Indoor Research On Transformation Between Shallow Phreatic Water And Soil Water Under Freeze-Thaw Action

There is a shortage of water resources in the arid and semi-arid areas of northern China.The surface evaporation in these areas is intense and the soil undergoes seasonal freezing and thawing process in winter and spring.In the shallow groundwater areas,the transformation between phreatic water and soil water are intensely during the seasonal freeze-thaw period,which not only affects the change of groundwater table depth and the accurate evaluation of water resources,but also aggravates the soil salinization.Therefore,the research on the transformation law between shallow phreatic water and soil water under the action of freeze-thaw has certain theoretical significance for scientific evaluation of groundwater resources and prevention of soil salinization.The transformation experiment between phreatic water and soil water with different groundwater table depths under man-controlled freezing temperature was conducted by indoor freeze-thaw simulation test device in this research.Based on the indoor experiment data,the variation characteristics of soil profile temperature,phreatic water evaporation and phreatic water recharge of five kinds of homogeneous soil(0.1?0.5 mm,0.5?1.0 mm,1.0?1.5 mm,1.5?2.0 mm,2.0?2.5 mm)under three kinds of groundwater table depths(0.5 m,0.8 m,1.0 m)were studied,and the relationship between phreatic water evaporation and soil particle size and negative accumulated freezing temperature during soil freezing process were analyzed.The reserach results are shown as follows:1.The soil profile temperature was affected by freezing temperature,groundwater table depth and soil particle size during the process of soil freezing and thawing.The soil temperature within 20 cm of the soil surface was obviously affected by the freezing temperature,and the temperature changed intensely.With the increase of the soil depth,the temperature gradient and the decreasing range of the soil profile decreased.The decrease range of soil profile temperature increased with the increase of groundwater table depth,and the soilprofile temperature decreased with the increase of groundwater table depth at the soil freezing stability stage.The larger the soil particle size was,the faster the soil temperature dropped,and the lower the stable temperature was.At the thawing stage,the initial thawing time was delayed with the decrease of groundwater table depth and the soil particle size,and the increasing range of soil profile temperature increased with the increase of groundwater table depth and the soil particle size.The deeper the groundwater table was and the larger the soil particle size was,the earlier the soil profile temperature was stable.2.The influence of particle size on the evaporation of phreatic water was different under different groundwater depth.The accumulated phreatic water evaporation increased with the increase of soil particle size during the freezing period of 0-8 days,and decreased with the increase of soil particle size after the11 th day of freezing under the groundwater table depth of 0.5 m,while the accumulative phreatic water evaporation decreased with the increase of soil particle size during the whole freezing period under the groundwater table depth of 0.8 m and 1.0 m.The accumulative phreatic water evaporation decreased exponentially with the increase of soil particle size.At the thawing stage,the larger the soil particle size was,the earlier the phreatic recharge reached stability.The accumulated phreatic water recharge increased with the increase of soil particle size first,and then decreased with the increase of soil particle size.3.The accumulated phreatic water evaporation and the accumulated phreatic water recharge decreased with the increase of groundwater table depth first,and then increased with the increase of groundwater table depth.The stable time of phreatic water evaporation and phreatic water recharge was delayed with the increase of groundwater depth.4.The decrease of freezing temperature has a significant effect on the evaporation of phreatic water.The evaporation rate of phreatic water increased obviously,and the accumulated phreatic water evaporation increased continuously when the freezing temperature decreased from minus 10? to minus 20?.The accumulated phreatic water evaporation and the absolute value of the negative accumulated freezing temperature were logarithmic relationship,and the regression coefficient a increased with the increase of groundwater tabledepth.When the accumulated negative freezing temperature exceeds minus800?·d,the effect of freezing temperature on the phreatic water evaporation was weakened.