ABSTRACT
Reflectometry) probes. These sensors are based on the measurement of soil dielectric constant, or dielectric permittivity (80 for pure water at 20°C (Lide 2001) and up to 14 for clay minerals (Fabbri et al. 2006)). So any change in the water content induces a change in its overall dielectric constant ′εr (Cosenza & Tabbagh 2004). Several authors have proposed universal relations to link the dielectric constant to water content in different type of soils (e.g. Topp et al. 1980). These relations are good in first approximation but not enough accurate at low saturation. In addition they are only valid at a given temperature which is problematic in the case of on-site measurement. A TDR probe does not directly evaluate the dielectric constant but the travel time (τ) for the reflection of an electromagnetic wave between conductive rods of length (L).
