Soil Water Heat of Transport

Abstract

Competing theories used to describe coupled transport of heat and water in soil are irreversible thermodynamics (IT) and the Philip–de Vries (PD) theory. In practice, PD has dominated the field and advanced understanding through models used to predict the thermal status of soil and soil-atmosphere interaction. However, the principle of Onsager reciprocity in IT has also found considerable acceptance in thermal physics. The applicability of IT reciprocity to soil physics has been extensively scrutinized without substantial agreement being reached. Examination of PD equations reveals that they do not satisfy Onsager reciprocity. One coefficient is missing from PD that would produce equations consistent with reciprocity. The coefficient is associated with the thermal water flux, but it fails to appear a second time in the heat flux equation as expected with reciprocity. This missing coefficient can be readily calculated for inclusion in models based on PD. The value of the coefficient varies with temperature and water potential from negligible to several times the PD coefficient to which it must be added. Heat of transport is also calculated and found to vary from 0.33 J/g in relatively wet soil to 9,071 J/g in drier soil, increasing slightly also with temperature.