Systemic Modelling of Soil Water Thermodynamics

In this chapter, we provide the systemic definition of three key thermodynamic variables, building on previous theorizations of the systemic approach to natural systems like soils. Temperature is defined as the internal energy of the molecules that make up a fluid phase (" "); entropy is defined as the ratio of two organizational variables of the phase ("the occupational volume of molecules and their own volume in the phase of volume ; and the internal molecular chemical potential is defined as the ratio of a phase molecule's temperature to its own mass. This allowed the following conceptual advances : i) establishing the definitional equations of the 3 equivalent forms of the Gibbs free energy of the aqueous phase of the soil pedostructure, ii) establishing that the general equilibrium criterion of the system is the internal molecular potential rather than the temperature , iii) Elimination of confusion between the internal and external pressures of the system and subsystems, which did not distinguish the two types of energy carried by a molecule of the fluid phase: the internal energy of the molecule which is oscillatory and the external kinetic energy which is due to Brownian motion in the phase. We corrected accordingly the differential equations of the usual thermodynamic potentials as well as the Gibbs-Duhem equation. The application to the soil-water-air system is given followed by some comments on the new vision of thermodynamic equilibrium modeling that emerges from this theory.