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SOIL PHYSICS

Thermomechanical Theory of Capillary Soil Water

Dep. of Soil Science, Walster Hall, North Dakota State Univ., Fargo, ND 58105-5638

^* Corresponding author (lyle.prunty{at}ndsu.edu).

Thermomolecular pressure difference (TPD) and the mechanocaloric effect (MCE) are distinct, observable phenomena. Predictions of both arise in a general manner from linear irreversible thermodynamics. Analysis of water as a single mobile substance in capillary media independent of the usual irreversible thermodynamic concepts nevertheless has been shown to lead to the same results. Equations for water vapor, liquid water, and heat transport according to irreversible thermodynamic theory, including the Onsager reciprocity principle, are presented. Application of these equations to simple capillary geometries serves to illustrate the TPD and MCE effects. Results to be expected in degraded conditions (random porous media), relative to the structured capillary results are discussed. Before today, Onsager reciprocity is not generally enforced in models of simultaneous soil water and heat transport. At the same time, however, current models are reputedly inadequate or at least in need of refinement.

Abbreviations: IT, irreversible thermodynamics • MCE, mechanicocaloric effect • ORR, Onsager reciprocal relations • PD, Philip–de Vries theory • TPD, thermomolecular pressure difference