Frequency domain analyses of tdr waveforms for soil moisture measurements

When time domain reflectometry (TDR) is applied to the field characterization of soil moisture, the waveforms have typically been analyzed using traveltime along the waveguide. The apparent dielectric constant (K_a) traditionally determined by the traveltime analysis using a tangent-line method does not have a clear physical meaning and is influenced by several system and material parameters. The frequency domain analysis, however, can determine the actual frequency-dependent dielectric permittivity and can be performed using a very short probe. The primary objectives of this paper are to (1) examine how several system parameters affect the apparent dielectric constant when the real dielectric relaxation spectrum of the material is fixed, (2) elucidate the relationship between the apparent dielectric constant and the dielectric relaxation spectrum, and (3) relate soil water content to a more definite quantity in the dielectric relaxation spectrum that is independent of the factors that influence the traveltime analysis.