An Empirical Model for Estimating Soil Thermal Diffusivity from Texture, Bulk Density, and Degree of Saturation

Soil thermal diffusivity kappa is an essential parameter for studying surface and subsurface heat transfer and temperature changes. It is well understood that kappa mainly varies with soil texture, water content theta, and bulk density rho(b), but few models are available to accurately quantify the relationship. In this study, an empirical model is developed for estimating kappa from soil particle size distribution, rho(b), and degree of water saturation S-r. The model parameters are determined by fitting the proposed equations to heat-pulse kappa data for eight soils covering wide ranges of texture, rho(b), and S-r. Independent evaluations with published kappa data show that the new model describes the kappa(S-r) relationship accurately, with root-mean-square errors less than 0.75 x 10(-7) m(2) s(-1). The proposed kappa(S-r) model also describes the responses of kappa to rho(b) changes accurately in both laboratory and field conditions. The new model is also used successfully for predicting near-surface soil temperature dynamics using the harmonic method. The results suggest that this model provides useful estimates of kappa from S-r, rho(b), and soil texture.