Regularity Of Water-Air Two-Phase Flow Displacement In Unsaturated Zone Under Infiltration Condition

The unsaturated zone is the key link of water and energy cycle between saturated zone and atmosphere.Besides the solid skeleton,the pores in the unsaturated zone are completely filled with water and gas,it is a typical water-air two-phase flow system..Many studies have shown that the gas phase has a significant influence on the water movement and material exchange in unsaturated zone,the process of rainfall infiltration is the main factor which causing the water-air two-phase movement in unsaturated zone,the infiltrating water will compress the gas in unsaturated zone,then the compressed gas will resist the water infiltration.Therefore,it is very important to study the regularity of water-air two-phase flow displacement in unsaturated zone under rainfall conditions in order to make out the movement process of water resources in the atmosphere-unsaturated zone-saturated zone system.However,due to the complexity of this theory and the limitation of experimental conditions,it is still necessary to study the regularity of water-air two-phase flow displacement in unsaturated zone.Based on the theory of two-phase flow in unsaturated zone,this paper designed one-dimensional experiments under indoor and in-situ conditions,we used foreign advanced sensors to observed data.Then analyzing the measured data to explore the influence of lithology and groundwater level on the movement of two-phase flow and it's correlation between different meteorological factors and gas pressure.In addition,simulating the coupled model of water-air two-phase flow in unsaturated zone under rainfall condition and explored the reasons for the disadvantage of simulation software.The following results are obtained:(1)Keeping a constant underground water table,profile moisture contents increases with the increase of rainfall intensity.The difference of the upper unsaturated zone water content is obvious,relatively,the difference of capillary zone is small.When the rainfall intensity is constant and the water level is deeper,the velocity of infiltration became faster.No matter how deep the groundwater table is,the maximum gas pressure always appear in the bottom of unsaturated zone.And the shallower the groundwater level is,the greater the compressibility of the gas is,the bigger pressure value is.(2)Analyzing the cumulative rainfall infiltration under different conditions,the infiltration process has the following characteristics:in the short period after rain,the infiltration recharge increased rapidly,then the increased value became smaller until zero.There is a multiple relationship between the infiltration total time of different groundwater tables.Regard 0.5m as the basis,the infiltration total time under 0.7m depth is 2.2 times of that,and the infiltration total time is about 3.2 times of that under the condition of 1.0m depth.(3)Wind speed is positively correlated with gas pressure,and atmospheric pressure fluctuation and relative humidity are negatively correlated with gas pressure.The correlation between the gas pressure and the meteorological factors is constant with the change of lithology.The atmospheric boundary should be taken as the upper boundary of the water-air two-phase flow in unsaturated zone.(4)The change of gas pressure is not only related to meteorological factors,but also related to the compressibility of gas;According to the variation of the gas pressure in unsaturated zone under different soil types,the unsaturated zone is divided into wind speed influenced zone,transition zone and compression influenced zone,in the range of 0~50cm from the surface is wind speed influenced zone,the sorting among the three kinds of lithologic's gas pressure is fine sand > middle sand > coarse sand,this zone is mainly affected by wind speed;In the range of 50cm~100cm is transition zone,the sorting among the three kinds of lithologic's gas pressure is not obvious.Within the range of 100 cm below the surface is compression influenced zone,coarse sand has the most gas pressure,medium sand and fine sand is relatively smaller,this zone is mainly affected by compression effect.(5)Comparing the influence of lithology on profile water content under rainfall condition,we can found that: the response time of water content to rainfall is longer if the thickness of the unsaturated zone is bigger.There are many factors influenced the water content and soil surface pressure;Lithologic factors not only affect the amount of water content in unsaturated zones,but also affect the time of moisture content response to rainfall from the 100 cm under the surface.The water in the coarse sand first reaches the bottom of the unsaturated zone.(6)Analyzing the influence of different lithology on rainfall infiltration,we can foundthat:The infiltration water will recharge to groundwater after retenting on unsaturated zone for a period of time.(7)By comparing the numerical simulation results and the measured data,we can found that:The simulation results of the gas phase pressure and water content can well verify the measured data in trend.But TOUGH2 overestimates the gas phase pressure,and the shallower the groundwater level is,the bigger difference will exist in the simulated and measured values.The simulation of water content is accurate by TOUGH2,the simulation error is small in the deepper range of but larger on surface.