WET Sensor Performance in Organic and Inorganic Media with Heterogeneous Moisture Distribution

Interest in electronic monitoring of soil water has grown as increased demand for water creates a greater need for effective water management. Relatively inexpensive commercial soil water sensors that use measured soil dielectric properties to calculate soil water content have been developed to address this need but have received little independent testing of critical calibration procedures and parameters. The WET sensor (Delta‐T Devices, Cambridge, UK) is one such sensor. The two primary objectives in this study were to: (i) determine appropriate calibration parameters for a variety of inorganic soils and organic media; and (ii) determine how WET sensor readings are affected in media with strongly contrasting water contents. We found that the two‐point calibrations suggested by the manufacturer yielded parameters that were not significantly different (α = 0.05) from those suggested by the manufacturer and yielded values consistent with well‐established oven‐dry values. For the organic media, we found that accurate, media‐specific calibration could be obtained for all media tested and that the general form of the calibration equation used for soils applies to organic media. Experiments in a variety of layered media showed that, in contrast to time domain reflectometry results, the WET‐measured dielectric value is the arithmetic average of the two contrasting layers. In soils, the linear correlation between measured and arithmetic average dielectric constants was very high ( R 2 > 0.99) for all four soils tested. This indicates that the upward infiltration calibration technique is generally inappropriate for the WET sensor.