A new TDR probe for measurements of soil solution electrical conductivity

SummaryThe measurement of the soil solution electrical conductivity (σw) is critical for a better management of irrigation water and the effective monitoring and control of soil salinity. The objective of this work is to present the design and validation of a new time domain reflectometry (TDR) probe (WECP) for accurate and non-destructive measurements of σw. The probe consists in 14 porous ceramics disks (0.5 bar bubbling pressure) arranged along the axis of a three-rod TDR probe. Using the Mualem and Friedman (1991) model, σw was estimated from the volumetric water content (θ) and the bulk electrical conductivity (σa) measured in the ceramic disk set of known pore-geometry. The τ and β factors, which describe the complex geometry of the ceramic matrix, were calculated by immersing the probe in NaCl solutions of different electrical conductivities, and in a pressure cell wetted and drained with these NaCl solutions, respectively. The reliability of the WECP was validated under laboratory and field conditions. The laboratory experiment consisted of the TDR probe inserted in a pressure cell packed with mixed sand and 2-mm sieved loam soil that was subsequently wetted and drained with different NaCl solutions at various pressure heads. The σw estimated by WECP was compared to the σw measured in the draining solutions after they stabilized in the soil porous system. The field experiment compared the σw estimated by WECP with the corresponding σw values measured in the soil solution extracted with three ceramic tension lysimeters (TLs) after successive wetting and drainage cycles. The τ and β factors calculated for the ceramic disks set were 1.957 and 4.282, respectively. High and significant correlations were found in both laboratory (R2 = 0.98; P < 0.001) and field (R2 = 0.97; P < 0.001) experiments between the σw estimated by the WECP and the corresponding σw values measured in the column-drainage or TL-extracted soil solutions, respectively. These results demonstrate that the WECP is a feasible instrument to accurately estimate soil solution salinity independently of the soil water content and the porous medium in which the TDR probe is installed.

► A new TDR probe (WECP) for measurements of σw is presented.
► WECP consisted of a TDR probe inserted in 14 ceramic discs.
► The WECP was calibrated and successfully validated in laboratory.
► Field experiments compared σw of WECP with that measured with suction cups (TL).
► Excellent correlation (R2 = 0.97) was found between σw estimated with WECP and TL.