On the relationship between matric potential and dielectric properties of organic free soils: a sensitivity study

High-frequency electromagnetic determination of moisture in porous media, (e.g., soil) is based on the strong relationship between volumetric water content and relative dielectric permittivity. In particular, in fine-grained soils the movement of water is influenced by different surface-bonding forces due to interface processes. The interface effects lead to a number of dielectric relaxation processes (free- and bound-water phase, Maxwell-Wagner effect, counterion relaxation effects). These relaxation processes are the reason for the strong frequency dependence of the electromagnetic material properties below 1 GHz. The matric potential is a measure of the bonding forces on water in the soil. Based on a thermodynamic relationship between soil matric potential and dielectric relaxation behaviour of water in different binding states, a broadband dielectric relaxation model was developed that considers low-frequency dispersion up to 1 MHz as well as losses due to direct-current conductivity. The sensitivity of the model on soil suction was systematically analyzed based on a pedotransfer function (PTF) for soil textures ranging from pure sand up to pure clay. The results are compared with known empirical and semiempirical calibration functions, as well as theoretical mixing models. Key words: soil matric potential, dielectric spectroscopy, time-domain reflectometry (TDR), soil moisture. La determination de l'humidite dans un medium poreux (par exemple le sol) avec des hautes frequences electromagne tiques est basee sur la forte relation entre la teneur en eau volumique et la permittivite dielectrique relative. Dans les sols fins en particulier, le mouvement de l'eau est influence par les differentes forces d'attachement de surface dues aux processus d'interface. Les effets d'interface amenent a plusieurs processus de relaxation dielectriques (phase aqueuse libre ou liee, effet Maxwell-Wagner, effets de relaxation contre-ion). Ces processus de relaxation sont responsables de la forte dependance de frequence des proprietes electromagnetiques d'un materiau sous 1 GHz. Le potentiel de la matrice est une mesure des forces d'attachement sur l'eau contenue dans le sol. Selon une relation thermodynamique entre le potentiel de la matrice du sol et le comportement de l'eau en relaxation dielectrique dans differents etats de liaison, un modele a bande large de la relaxation dielectrique a ete developpe qui tient compte la dispersion a basses frequences jusqu'a 1 MHz ainsi que des pertes dues a la conductivite directe du courant. La sensibilite du modele a ete analysee systematiquement a partir d'une fonction de pedotransfert (<< PTF >>) pour des sols ayant une texture variant du sable pur jusqu'a l'argile pure. Les resultats sont compares avec des fonctions de calibrage empiriques et semi-empiriques connues, ainsi qu'a des modeles theoriques de melanges. Mots-cles : potentiel de la matrice du sol, spectroscopie dielectrique, reflectometrie dans le domaine temps (RDT), humidite du sol. [Traduit par la Redaction]
Introduction High-frequency electromagnetic measurement techniques are used to quantify spatial and temporal variation of soil moisture within scientific and practical issues of geotechnics, hydrology, and soil physics (Rowe et al. [...]