Thermal property values of a central Iowa soil as functions of soil water content and bulk density or of soil air content

Soil thermal properties play important roles in dynamic heat and mass transfer processes, and they vary with soil water content (θ) and bulk density (ρ_b). Both θ and ρ_b change with time, particularly in recently tilled soil. However, few studies have addressed the full extent of soil thermal property changes with θ and ρ_b. The objective of this study is to examine how changes in ρ_b with time after tillage impact soil thermal properties (volumetric heat capacity, C_v, thermal diffusivity, k, and thermal conductivity, λ). The study provides thermal property values as functions of θ and ρ_b and of air content (n_air) on undisturbed soil cores obtained at selected times following tillage. Heat pulse probe measurements of thermal properties were obtained on each soil core at saturated, partially saturated (θ at pressure head of −50 kPa) and oven-dry conditions. Generally, k and λ increased with increasing ρ_b at the three water conditions. The C_v increased as ρ_b increased in the oven-dry and unsaturated conditions and decreased as ρ_b increased in the saturated condition. For a given θ, a larger ρ_b was associated with larger thermal property values, especially for λ. The figures of C_v, k and λ versus θ and ρ_b, as well as C_v, k and λ versus n_air, represented the range of soil conditions following tillage. Trends in the relationships of thermal property values with θ and ρ_b were described by 3-D surfaces, whereas each thermal property had a linear relationship with n_air. Clearly, recently tilled soil thermal property values were quite dynamic temporally due to varying θ and ρ_b. The dynamic soil thermal property values should be considered in soil heat and mass transfer models either as 3-D functions of θ and ρ_b or as linear functions of n_air. Highlights: Thermal property values for a range of θ and ρ_b were measured on undisturbed soil cores. Freshly tilled soil thermal property values were quite dynamic temporally. The thermal property values of a tilled soil were described as 3-D surfaces with θ and ρ_b. The thermal property values of a tilled soil varied linearly with n_air.