Dielectric Relaxation of Bound Water versus Soil Matric Pressure

Abstract

The electrical permittivity of soil is a function of the water content, which facilitates water content measurements. The permittivity of soil is also a function of the frequency of the applied electric field. This frequency dependence can be described by the relationship between the dielectric relaxation frequency and the activation enthalpy of the water, which in turn is related to the soil matric pressure. The activation enthalpy or soil matrix pressure is a measure of the binding forces acting on a water molecule in the soil matrix. Each water molecule is differently bound, varying from tightly bound to free water. The permittivity of the bulk soil results from the contribution of all the water molecules in the soil matrix. Therefore, the permittivity of soil as a function of frequency is related to the soil matrix pressure. It is realistic to consider hygroscopic water as ice like. A relatively sharp transition can be observed from free to hygroscopic water at matric pressure – 100 MPa corresponding to relaxation frequency fr ≈ 8 GHz. Therefore, for the interpretation of dielectric data using a dielectric mixture equation, the water content of soil can be split conveniently in “free” water and “hygroscopic” water.

Spatial time domain reflectometry and its application for the measurement of water content distributions along flat ribbon cables in a full‐scale levee model

“…[21] The relationship between volumetric water content and dielectric permittivity is affected by various factors such as measurement frequency, temperature, mineralogical composition, structure, texture, bulk density and chemical composition of the pore fluid [Shen et al, 1987;Ishida et al, 2000;Boyarskii et al, 2002;Gutina et al, 2003;Hilhorst et al, 2001;Topp et al, 1980;Logsdon, 2005]. Therefore, the objective of numerous experimental, numerical and theoretical investigations is the development of general calibration rules for a broad range of soil textures and structures [Wang and Schmugge, 1980;Topp et al, 1980;Roth et al, 1990Roth et al, , 1992Jacobsen and Schjønning, 1993;Friedman, 1998].…”

Non-Vegetated Playa Morphodynamics Using Multi-Temporal Landsat Imagery in a Semi-Arid Endorheic Basin: Salar de Uyuni, Bolivia

“…When the sediment is fine sand or silt, the reflectance is high due to little scattering effect and a large specific area. However, with an increasing content of clay, the capacity of the sediment for storing water increases because of the large internal surface, which is related to adhesive, cohesive and osmotic forces [22]. Due to strong evapo-transpiration in the study area (1500 mm/year), the water in the sediments precipitates salt in the long dry period (normally from April to November in the study area).…”

Efficient reconstruction of dispersive dielectric profiles using time domain reflectometry (TDR)

“…Because the dielectric permittivity of soil material typically depends considerably on frequency, particularly if there are clay and loam components (Hoekstra and Delaney, 1974;Sposito and Prost, 1982;Ishida et al, 2000;Huisman et al, 2004;Robinson et al, 2005), in a third phase methods have been studied to recover the average dispersive dielectric parameters from TDR traces (Heimovaara, 1994;Heimovaara et al, 1996;Hilhorst et al, 2001;Lin, 2003). Clearly, the next logical step are methods to extract the full dielectric profile from a TDR trace.…”