Abstract

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 (σa) 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 σa was examined using a saturated solute displacement experiment with both probe technologies installed in a column packed with Pullman clay loam. A range of σa (0.65–2.8 dS m−1) was established by equilibrating the column with 0.25 dS m−1 CaCl2 and introducing a step pulse of 7.3 dS m−1 CaCl2. Permittivity calibrations of the TDR-315 could be accomplished with conventional TDR methods and with similar sampling errors. Conventional calibrations of σa using long time amplitudes yielded a linear response for σa ≤ 3 dS m−1, above which the response was nonlinear. The fitted water content calibrations of the Pullman clay loam for the TDR-315 were nearly indistinguishable from conventional TDR calibrations with similar root mean square errors (0.017–0.020 m3 m−3). Response of the two measurement technologies in a lossy soil during changing solution conductivities demonstrated that, in contrast to conventional TDR, travel time measured using acquired TDR-315 waveforms was insensitive to σa up to 2.8 dS m−1.

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