ABSTRACT
Fig. 1 shows a schematic representation of the soil horizon showing its four hierarchical structural levels: the horizon, the pedostruture, the primary peds, and the primary particles. In this representation, we define hierarchical representative elementary volumes (REVs) where different physical laws such as Darcy’s law can be applied. The REV of a soil horizon is large enough to comprise cracks or fissures that are opened to air when soil dries and includes vertical porosity. It is delimited vertically by the soil horizon, thus its volume change is only one-dimensional (vertical). The REV of the pedostructure is composed of the primary peds, Vmi, and the pore space created by their assembly, Vpma; its volume change is three-dimensional and isotropic. In that pore space, two water pools are differentiated and defined by the shrinkage curve (Fig. 2): a ‘‘swelling’’ water wip, which leaves the pore system without air intake (peds approaching each other) and a non-swelling water wst, which leaves the same pore system while being replaced by air (peds are jointed). The primary peds pore system is quantitatively defined by its air entry point that is clearly identified on a continuously measured SC. At point B of the SC (Fig. 2),
the specific pore volume of the primary peds (Vpmi) are equal in value to the water content WB (Vpmi ¼ WB/rw, where rw is the water bulk density). The primary peds are composed of the primary particles, of specific volume, Vs, and of the micropore space between them. In this micropore space, two water pools are also defined using the SC: swelling water, wbs, and a non-swelling water, wre. The subscripts ip, st, and bs are referred to as interped, structural, basic, and residual, which are the classical names of the different linear shrinkage phases to which a single water pool is associated (at inflection point of the structural shrinkage, e.g., dW ¼ dwst).
