A combined equation to estimate the soil pore-water electrical conductivity: calibration with the WET and 5TE sensors
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Affordable, commercial dielectric sensors of the frequency domain reflectometry (FDR) and capacitance-conductance (CC) types estimate the dielectric permittivity (epsilon(b)) and electrical conductivity (sigma(b)) of bulk soil. In this work, an equation was obtained to estimate the pore-water electrical conductivity (sigma(p)), which is closely related to the soil salinity in contact with plant roots, from epsilon(b) and sigma(b) data, by combining the simplified dielectric mixing (SDM) model that relates epsilon(b) to the soil volumetric water content (theta), with the Rhoades equation that relates theta and sigma(b) to sigma(p). This equation was calibrated with measurements of epsilon(b) and sigma(b) obtained with the Delta-T WET (FDR) and the Decagon 5TE (CC) sensors, in 20 pots filled with a clay loam soil and arranged as combinations of four levels of soil moisture with five levels of soil salinity. The calibrations were performed against reference theta and sigma(p) values. The sp was calculated with the chemical equilibrium model SALSOLCHEMEC and used as a more reliable reference than the electrical conductivity of the soil wetting water. For both sensors, the SDM model on the one hand, and the Rhoades equation on the other, provided the most accurate estimations using the least number of parameters regarding their respective alternatives, i.e. the third-order polynomial and the Hilhorst equation. The combined equation for estimation of sigma(p) subsequently provided root mean square deviations of 3.1 (WET) and 4.1 (5TE) dSm(-1), which decreased to 1.5 and 2.6 dSm(-1) for theta > 0.22m(3)m(-3), and sigma(b) 0.22m(3) m(-3) and sigma(b) < 3.7 dSm(-1).