Time and frequency domain reflectometry for the measurement of tree stem water content: A review, evaluation, and future perspectives

A New Non-Inserted and Portable FDR Instrument for Measuring Surface Soil Water Content

“…The critical technique of the instrument was an improved adjustable high-frequency oscillation method [21], based on the FDR principle [17,19]. In this study, the maximum resonance frequency was monitored by considering the soil as a capacitor, and the normalized resonance frequency (reflecting the soil dielectric constant) was related to the target SWC.…”

“…To conduct multiple linear regression analysis, the Pearson correlation analysis was used to evaluate the correlation coefficient between the NF and SWC. The formula for calculating the correlation coefficient is shown in Equation (1) [17]: This method was first used to measure the SWC of different soil depths by adjusting the distance between the oscillating circuit electrodes and the starting frequency of the vibration reference. The main advantages of this device are that it is non-inserted, portable, and does not break the soil structure.…”

“…To conduct multiple linear regression analysis, the Pearson correlation analysis was used to evaluate the correlation coefficient between the NF and SWC. The formula for calculating the correlation coefficient is shown in Equation (1) [17]:…”

“…Among them, monitoring SWC at a point scale (less than 0.01 cm 2 ) has advanced with various in-situ sensors, and the FDR method has been widely used [16]. The FDR method can measure the soil apparent dielectric constant using Water 2021, 13, 2712 2 of 8 the electromagnetic pulse principle, according to the frequency of the electromagnetic waves propagating in the soil [17][18][19][20]. Because soil dielectric content is related to the SWC, the dynamic SWC can be calculated using the relationship curve generated by the FDR method.…”

“…Because soil dielectric content is related to the SWC, the dynamic SWC can be calculated using the relationship curve generated by the FDR method. Most previous studies have suggested that FDR instruments are convenient for measuring the SWC [17,19]. However, the FDR method requires instrument sensors to be inserted into the soil to obtain the SWC data while breaking the original soil structure at the point scale.…”