The Influence Of Crack Aperture Deformation On Soil Infiltration Process: A Water And Salt Migration Model

Soil shrinkage and cracking is a common natural phenomenon in arid and semi-arid regions.The appearance and development of cracks can damage soil structure,leading to unstable soil structure,uneven water infiltration,decreased fertility,and reduced crop yield.These problems seriously restrict the sustainable development of agriculture.Therefore,in-depth research on the spatial and temporal dynamics of preferential flow and salt in cracked soil is crucial to understanding the impact of soil shrinkage and cracking on soil structure,water cycle,and crop growth.Such research provides a scientific basis for formulating soil improvement and management strategies,which can promote the sustainable development of agriculture.This article focuses on the impact of salinization and climate change on agricultural production in arid and semi-arid areas.The study examines salt-cracked soil and uses four types of soils with different salt concentrations(0,5,30,and 100 g/L)and two types of crack geometries(crack depth:5,10,15 cm;crack burial depth:0,5,10 cm).Through experiments on soil hydraulic parameters,crack soil infiltration,and crack development monitoring during dry-wet cycles,the study analyzes the impact of salinity on hydraulic properties and water and salt migration behavior in the presence of cracks.A water movement model considering crack variation characteristics is proposed based on the water-salt-crack interaction mechanism.(1)The soil water holding capacity is significantly reduced by the introduction of Na~+,with a corresponding decrease in capacity as Na~+concentration increases.The van Genuchten model effectively describes the impact of Na~+on soil moisture characteristic curves.Na~+has a notable influence on soil saturated and residual water content,which both decrease with increasing Na~+concentration.However,its impact on the air entry value is negligible.Salt content shrinkage and suction exhibit a logarithmic relationship,and Na~+introduction reduces shrinkage strain,thereby effectively minimizing soil shrinkage.The three-line model is well-suited for characterizing the shrinkage properties of saline soil,although its effect on the characteristic water content of the air entry point and expansion limit point is insignificant.(2)Infiltration characteristics,wetting body shape,and spatial-temporal salt distribution are strongly influenced by the geometric morphology of cracks.The depth of cracks is positively correlated with both soil wetting effect and cumulative infiltration.Moreover,the buried depth of cracks has a time lag effect on infiltration and wetting body shape,meaning that changes in infiltration characteristics only occur after water has penetrated to the cracks.Soil salt content,on the other hand,has a negative effect on cumulative infiltration and soil wetting.The Kostiakov infiltration model poorly fits the relationship between cumulative infiltration-infiltration duration and infiltration rate-infiltration duration in all treatments,as cracked soil exhibits a higher initial infiltration rate compared to uncracked soil,leading to rapid and complete soil wetting in a short period of time.(3)Soil moisture absorption and desorption directions lead to different rules for crack opening and closing.Dry-wet cycle experiments reveal that soil cracking process can be divided into constant-rate,decelerating-rate,and residual evaporation stages,while soil salinity inhibits water loss.Crack geometric morphology is influenced by soil salinity,water processes,and history,and crack line/area density increases with increasing soil salinity.During desorption,crack opening rate is slow,while during absorption,crack closure is rapid.As dry-wet cycles increase,soil fragmentation intensifies,and crack area and line density increase,while crack aperture width opens and closes rapidly.The logistic and Boltzmann models fit well to the relationship between crack area density,line density,and average aperture with moisture content during the absorption and desorption process.(4)A coupled model of crack geometric morphology and soil water-salt migration was established,discretizing the cracked soil into a matrix domain and a zero-thickness plane crack domain using different water-salt movement laws.The correlation between the crack aperture and soil water content was established,and the processes of crack aperture variation,water movement,and salt migration were coupled.Results revealed that the water movement model that neglects crack deformation overestimates soil permeability,and crack closure weakens the preferential flow effect caused by cracks.The spatial non-uniformity of crack closure leads to uneven water distribution,with crack aperture being the most significant factor influencing soil infiltration among crack number,area density,and average aperture.