Soil water content and salinity determination using different dielectric methods in saline gypsiferous soil

Measurements of dielectric permittivity and electrical conductivity were taken in a saline gypsiferous soil collected from southern Tunisia. Both time domain reflectometry (TDR) and the new WET sensor based on frequency domain reflectometry (FDR) were used. Seven different moistening solutions were used with electrical conductivities of 0.0053-14 dS m(-1). Different models for describing the observed relationships between dielectric permittivity (K-a) and water content (theta), and bulk electrical conductivity (ECa) and pore water electrical conductivity (ECp) were tested and evaluated. The commonly used K-a-theta models by Topp et al. (1980) and Ledieu et al. (1986) cannot be recommended for the WET sensor. With these models, the RMSE and the mean relative error of the predicted theta were about 0.04 m(3) m(-3) and 19% for TDR and 0.08 m(3) m(-3) and 54% for WET sensor measurements, respectively. Using the Hilhorst (2000) model for ECp predictions, the RMSE was 1.16 dS m(-1) and 4.15 dS m(-1) using TDR and the WET sensor, respectively. The WET sensor could give similar accuracy to TDR if calibrated values of the soil parameter were used instead of standard values.