The greater difference between day and night temperatures in arid and semi-arid areas influences water movement and heat transport in the vadose zone, and further influences the water and heat fluxes between the water table and the atmosphere. An evaporator and lysimeter, designed by the authors, and combined numerical simulation technology were used to study water movement and heat transport in the vadose zone, and the evaporation of phreatic water under the influence of surface temperature for different groundwater depths. The differences between water movement of the vadose zone and phreatic water evaporation calculated by isothermal and anisothermal models were also compared. The results of experiment and numerical simulations show that the surface temperature has a great influence on both water movement and heat transport in the vadose zone, as well as on evaporation intensity and the evaporation depth of phreatic water when the surface temperature is more than 25°C. The influential depth for the soil water content of vadose zone and the temperature of unsaturated and saturated zones is about 70 cm, but the greatest change is in the top 35 cm. The limited evaporation depth of phreatic water was about 70 cm for the experimental medium (silt/fine sand). The evaporation intensity of phreatic water was found to be maximum for a groundwater level of 20–40 cm (about 0·096 cm/h for silt/fine sand). The error of more than 8% was due to water movement of the vadose zone and the evaporation intensity of phreatic water calculated using an isothermal model. A coupled water and heat model was used to simulate water movement of the vadose zone and the exchange flux between the water table and atmosphere for surface temperatures higher than 25°C. For surface temperature below 25°C, the results of the isothermal and anisothermal models were coherent. There is thus no need to consider the influence of surface temperature on water movement of the vadose zone or the flux between the water table and the atmosphere.
Due to frequent changes in the humid and hot environment, the residual soil with a particle-size distribution (PSD) from gravel to clay experiences multiple drying–wetting cycles. The pressure plate test and nuclear magnetic resonance (NMR) spectroscopy were used to investigate the influence of drying–wetting cycles on the soil–water characteristic curve (SWCC) and pore-size distribution (POSD) of undisturbed residual soil. The results showed that the water-holding capacity of the residual soil decreased as the number of drying–wetting cycles increased and gradually stablilized, and then the van Genuchten (VG) model was found to perform well on the SWCC during the drying–wetting processes. The NMR results indicated a double-pore structure, and the porosity of the residual soil as well as the internal water content increased smoothly with more drying–wetting cycles. The obtained POSD curve of soil implied that drying–wetting cycles had a more obvious effect on small pores and macro-pores than on micro-pores and meso-pores. Theoretical calculations evinced that the product of the matric suction and relaxation time should be constant at a constant temperature. However, the experimental results did not effectively reflect such a relation between the matric suction and relaxation time. A modified VG model based on the cumulative pore volume was utilized to describe the POSD under drying–wetting cycles. Subsequently, the proposed Rational2D surface equation was used to accurately reflect the internal relationship between the SWCC and POSD curve under different numbers of drying–wetting cycles. Moreover, the fractal model for the SWCC derived from the capillary theory confirmed that the matric suction had a strong linear relationship with the relative volumetric water content in the log-log scale. Also, the fractal dimension can be approximated as a constant, because its attenuation is small with more drying–wetting cycles.
Based on the analysis of geological conditions of water resources site, generalized hydrological model was built, which was divided into three layers with different parameters. By using Visual Modflow, the groundwater seepage field was simulated for different mining yields, which give suggestions for the reasonable mining schedule of water resources site.
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