Comparisons of coupled water and heat transport models in the vadose zone

Understanding and simulating coupled water and heat transfer appropriately in the shallow subsurface is of vital significance for accurate prediction of soil evaporation. The theory of Philip and de Vries (1957), accounting for water vapor diffusion only, was considered physically incomplete and consequently extended and improved by several researchers (e.g., Parlange et al., 1998; Grifoll et al., 2005; Smits et al., 2011) by explicitly taking water vapor convection, dispersion or air flow into account. It is generally believed that the soil moisture is usually low in the near-surface layer under highly transient field conditions, particularly in arid and semiarid regions, and that accurate characterization of water vapor transport is critical when modeling simultaneous water and heat transport in the shallow field soils. The objectives of this study are thus: (a) to test existing coupled water and heat transport theories, (b) to develop reasonable and simplified numerical models, and (c) to perform sensitivity analysis of soil hydraulic and thermal parameters using field experimental data collected under different hydro-climatic conditions. This model comparison study identified the necessity of including explicitly the air flow into traditional coupled water and heat models in the shallow vadose zone under transient field environments for better agreement between simulated and measured soil moisture and temperature. Comparing the magnitude of different moisture fluxes facilitated deriving simplified and appropriate models which are suitable for large scale simulation. Parameter sensitivity analysis indicated the importance of using full-range soil water retention and unsaturated hydraulic conductivity function in modeling coupled water and heat movement in vadose zone.