Measurement of the Dielectric Properties of Wyoming Soils Using Electromagnetic Sensors

We developed soil-specific calibrations for a time domain reflectometry (TDR) sensor, a water content reflectometer (WCR), and an impedance sensor for 19 soil moisture monitoring sites in Wyoming. The main objective was to calibrate the WCR sensors that form a statewide distributed soil moisture network. A secondary objective was to use the TDR and impedance sensor data to better understand the WCR readings. All calibrations were conducted in the laboratory by mixing different amounts of water in the soils to obtain a range of soil water contents and by packing the soils in a bucket. The observed apparent permittivity as a function of sod water content was relatively high for the WCR and impedance sensors (up to 187 and 150% higher, respectively, than TDR). This was attributed to the relatively low operating frequency (< 175 and 50 MHz, respectively) of the WCR and impedance devices. Soil texture had little impact on sensor readings. Differences between soils for the WCR and impedance sensors were due to differences in bulk soil electrical conductivity (EC) and dielectric relaxation losses. Relaxation losses were found to be significant at 50 MHz for all soils. Regression equations (R-2 > 0.643) were developed for the WCR and impedance sensors that relate the slope of the soil water content vs. permittivity relationship to the bulk soil EC (WCR and impedance sensors) or the imaginary permittivity (impedance sensor).