Impacts of the real and imaginary components of relative permittivity on time domain reflectometry measurements in soils

Time domain reflectometry (TDR) is widely used for routine field monitoring of water content and salts in soils. Most estimates of water content assume the TDR-measured apparent relative permittivity, ε_a, is a good approximation for the real component, ε′_r of the soil's complex relative permittivity with the magnitude of ε′_r being determined primarily by water content. We examine this assumption and show that ε_a is influenced by both the real and imaginary components of the relative permittivity. Increases in ε_a resulted from the dc conductivity and dielectric loss arising from the presence of ions in solution and clay content. At water contents above 0.15 m³ m^-3 in soils with high clay content and/or salt, specific calibrations are needed for precise determinations of water content from TDR. We use the wave propagation equations to separate the real and imaginary component contributions to ε_a. The Giese and Tiemann interpretation for dc conductivity was again shown to be within 10% of that from a conductance meter and this fact was used to propose a method using only TDR data to separate real and imaginary components of the relative permittivity. It was found that the dielectric losses and conductive losses did not differ according to the source of conductivity, whether from clay content in the soil matrix or electrolyte in the soil solution.