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

摘要

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 (K-alpha) and bulk electrical conductivity (sigma(alpha)) measurements. Also, soil water content calibrations were completed for a Pullman (fine, mixed, superactive, thermic torrertic Paleustolls) clay loam soil. Lastly, the sensitivity of K-alpha to sigma(alpha) 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(alpha) (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 sigma(alpha) using long time amplitudes yielded a linear response for sigma(alpha) < 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 m(3) 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 sigma(alpha) up to 2.8 dS m(-1).