Experimental study of the effect of shallow groundwater table on soil thermal properties

In plains areas with semi-arid climates, shallow groundwater is one of the important factors affecting soil thermal properties. In this study, soil temperature and water content were measured when groundwater tables reached 10 cm, 30 cm, and 60 cm depths (Experiment I, II, and III) by using sensors embedded at depths of 5 cm, 10 cm, 20 cm, and 30 cm for 5 days. Soil thermal properties were analyzed based on the experimental data using the simplified de Vries model. Results show that soil water content and temperature have fluctuations that coincide with the 24 h diurnal cycle, and the amplitude of these fluctuations decreased with the increase in groundwater table depth. The amplitude of soil water content at 5 cm depth decreased from 0.025 m(3).m(-3) in Experiment II to 0.01 m(3).m(-3) in Experiment III. Moreover, it should be noted that the soil temperature in Experiment III gradually went up with the lowest value increasing from 26.0 degrees C to 28.8 degrees C. By contrast, the trends were not evident in Experiments I and II. Results indicate that shallow groundwater has a "cooling" effect on soil in the capillary zone. In addition, calculated values of thermal conductivity and heat capacity declined with the increasing depth of the groundwater table, which is consistent with experimental results. The thermal conductivity was stable at a value of 2.3W.cm(-1)K(-1) in Experiment I. The average values of thermal conductivity at different soil depths in Experiment II were 1.82W.cm(-1).K-1, 2.15W.cm(-1).K-1, and 2.21W.cm(-1).K-1, which were always higher than that in Experiment III.