Evaluation of a Direct-Coupled Time-Domain Reflectometry for Determination of Soil Water Content and Bulk Electrical Conductivity

Signal degradation in coaxial cables and interconnects is a long-standing problem in the practical deployment of time-domain reflectometry (TDR) for soil water monitoring. Acclima, Inc. has recently commercialized a TDR sensor (TDR-315) with all electronics required for waveform acquisition embedded in the probe head. We calibrated ten TDR-315 sensors and conventional TDR for apparent permittivity (Ka) and bulk electrical conductivity (sigma_a) measurements. Also, soil water content calibrations were completed for a Pullman (fine, mixed, superactive, thermic torrertic Paleustolls) clay loam soil. Lastly, the sensitivity of Ka to sigma_a was examined using a saturated solute displacement experiment with both probe technologies installed in a column packed with Pullman clay loam. A range of sigma_a (0.65–2.8 dS/m) was established by equilibrating the column with 0.25 dS/m CaCl2 and introducing a step pulse of 7.3 dS/m CaCl2. Permittivity calibrations of the TDR-315 could be accomplished with conventional TDR methods and with similar sampling errors. Conventional calibrations of sigma_a using long time amplitudes yielded a linear response for sigma_a