Design and Field Tests of a Directly Coupled Waveguide-On-Access-Tube Soil Water Sensor

Sensor systems capable of monitoring soil water content can provide a useful tool for irrigation control. Current systems are limited by installation depth and labor, accuracy, and cost. Time domain reflectometry (TDR) is an approach for monitoring soil water content that relates the travel time of an electromagnetic pulse on a waveguide to the water content of the soil. This article discusses the design, installation, lab testing, and field testing of a novel TDR sensor, using a multi-segmented, waveguide-on-access-tube (WOAT) geometry. The segmented WOAT approach allows for ease of assembly and installation of the sensor stack to the desired depth. Additionally, this sensor incorporates circuits embedded in the sensor body itself, directly coupled to the waveguides, eliminating problems associated with cabling encountered in previous WOAT embodiments. Despite some initial problems with mechanical strength of the prototype sensors, the WOAT equipment provided quality TDR waveforms and long-time reflection coefficients on a daily basis at multiple depths over the course of several months, providing data for both soil water content and bulk electrical conductivity estimates. When properly calibrated, the WOAT data were comparable to concurrent measurements of soil water content using a neutron probe, though there were differences due to the disparity in sensing volumes. Overall, we show that this is a promising new sensor design.