Robert C. Schwartz1, Steven R. Evett2, Scott K Anderson3 and David J Anderson3, (1)2300 Experiment Station Rd, USDA-ARS, Bushland, TX (2)USDA-ARS Conservation and Production Research Laboratory, Bushland, TX (3)Acclima, Inc., Meridian, ID
Signal degradation in coaxial cables and interconnects is a long-standing problem in the practical deployment and calibration of time domain reflectometry (TDR) for soil water content monitoring. Acclima, Inc. has recently commercialized a low-cost TDR sensor (TDR-315) with all electronics required for waveform acquisition embedded in the probe head thereby avoiding signal degradation. Firmware acquires pertinent waveform features and calculates apparent permittivity (Ka), bulk electrical conductivity (σa), and water content values that are transmitted to the data logger via SDI-12 communications. We calibrated ten TDR-315 sensors and conventional TDR for Ka and σa measurements using acquired waveforms in air, deionized water, and electrolytic solutions of CaCl2. Additionally, soil water content calibrations were carried out for TDR-315 probes and conventional TDR using a Pullman clay loam soil. Lastly, a column displacement experiment using Pullman clay loam and electrolytic solutions of CaCl2 (0.25 and 7.3 dS m-1) was carried out to evaluate the sensitivity of Ka to σa (0.65 to 2.8 dS m-1) for both measurement technologies. Permittivity calibrations of the TDR-315 could be accomplished with conventional methods of waveform interpretation used for TDR. Conventional calibration of σa using long time amplitudes yielded a linear response for σa ≤ 0.3 S m-1 above which the response was nonlinear; but successfully accounted for by the firmware conductivity calibration. The fitted water content calibrations of the Pullman clay loam using the acquired waveforms and firmware-reported Ka of the TDR-315 were nearly indistinguishable from conventional TDR calibrations. 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. Firmware-calculated Ka was satisfactory at σa < 2 dS m-1compared with estimates evaluated using full waveforms and could be improved at greater conductivities by sampling at finer time resolutions.