Water potential in nonrigid unsaturated soil-water medium

This paper presents a development of water potential functions for a dual porosity nonrigid aggregated soil medium defined and characterized by its characteristic shrinkage curve. Fractal and thermodynamic approaches to modeling soil water potential are evaluated using simultaneous and continuously measured soil shrinkage and tensiometric curves on a standard laboratory soil sample. Water potential relationships are developed and analyzed for five Ivory Cost soils with clay content ranging from 10 to 50%. Analysis of modeled and observed results showed that the tensiometric curve, for its entire range of measurement, is directly linked to the interped water pool. The tensiometric curve, which is a function of the interped water pool, was modeled using both the fractal and the thermodynamical equations. Fits to the measured tensiometric curves were very well. This study demonstrates the link between the tensiometric curve with some of the pedostructure (soil-water medium) variables and parameters determined by the shrinkage curve, which leads to physically based equations of soil water potential. An interpretation of the origin of the swelling pressure inside and outside the primary peds and of the resulting suction pressure in unsaturated aggregated soils is proposed and discussed. This interpretation explains the link between the generalized Low's [1979] equation for the thermodynamic properties of the hydration layers of water surrounding particles in primary peds and the equations given by Berezin et al. [1983] for the swelling and the suction pressures in unsaturated aggregates.