| As the most active factor in soil three-phase composition, soil water is the link between surface water and groundwater. During seasonal freeze-thaw period (from November to March of next year), the soil water migration and transformation between the groundwater and soil water are more complicated influnced by soil freezing and thawing in shallow groundwater area. Based on monitoring of soil temperature, soil moisture, evaporation and infiltration of lysimeter at Taigu Water Balance Experimental Field, Bureau of Hydrology and Water Resources Survey of Shanxi Province, and combing with indoor unidirectional freezing test, the soil freezing and thawing process, the varying characteristics of soil profile temperature and moisture, the mutual transformation between phreatic water and soil water, surface soil evaporation and variation of soil water storage under different groundwater level depth(GLD)during seasonal freeze-thaw period are studied with statistical analysis method and numerical simulation. The research results are significant for the prevention of soil salinization and evaluation of groundwater resources in shallow groundwater area. The main findings are as follows:1. Characteristics of soil profile temperature variation under different GLD.Soil temperature at the depth of 0 to 30 cm varies dramatically influenced by external factors. With the increase of soil depth, the profile soil temperature gradient and the fluctuation becomes smaller and smaller. With the increase of GLD, the fluctuation of soil temperature at the depth of 0 to 30 cm increases significantly. The fluctuation of soil temperature of loamy sand(LS) is bigger than that of sandy loam(SL) at the same depth. Under the different freezing temperatures, the surface soil temperature of SL is lower than that of silty clay loam(SCL), and the final temperature difference between SL and LS is also increased with the increase of freezing temperature decreasing range, which is 0.88 ?, 2.06 ? and 2.81 ? respectively for the three kind of freezing temperature decreasing range.2. Characteristics of soil freezing and thawing under different GLD. The whole soil profile is almost freezing when GLD is 0.5 m. When GLD is 1.0 m,the freezing depth is deepest and the thawing process is slow, the time that soil thaws completely lags behind natural thawing time about 12 days. The maximum frozen depth is sensitive to outside temperature when GLD is 1.5 m and 2.0 m. There is a linear relationship between soil freezing depth (Hf) and the square root of accumulated surface negative soil temperature (Tn0.5) under different GLD.3. Distribution of soil profile moisture during seasonal freeze-thaw period.When GLD is 0.5 m, the soil profile moisture gathered at the depth of 25 cm to 35 cm and 20 cm to 30 cm where the soil moisture content is about 52% and 45% for SL and LS respectively. The soil profile moisture gathered at the depth of 50 cm to 70 cm for SL and LS when GLD is 2.0 m, however, the soil moisture of soil water gathering zone is decreased significantly, it is about 24%for SL and 11% for LS respectively. When GLD is 0.5 m, the active layer of soil water variation is about 0 to 20 cm, while the active layer of soil water variation is mainly from 0 to 60 cm when GLD is more than 1.0 m.4. Influence of different freezing temperature decreasing range on soil moisture migration. Under same freezing temperature, the soil freezing rate of SL is quicker than that of SCL, soil moisture gathered at the depth of 15 cm to 25 cm for SL, which is shallower than SCL that soil moisture gathered at the depth of 25 cm to 35 cm. The surface soil moisture freezing in its place is slow when the freezing temperature with smaller decreasing range, so the soil moisture of 0 to 10 cm is higher than other freezing scheme about 1.5 % to 7.3% for SL and 2.3 % to 2.7 % for SCL.5. Laws of annual phreatic evaporation. The annual phreatic evaporation coefficient (?) and GLD (Hg) is well fitting to the exponential relationship, that is ?=?×e?×Hg,in which a signifies the surface soil evaporation coefficient, and? indicates the attenuating intensity of the phreatic evaporation coefficient when GLD is increasing, ? and ? changes linearly with the average soil particle size(du). Phreatic evaporation coefficient is relatively complex during freeze-thaw period. With the increase of GLD, phreatic evaporation coefficient for SL is decreased overall, however, the phreatic evaporation coefficient for LS and sandy soil is largest when GLD is 1.0 m.6. Phreatic water inflow during seasonal freeze-thaw period. The cumulative phreatic inflow is well related to freezing time in freezing period,which is fitting to the exponential relationship. When GLD is less than 2.5 m,the total amount of phreatic inflow for SL decreased almost linearly with the increase of GLD during freeze-thaw period. Under the same freezing temperature, the amount of phreatic inflow for SL is higher than that of SCL.The difference of phreatic inflow between SL and SCL is increased with the increase of freezing temperature decreasing range, which is 1150 ml, 1550ml and 1900ml respectively for the three kind of freezing temperature decreasing range.7. Transformation between soil water and phreatic water. The phreatic water is losed seriously with the lower GLD and the soil water migrating in the soil profile appears evaporation type during freeze-thaw period. On the contrary,if GLD is higher than 3.0 m, the soil water migrating in the soil profile appears infiltration type. The difference between recharge and consumption of phreatic water (DRC)(Qbh) is closely related to GLD (Hg) during freeze-thaw period,their relationship fits to the model such as Qbh? alnHg+b, in which the a, b and the average soil diameter (du) satisfied a linear relationship. The depth of zero DRC for SL, LS and SCL is 2.79 m, 2.40 m and 2.12 m respectively during freeze-thaw period.8. Surface soil evaporation. At unstable freezing stage and thawing stage,the surface soil evaporation accounts for about 81.8% to 91.7% of whole freeze-thaw period. Surface soil evaporation is reduced with the increase of GLD during freeze-thaw period, but it is inverse at stable freezing stage. At stable freezing stage, soil evaporation is almost zero because the concrete frozen layer almost located at the surface when GLD is 0.5 m. When GLD is 2.0 m, the compactness of upper frozen layer soil is decreased and the surface soil evaporation accounts for about 8.3% for SL and 18.2% for LS of whole freeze-thaw period.9. Characteristics of soil water storage variation. Soil water storage is increased overall under different GLD during freeze-thaw period. Soil water storage is increased considerably when GLD is 1.0 m and it is increased about 17.71 mm for SL and 25.96 for LS. |
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