therefore,the layered soil configurations and the water quality characteristics of rainfall play an important role in rainfall dominated soil erosion processes.In addition,the existence of layered soil configurations make soil water flow different between homogeneous and la yered soils,and soil physical and chemical properties change with different qualities of rainwater.The influences of layered soil structure and various types of water qualities on soil infiltration have been confirmed,but little research has been conducted on soil erosion processes.Moreover,studies on soil infiltration process of different irrigated and rainwater qualities in the layered soil configurations have not been reported.Therefore,the present study used Lou soil,loessal soil and sandy soil t hat are widely distributed on the Loess Plateau as study materials to investigate(1)the influences of different layered soil configurations on soil infiltration process and slope soil erosion,(2)the process of rainwater infiltration,runoff characteristics and sediment yield under different rainwater qualities and(3)the indoor layered soil configurations and soil moisture movement in the infiltration process of various water qualities and erosion process simulation of layered soil through simulated rainfall.Results of the present study show that(1)The layered soil configuration concerning a combination of "upper sand soil and lower clay soil" could reduce the degree of rainfall erosion.Through artificial rainfall simulation experiment,rainfall infiltration and runoff and sediment yield characteristics under the conditions of 3 soil types and different layered structure combination thereof were studied: with increased rainfall intensity,the runoff yield time of different layered soil was shortened,but the runoff coefficient increased,and the soil erosion intensity increased;under the same rainfall intensity,a layered soil configuration concerning "upper sand soil and lower clay soil" was stable and had increased infiltration rate and relatively strong anti-erosion ability,but a layered soil configuration concerning "upper sand soil and lower clay soil" had large sediment yield and strong soil erosion intensity in the process of rainfall erosion;the soil infiltration rate was co-determined by total soil-water potential and soil hydraulic conductivity of the layered soil,the infiltration rate of three-layer soil combination was ranked as: sandy soil-loessal soil-Lou soil >sandy soil-Lou soil-loessal soil >Lou soil-sandy soil-loessal soil> Lou soil-loessal soil-sandy soil.The infiltration rate of two-layer soil combination was ranked as: sandy soil-Lou soil > Lou soil-sandy soil.The soil-water potential played a leading role in the infiltration of soil moisture,therefore,in the rainfall erosion process,pores were connected to the atmosphere during unsaturated soil water movement,the soil infiltration rate was determined bysoil water pressure headof the layered soil configurations,so that water contents of the layered soil configurations were higher than those of homogeneous soil.The total amount of runoff and sediment yield in the layered soil configurations was ranked as: three-layer combination < two-layer combination< single-layer soil,indicating that multi-layer combinations may be associated with increased anti-erosion ability;a infiltration model formula of the layered soil under the condition of rainfall can be obtained according to Smith rainfall infiltration modeland Kostiakov infiltration model,and simulation verification was carried out by experiments.(2)Different water quality of rainfall resulted in different erosion process and erosion degree.The soil infiltration and runoff and sediment yield characteristics under the simulated rainfall conditions in different aqueous solution(deionized water,natural rainfall,natural accumulated water,tap water and brackish water)were evaluated through simulated rainfall experiments:the soil erosion intensity initially decreased and then increased with the increase of sodium adsorption ratio(SAR)and electrical conductivity;under the action of natural accumulated water,the erosion degree was the lowest,so it is the critical value of the SAR and the electrical conductivity of the rainfall,higher or lower than this value,the erosion degree was intensified;the infiltration rate and cumulative infiltration of the Lou soil were ranked as(from high to low): natural accumulated water > brackish water > tap water >natural rainfall >deionized water,the cumulative infiltration was the highest regarding the natural accumulated water,and the lowest regarding deionized water.The influence characteristics of the water quality on the rainfall erosion were also affected by soil texture and configuration,water quality influenced rainfall erosion was more significant in the Lou soil with higher clay content compared with the loessal soil with lower clay content,and the rainfall erosion degree of a layered soil configuration concerning Lou soil-sandy soil was consistent with erosion rules of homogeneous Lou soil,but the erosion degree was stronger in comparison with the homogeneous Lou soil;and Philip model applied to the simulation of rainfall water quality infiltration had more accurate calculation results for the water quality with relatively low SAR,but the calculation results had a certain deviation for the water solution with relatively high SAR.In other words,SAR of water quality had a certain threshold range,and the range of this experiment was that when the SAR of water quality was equal to the SAR of natural accumulated water,the rainfall erosion was the lowest.(3)Theponded water infiltration process of different water quality was controlled by the soil configurations.Three soil configurations(homogeneous loess soil,Lou soil interbedded with sandy soil and sandy soil interbedded with Lou soil)were treated with four kinds of water with different water quality(deionized water,natural rainfall,natural accumulated water and tap water)to study the processes ofponded water filtration in the homogenous soil and the layered soil:Philip infiltration model simulated infiltration process of different water quality in the homogeneous Lou soil had relatively small root-mean square error(RMSE)and high fitting precision;the infiltration time of four kinds of water solution in the three soil configurations was ranked as: deionized water > natural rainfall > tap water > natural accumulated water;and homogeneous Lou soil >Lou soil interbedded with sandy soil>sandy soil interbedded with Lou soil.Advancing distance of wetting front within the same time: homogeneous Lou soil < Lou soil interbedded with sandy soil < sandy soil interbedded with Lou soil;the influence degree of water so lution containing different SAR and electrical conductivity on the advancing distance of the wetting front of the layered soil was determined by soil layered combination patterns.Layered soil structure exists very often,erosion and infiltrationcharacteristics of the layered soil configurations and ponded water infiltration of the Lou soil interbedded with sandy soil and the sandy soil interbedded with Lou soil are different than uniform soils under artificial rainfall simulation and one-dimensional infiltration,the erosion characteristics of the layered soil configurations,the infiltration rates of layered soil with different combination of texture layers and the influence degree of the rainwater quality on soil erosion were obtained,which can reveal some infiltration mechanism of the layered soil,provide reference for improving the utilization efficiency of water resources and prevent soil erosion related hazards in arid and semi-arid regions.The findings have practical implications foragriculture practices. |
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