Evaporation Mechanism Of Soil With Different Sand Content And Numerical Simulation Of Water-salt Transport

Evaporation is a major water balance component of the hydrological cycle in arid and semi-arid regions.Water shortage and soil salinization caused by evaporation pose a major threat to the ecological environment of the irrigation area,which greatly limits agricultural production in arid and semi-arid areas.But the soil in the natural world has undergone complex process evolution,the soil sand content varies greatly,and the soil sand content is different,its physical and chemical properties and pore structure are also different,which makes the evaporation process of soils with different sand content very different.Therefore,accurately determining energy and mass transfer in the process of evaporation is the basis of clarifying the law of water-salt movement of different soils.Studying the evaporation characteristics and laws with different sand content,exploring its ability to transport and retain soil moisture and salinity can provide technological support for the control of evaporation under different geological conditions.It is of great significance to prevention of soil salinization and protection of water resources in arid and semi-arid areas.In this paper,the loess in Nanyuan,Jingyang County,Shaanxi Province is selected as the experimental soil.We mixed sand with different contents to simulate the soil with different textures widely existing in nature.Using the laboratory experiments combined with theoretical analysis under the condition of fixed temperature(30 ?)and humidity(40%),systematically study the physical and pore characteristics and evaporation laws and water-salt movement processes of soils with different sand content.Then the Ku-p F unsaturated hydraulic conductivity measurement system and soil column test are used to measure the soil water-salt movement parameters,and the Hydrus numerical simulation software is used to establish the wate-salt transport model.From experimental analysis to mechanism research then to numerical simulation,quantitatively study the evaporation characteristics,water-salt movement process and its influencing mechanism of soils with different sand content.The following main conclusions are obtained:(1)Experimental results show that based on the stable evaporation(the amount of evaporation does not change with the duration of evaporation),the day evaporation of soil with different sand content decreases with the increase of sand content.When the sand content >20%,with the extension of evaporation duration,the continuous evaporation process makes the water morphology and energy change,which makes the soil daily evaporation gradually decreases.When the sand content at about 0%?20%,the evaporation is remain stable under the synthetic effects of soil capillary,water morphology and effective matrix suction.(2)The greater the soil sand content,the larger the inhibitory effect(the difference between the average evaporation of 0% soil column and each sand column)on evaporation When the sand content of soil increases from 0% to 100%,the inhibitory effects of sand on evaporation is 0.7(20% sand content),1.39(40% sand content),2.55(60% sand content),2.94(80% sand content)and 3.72(100% sand content),respectively.(3)The water and salt contents on the surface of each soil column have a positive correlation with the soil evaporation capacity.That is,the stronger the soil evaporation capacity,the higher the soil surface moisture content and the larger the degree of surface salt accumulation.The moisture content of the profile increase with the depth of soil column,and the change of it becomes more intense as the sand content increase.Except for the 100%sand column,each column has strong salt accumulation on the surface and desalination at bottom,and the accumulation degree increases with the decrease of sand content.According to the distribution characteristics of moisture content and the position of salt accumulation on each soil column,it can be concluded that the capillary supply rate and rising height of six texture soils are 0%>20%>40%>60%>80%>100%.(4)When the soil sand content increases,the macropore structure in the soil increases,which can affect the water characteristic curves,permeability coefficient and hydrodynamic dispersion coefficient of each texture soils as a whole.With the increase of soil sand content,the curve of soil water characteristic shifts downward,and the coefficient of permeability and dispersion increase with the addition of sand.(5)The hydraulic parameters determined by the measured data rate can be better applied to the Hydrus water and salt migration model.The water movement models and salt transport models of soils with different sand content are in good agreement with the measured results.According to the results of RMSE between the measured and calculated values,the RMSE of the water movement models are below 0.5,while the salt models are below 2.Moreover,when the sand content between 20% and 80%,RMSE values are generally small.The water movement simulation results are better than the salt movement simulation results.