Numerical Evaluation of Transient Bulk Density Impact on Surface Energy Balance and Coupled Heat and Water Transfer in Soils.

Modeling of soil mass and heat transfer and surface energy balance has played an important role in many hydrologic, environmental, and climatic studies. In most existing models, soil bulk density is considered to be static, which is not true in some cases. Several studies report that soils experience dynamic soil bulk density (e.g., the change can be as much as 0.3 Mg m^-3) within a year due to disturbance associated with agriculture, compaction, wetting-drying, freeze-thaw, and erosion-deposition. The importance of transient soil bulk density on soil heat and mass transfer needs to be evaluated. The objectives of this numerical study are 1) to quantify the effects of soil bulk density variation on fundamental soil properties including soil volumetric heat capacity, thermal conductivity, thermal diffusivity, water retention curve, hydraulic conductivity, and vapor conductivity, and 2) to evaluate the impact of changing soil bulk density and associated properties on surface energy balance and coupled heat and water transfer in soils.

Clay loam, silt loam, and sandy loam soils are evaluated in this study. Bulk density ranges for each soil are obtained from the literature. With commonly used models, the soil properties as a function of bulk density (1.1-1.4 Mg m^-3) and soil water contents (0.10-0.40 m³ m^-3) are presented. HYDRUS 1D is used to simulate coupled heat and mass transfer and surface energy balance of several bulk density values for each soil. Measured weather data are used for the surface boundary condition. Simulation with time change in bulk density are also performed.

Impacts of bulk density variation on soil properties, coupled heat and water transfer in soils, and surface energy balance are quantified. And importance of transient soil bulk density on soil heat and mass transfer is discussed.