Soil water thermodynamic to unify water retention curve by pressure plates and tensiometer

Abstract

The pressure plate method is a standard method for measuring the pF curves, also called soil water retention curves, in a large soil moisture range from saturation to a dry state corresponding to an applied pressure of near 1500 kPa. However, the pressure plate can only provide discrete water retention curves represented by a dozen measured points. In contrast, the measurement of the soil water retention curves by tensiometer is direct and continuous, but limited to the range of the tensiometer reading: from saturation to near 70-80 kPa. The two methods stem from two very different concepts of measurement and the compatibility of both methods has never been demonstrated. The recently established thermodynamic formulation of the pedostructure water retention curve, will allow the compatibility of the two curves to be studied, both theoretically and experimentally. This constitutes the object of the present article. We found that the pressure plate method provides accurate measurement points of the pedostructure water retention curve h(W ), conceptually the same as that accurately measured by the tensiometer. However, contrarily to what is usually thought, h is not equal to the applied air pressure on the sample, but rather, is proportional to its logarithm, in agreement with the thermodynamic theory developed in the article. The pF curve and soil water retention curve, as well as their methods of measurement are unified in a same physical theory. It is the theory of the soil medium organization (pedostructure) and its interaction with water. We show also how the hydrostructural parameters of the theoretical curve equation can be estimated from any measured curve, whatever the method of measurement. An application example using published pF curves is given.

The changes in the physical and hydraulic properties of a loamy soil under irrigation with simpler-reclaimed wastewaters

“…Pore size distribution was determined using the soil water retention curve (pF curve). The pressure plate method as a standard method was used to get the pF curve (Braudeau et al, 2014). For drawing the pF curve, the volumetric water contents retained at different tensions were plotted against the matric potentials.…”

Comparação Entre Curvas De Retenção De Água Obtidas Em Laboratório E a Campo

“…Segundo Wassar et al (2016) uma das limitações no uso de tensiômetros está ligado aos limites de determinação da tensão de água do equipamento. Braudeau et al (2014), no entanto, consideram esse intervalo suficiente para representar variações em pressões, em pelo menos 1.500 kPa.…”

Hydrostructural Pedology, New Scientific Discipline Allowing for Physical Modelling of ‘Green Water’ Dynamics in the Soil-Plant-Atmosphere System

“…The use of the new hydro structural pedology paradigm allowed discovering the exact equations of state of the pedostructure that characterize the thermodynamic and hydrostructural equilibrium between the two types of pedostructural water (W mi and W ma , internal and external to primary peds in the pedostructure) [9]. This, in turn, led to the establishment of theoretical equations of the four pedostructural characteristic curves that fully represent the hydrostructural properties of soils [9,10] and [11]; these are:…”

“…• The soil water retention curve, ℎ = : the tension of the soil water expressed in suction pressure (hPa), function of the pedostructural water content (in kg water/kg soil structure) [9,10].…”

“…These four curves are dependent on the thermodynamic interaction between the water molecules and the surface charges of soil particles, both organic and mineral, constituting the non-rigid structure of the soil medium. The new apparatus, named TypoSoil® [12], which measures accurately and simultaneously in laboratory the two first hydrostructural characteristic curves of the pedostructure, namely, the shrinkage curve and the water retention curve, allowed us validating the theoretical equations of these curves [13,9,10]. It was recently put available on the market (2014).…”