Thermodynamic Formulation of the Pedostructure Water Retention and Shrinkage Curves: An Approach to Soil Medium Organization

The two characteristic soil moisture curves: the soil water retention curve and the shrinkage curve, necessary for the physical modeling of the hydrological and agronomic properties of the soil are, still today, entirely empirical. They have been established so far without referring to the hydrostructural and thermodynamic properties of the pedostructure, nor to the internal and hierarchical organization of the soil pedon and its horizons which are described in Hydrostructural Pedology. On the contrary, this article presents the development of these equations according to the systemic and thermodynamic paradigm of this new scientific discipline of earth sciences. We show how the aqueous phase of the pedostructure is divided into two types of water positioned differently in the pedostructure: one in the plasma porosity of the primary aggregates and the other outside the primary aggregates in the inter-aggregate porosity. The thermodynamic equilibrium established between these two types of water has been put into equations to represent the pedostructural water retention curve h(W) resulting from this equilibrium: the suction pressure of the pedostructure as a function of the water content. Using information from simultaneous measurements of both curves, the new equations were put to the test and the theory was verified. The implications of these findings for physical modelling in agro-environmental sciences are discussed.