Dielectric relaxation of bound water versus soil matric pressure

The electrical permittivity of soil is a function of the water content, whi ch facilitates water content measurements. The permittivity of soil is also a function of the frequency of the applied electric held, This frequency d ependence can be described by the relationship between the dielectric relax ation 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 tig htly bound to free water. The permittivity of the bulk soil results from th e contribution of all the water molecules in the soil matrix. Therefore, th e permittivity of soil as a function of frequency is related to the soil ma trix 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 f(r) ap proximate to 8 GHz, Therefore, for the interpretation of dielectric data us ing a dielectric mixture equation, the water content of soil can be split c onveniently in "free" water and "hygroscopic" water.