Evaluation of a Direct-Coupled TDR for Determination of 1 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 (Kz) and bulk electrical conductivity (oa) measurements. Also, soil water content calibrations were completed for a Pullman clay loam soil. Lastly, the sensitivity of K3 to oa was examined using a saturated solute displacement experiment with both probe technologies installed in a column packed with Pullman clay loam. A range ofaa(0.65to2.8dSm_1) was established by equilibrating the column with 0.25 dS m' CaCl2 and introducing a step pulse of 7.3 dS m' CaC^. Permittivity calibrations of the TDR-315 could be accomplished with conventional TDR methods and with similar sampling errors. Conventional calibrations of oa using long time amplitudes yielded a linear response for Ga < 3 dS m' 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 to 0.020 mJ m~ ). 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 oa up to 2.8 dS m' .
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