Assessment of Soil Water Thermodynamic to Unify Water Retention Curve by Pressure Plates and Tensiometer

In soil science, there are two well-known standard methods for measuring the soil moisture characteristic curve: the pressure plate method providing the pF curve and the tensiometer providing the soil suction pressure curve. These two empirical curves are generally used for characterizing and modeling the soil water properties. The two approaches are based on two very different measurement concepts, and physical compatibility of the measured variables has never been demonstrated. The recently thermodynamic formulation of the pedostructure water retention curve h(W) allows us studying this compatibility, both theoretically and experimentally. This constitutes the object of the present article. We found that the pressure plate method provides correct measurement points of the water retention curve of the pedostructure h(W) but on the condition that h is not calculated as usual, as equal to the air pressure applied to the sample. Instead, according to the thermodynamic theory developed here, h must be calculated as the logarithm of the air pressure applied to the pedostructure sample to correspond to the suction pressure directly measured by the tensiometer. We then demonstrate how the hydrostructural parameters of the theoretical curve equation can be estimated from any measured curve, regardless of measurement technique. An application example using published pF curves is given.