Soil Water Infiltration And Evaporation Models Considering Solute Potential And Salt Crystallization Processes

Soil salinization is one of the five major soil problems plaguing the world today.Preventing salinization depends on in-depth research into the laws of water and salt movement.However,current mathematical models of water and salt movement do not fully consider the influence of solute potential changes and the crystallization of salts on the surface during the water and salt evaporation process.Focusing these scientific problems,this paper conducts research on the soil moisture characteristic curves of loam and sandy loam,as well as Na Cl/KCl and Na₂SO₄,analyzing the effects of soil salinity and moisture content on soil solute potential.A soil infiltration process water-solute dynamics model accounting for solute potential changes caused by salt movement,and a saline soil evaporation water-vapor-salt-heat coupling dynamics model that considers evaporation and salt crystallization processes,were constructed.The paper proposes solution methods for these models and validates them,revealing the influence patterns of soil solute potential and salt crystallization on soil infiltration and evaporation processes.The main research results are as follows:（1）Two types of soil textures,Loam and Loamy Sand,were selected to construct a water characteristic curve and solute potential estimation model for saline soils.Experiments with various concentrations of Na Cl/KCl and Na₂SO₄ in soil were performed for validation.A universally applicable method for estimating soil solute potential was developed,unveiling the response patterns of soil solute suction,matrix suction,and total suction with changes in soil water content and solution salinity.Experiments with Loam and Loamy Sand demonstrated that as salinity increases,the total suction of the soil significantly increases;the soil solute potential is primarily determined by the salt content of the soil solution.When the water content of the soil sample decreases and the solution in the pores approaches saturation,the solute potential reaches different saturated suction values for different salt solutions.For Na Cl and KCl solutions,their solubility remains relatively stable with temperature changes,so the soil suction and solute suction do not significantly vary with temperature.However,for Na₂SO₄ solutions,temperature has a substantial impact on salt solubility,and solute potential and total suction vary noticeably with temperature.The solute suction of saline soil can be estimated by considering the salt content in the soil solution as if it were in pure water,using a theoretical model.This provides a theoretical foundation for studying soil water and salt movement models that consider solute potential.（2）By combining physical-chemical semi-permeable membrane theory,a method for determining the effective coefficient of solute permeation in liquid water movement is established,and a soil infiltration model considering solute potential for mildly saline water is constructed.Through various soil experiments and model simulations,the mechanisms of reduced infiltration in mildly saline soil and its impact on soil water and salt distribution are clarified,and a simplified estimation model for infiltration parameters in mildly saline water considering the specific surface area of soil particles is constructed.The results show:1)Infiltration experiments and simulations with different salinity levels（Na Cl）in Loam,Loamy Sand,and mixed soil samples（1:1 mass ratio of Loam and Loamy Sand）indicate that the constructed model,despite some differences from the measured values,has a small discrepancy and simulates well,accurately reflecting the characteristic of decreasing soil infiltration rate with increasing salinity of mildly saline water.2)Soil solute potential has a significant impact on infiltration rate.Under a 60 mm irrigation condition,compared to clear water infiltration,the average infiltration time for typical soils（Sandy Loam,Silt Loam,Loam and Clay Loam）with mildly saline water salinity levels ranging from 1%～5%increased by1.05～1.64,1.14～2.48,1.14～2.71,and 1.44～8.45 times respectively under three initial moisture content conditions(50%θ_fc,60%θ_fc,and 70%θ_fc).Affected by solute potential,the water content distribution along the depth direction of the soil layer during infiltration shows some differences from the case where solute potential is not considered,mainly manifested in the bending of the infiltration front.Also,when considering and not considering solute potential,the distribution of salinity along the depth direction of the soil layer varies weakly,except for Clay Loam.3)The established mild saline infiltration parameter correction model considering the specific surface area of soil particles well reflects the influence of salt content of mildly saline water on infiltration rate.（3）By changing boundary conditions,a clear water infiltration dynamics model for saline soil considering solute potential is constructed,and through various soil experiments and model simulations,the mechanisms of soil water and salt distribution in saline soil clear water infiltration are clarified.The results show:1)The constructed infiltration model,although with some errors compared to experimental results,overall can reflect the changes in soil water content and salt content distribution during the infiltration process in saline soil.2)Series of simulation results with four typical soils and various salinity levels show that soil solute potential has little effect on the infiltration rate of saline soil,but as soil salinity increases,the infiltration depth increases slightly,and there are certain bends in soil water content distribution and matrix potential at the infiltration front.3)During the infiltration process,a soil salinity accumulation zone appears,and the peak value of soil salinity is positively correlated with the initial soil salinity.Considering solute potential,as the infiltration time increases,the depth and thickness of the soil salinity accumulation layer increase,while the peak value remains approximately unchanged.Therefore,existing models that do not consider solute potential may underestimate the depth and thickness of the soil salinity accumulation zone to some extent,especially in Clay soils.（4）Based on the equations of soil water,heat and salt movement,a soil water-vapor-heat-salt movement model is constructed,coupling the spatiotemporal distribution of liquid water flux,water vapor flux,heat flux,and solute transport flux in the soil,and considering the crystallization mechanism during soil evaporation.A solution method for the comprehensive equation set model is proposed.The results show:1)The simulated soil evaporation and soil water and salt distribution,although with minor errors compared to the measured values,overall have a small difference and can accurately reflect the characteristics of decreasing evaporation with increasing soil salinity,with little change in soil temperature.2)Series of simulation results with three models（C0 model that does not consider solute potential and crystallization process,C1 model that considers solute potential but not the crystallization process,and C2 model that considers both solute potential and crystallization process）in four typical soils and various evaporation conditions show that solute potential has little impact on soil temperature changes but has a significant impact on soil evaporation and surface salt accumulation,especially in high-salinity soils.Under the C1 model,the seven-day cumulative evaporation of four typical soils with initial soil salinity levels of 5,10,and 30 g/kg are 0.910,0.842,and 0.668 times the simulated values of the C0 model,respectively,while the ratio of simulated values between the C1 and C2 models is nearly consistent.3)Considering the soil crystallization process has little obvious impact on the evaporation process,soil evaporation,soil temperature,and salt content distribution,and can be ignored.Therefore,models that do not consider solute potential may overestimate soil evaporation to some extent.Soil water and salt movement is extremely complex and involves factors such as soil type,climate,and salinity.This paper mainly focuses on the impact of solute potential changes caused by salt variation on soil water,vapor,salt,and heat during soil infiltration and evaporation processes,while also considering the crystallization mechanism of salts during the evaporation process.This reveals the patterns of influence of soil solute potential and salt crystallization on soil infiltration and evaporation processes,providing a theoretical foundation and scientific basis for the improvement and utilization of saline-alkali soils and mildly saline water irrigation.However,due to time and experimental condition limitations,the types of soil and salts tested are limited,and the effects of crop planting and fertilizers on salt migration are not considered.The reliability and universality of the preliminary constructed model simulation results need further verification.Moreover,this study did not consider the long-term changes in the chemical properties and structure of soil particle surfaces due to the presence of salts in mildly saline water irrigation and saline-alkali soil management,which require further in-depth research in the future.