Time domain reflectometry models as a tool to understand the dielectric response of volcanic soils

The transmission of an electromagnetic wave through the soil using time domain reflectometry (TDR) permits the determination of its water content, θ, given the composite dielectric constant of the soil, εc, via Toppʹs equation. This is possible since the dielectric constant of water is much larger than that of the soilʹs solid phase (80 vs. 5) and, therefore, changes in soil moisture content lead to variations in εc. However, organic soils and those of volcanic origin do not obey the “universal” relationship εc–θ originally obtained by Topp. Hence, several authors have proposed alternative empirical εc–θ relationships. Volcanic soils are particular with respect to their low bulk density, large porosity and specific surface, mainly due to the strong aggregation of particles and high concentration of Fe-oxihydroxides, and also due to the presence of allophanic clays with large surface area and water affinity. Thus, it is likely that the water retention characteristics of volcanic soils will exhibit an atypical dielectric behaviour. This work shows that physical models, which incorporate parameters such as bulk density, porosity and surface area, can interpret the anomalous dielectric behaviour of volcanic soils. Among these models, we shall distinguish those which consider three phases (water–air–soil) and those which separate the water content in two phases, i.e. free and bound water (four-phase models). Four-phase models help identify two distinct linear dielectric regimes dominated by free and bound water, respectively, for at least some soils. Soil porosity is shown to play an important role in the dielectric behaviour of volcanic soils. We also found that soil andic parameters provide an a priori diagnosis criterion to evaluate the departure of volcanic soils from Toppʹs curve. Additionally, we propose an alternative general εc–θ calibration relationship for these kinds of soils, which may be incorporated in commercial TDR devices for laboratory and field water content determination.