Salt Accumulation In Saline Soil During Evaporation And Its Effect On Soil Hydraulic Parameters

The area of salinized soil in the arid and semi-arid regions of Northwest China accounts for 69%of the total salinized soil in China.Because of the low rainfall,strong evaporation,and common use of brackish water for irrigation,soil salinization is becoming severe in many arid regions.Fully understanding the mechanism of salt accumulation and its influence on the soil hydraulic parameters under evaporation is of great significance for saline soil reclamation and making full use of the productive potential of soil resources.Based on reviewing the previous related research,experimental data of four field lysimeters was collected and a set of indoor experiments were conducted to explore the salt accumulation mechanism in homogenous and heterogeneous soil and its feedback on soil hydraulic properties.The main conclusions are as follows:(1)HP1 model coupled HYDRUS-1D with the PHREEQC code can simulate the processes of dissolution of resident salts and the precipitation due to oversaturated,and significantly improve the prediction accuracy of salt distribution in the soil profile.The prediction of salt distribution in homogenous lysimeters in this study shows that ignoring the dissolution of resident salts in the soil,the salt content in a deep layer will be overestimated,while the salt content in the shallow soil will be underestimated.Besides,if the salt precipitation due to supersaturation is not considered,the predicted solution concentration will increase indefinitely to against nature.(2)Salt not only accumulated at the evaporating surface,but also at the top of the interlayer with lower permeability.The water storage in the shallow sand was reduced sharply by the presence of inter-layering silt loam in the indoor experiment.The resulting dry surface reduced the evaporation rate by about 70%in the layered columns compared to the homogenous sand column.That also led to the salt precipitated as a quasi-crust subsurface at the receded evaporating surface in the layered columns.Due to the decreased capillary flow rate in the vadose zone that can not meet the potential evaporation,the evaporative loss via vapor from the drying font elevated by the silt loam increased.It resulted that the salt also accumulated at the top of the silt loam layer.(3)Salt precipitation can not only decrease the actual hydraulic conductivity(K_s),but also increase the K_s.For the crust composed of salt and soil particles at the surface in the lysimeter experiment,the continuously increasing salt precipitation during the evaporation process can expand the pores in the salt-soil crust and loosen its structure.That resulted in an increase of K_s.As for the subflorescent salt in the shallow dry sand in the indoor heterogenous columns,when the amount of vapor blocked by the salt precipitation increased to a certain threshold,it would lead to a structural damage of the dry sand.It also can lead to an increase in the K_s.(4)The empirical parameters?and n,included in the models proposed by van Genuchten and Durner for the soil-water characteristic curve(SWCC,water content is on the y axis,pressure head is on the x axis),were negatively correlated with the solute concentration.In the early two stages of the desaturation process,higher solute concentration,which had a higher osmotic potential,led to a decrease in?related to the air entry value and led the SWCC to offset to the right.In the last stage,due to the water loss and the salt precipitation at the same time,the solution mass in porous media decreased sharply.The solution volume in the sand with a higher concentration of Na Cl decreased faster and led to a smaller value of n which is related to the slope at the inflection point on the SWCC.The studies in this thesis can provide a theoretical basis for improving the estimation accuracy of salt distribution in saline soil during evaporation,and guiding the reclamation of saline soil and irrigation schedules in arid and semi-arid areas.It is of great significance for the sustainable utilization of soil and water resources.