Verification of soil salinity index model based on 0.02–3 GHz complex dielectric permittivity spectrum measurements

Abstract

Determination of electrical conductivity of soil pore-water from measurements of soil bulk electrical conductivity and volumetric water content or dielectric permittivity, requires theoretical or empirical models connecting these quantities. One such model is the salinity index model, which in the original formulation requires the measurement of soil bulk electrical conductivity and apparent dielectric permittivity determined with the time-domain-reflectometry (TDR) technique. Currently, many popular soil moisture sensors are operated at a single frequency, usually in the low-frequency part of the typical TDR bandwidth. Yet, no systematic verification of the salinity index model applied with the use of dielectric permittivity measured at specific frequencies in a broad frequency range has been performed. The aim of this paper is to evaluate the salinity index model with the use of dielectric spectra of 277 samples of 14 soils obtained in a 20 MHz–3 GHz frequency range with the use of a coaxial transmission-line cell vector-network-analyzer system. A three-pole Debye model was fitted to the spectra in order to determine bulk electrical conductivity and the real part of dielectric permittivity at chosen frequencies in order to diminish the influence of measurement errors. Then, the salinity index model was applied at each examined frequency and its soil-specific parameters were calculated. The performance of the model was evaluated with the use of the leave-one-out cross-validation scheme, and the frequencies at which the model performed best were found within 0.5–2 GHz range. Finally, the salinity index model parameters obtained at the optimal frequencies were correlated with other soil properties.