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

Abstract

Core Ideas TDR‐315 sensors could be calibrated using conventional TDR methodologies. Soil water content calibrations using conventional TDR and TDR‐315 were comparable. Measured travel time using TDR‐315 was insensitive to soil 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 (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.