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

Abstract

<p>Soil thermal properties play important roles in dynamic heat and mass transfer processes, and they vary with soil water content (<em>θ</em>) and bulk density (<em>ρ</em> <em>b</em>). Both <em>θ</em> and <em>ρ</em> <em>b</em>change with time, particularly in recently tilled soil. However, few studies have addressed the full extent of soil thermal property changes with <em>θ</em> and <em>ρ</em> <em>b</em>. The objective of this study is to examine how changes in <em>ρ</em> <em>b</em> with time after tillage impact soil thermal properties (volumetric heat capacity, <em>C</em> <em>v</em>, thermal diffusivity, <em>k</em>, and thermal conductivity, <em>λ</em>). The study provides thermal property values as functions of <em>θ</em> and <em>ρ</em> <em>b</em> and of air content (<em>n</em> <em>air</em>) 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 (<em>θ</em> at pressure head of −50 kPa) and oven‐dry conditions. Generally, <em>k</em>and <em>λ</em> increased with increasing <em>ρ</em> <em>b</em> at the three water conditions. The <em>C</em> <em>v</em> increased as <em>ρ</em> <em>b</em>increased in the oven‐dry and unsaturated conditions and decreased as <em>ρ</em> <em>b</em> increased in the saturated condition. For a given <em>θ</em>, a larger <em>ρ</em> <em>b</em> was associated with larger thermal property values, especially for <em>λ</em>. The figures of <em>C</em> <em>v</em>, <em>k</em> and <em>λ</em> versus <em>θ</em> and <em>ρ</em> <em>b</em>, as well as <em>C</em> <em>v</em>, <em>k</em> and <em>λ</em> versus <em>n</em> <em>air</em>, represented the range of soil conditions following tillage. Trends in the relationships of thermal property values with <em>θ</em> and <em>ρ</em> <em>b</em> were described by 3‐D surfaces, whereas each thermal property had a linear relationship with <em>n</em> <em>air</em>. Clearly, recently tilled soil thermal property values were quite dynamic temporally due to varying <em>θ</em> and <em>ρ</em> <em>b</em>. The dynamic soil thermal property values should be considered in soil heat and mass transfer models either as 3‐D functions of <em>θ</em> and <em>ρ</em> <em>b</em> or as linear functions of <em>n</em> <em>air</em>.</p>