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

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 (K a ) and bulk electrical conductivity (s a ) 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 a to s a was examined using a saturated solute displacement experiment with both probe technologies installed in a column packed with Pullman clay loam. A range of s a (0.65-2.8 dS m −1 ) was established by equilibrating the column with 0.25 dS m −1 CaCl 2 and introducing a step pulse of 7.3 dS m −1 CaCl 2 . Permittivity calibrations of the TDR-315 could be accomplished with conventional TDR methods and with similar sampling errors. Conventional calibrations of s a using long time amplitudes yielded a linear response for s a £ 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 s a up to 2.8 dS m −1 .Abbreviations: AWIGF, adaptive waveform interpretation with Gaussian filtering; SDI, serial digital interface; TDR, time-domain reflectometry.

Field assessment of interreplicate variability from eight electromagnetic soil moisture sensors

“…Each of these sensors measures a property that is related to soil permittivity, which in turn is positively associated with θ v . TDR315 is a time domain reflectometer that generates its own EM pulses and analyzes its own waveforms to obtain travel times (Schwartz et al, 2016). CS616 and CS655 are both water content reflectometers that count the average times per second the reflection of the previous generated EM pulse returns to the sensor head to trigger the next generated EM pulse (Seyfried and Murdock, 2001;Kelleners et al, 2005).…”

Performance Assessment of Five Different Soil Moisture Sensors under Irrigated Field Conditions in Oklahoma

“…The impact of soil salinity on sensor readings of soil volumetric water content ( θ v ) (m 3 m −3 ) has been highlighted in several studies [ 15 , 16 , 17 ]. For example, Wyseure et al [ 16 ] reported that θ v error was acceptable at soil bulk electricity conductivity (EC) (dS m −1 ) levels below 2.0 dS m −1 , and Schwartz et al [ 18 ] found that θ v estimates were not affected at bulk EC levels below 2.8 dS m −1 . These thresholds are exceeded in many irrigated areas in arid/semi-arid regions, where there is a great need for improving irrigation management using sensor technologies.…”

Precision Agriculture and Irrigation: Current U.S. Perspectives

“…3, right), which can automatically guide the irrigation system or be modified by the irrigation manager before automatic applica- tion. The infrared sensors and sensor network were commercialized from research prototypes (O'Shaughnessy et al, 2013), and the soil water sensors were also developed with a commercial partner (Evett et al, 2015;Schwartz et al, 2016). This amounts to a 3R concept for irrigation (right place, right amount, and right time) and results in improved crop water productivity for several field crops in the U.S. Great Plains (cotton, maize, sorghum, and soybean) (O'Shaughnessy et al, 2016).…”