Abstract
Recently, it is noticed that the measurement method of soil water content from the dielectric constant of soil, and TDR (time domain reflectometry) method and FDR (frequency domain reflectometry) method are frequently utilized. However, these methods have some problems of cost and of their complex automatic measuring system. ADR (amplitude domain reflectometry) method which measures the dielectric constant of soil at low cost using a simplified impedance measuring technique has been developed to solve these problems and has a similar performance to TDR method and FDR method. In the measurement by the ADR sensor, the amplitude of the voltage standing wave produced on the transmission line is outputted as direct current voltage from the ADR sensor, and θ is calculated from the voltage. ADR method is very effective for measuring soil water content because of the easiness of automatic or multiplexing monitoring and the possibility of the use in deep underground.
In this study, calibrations with the soil moisture sensor based on ADR method were carried out by laboratory experiments and field measurements, and the applicability of the ADR sensor to the measurement of soil water content was examined.
The calibration curves with the ADR sensor were obtained applying the theory that the relationship between the volumetric water content θ and the square root of dielectric constant √ε can be approximated by a linear equation, and these calibration characteristics can be obtained easily with only two parameters, i. e., the slope and the intercept of the curve. The calibration characteristics between√εand θ obtained in this study are almost matched to that obtained for mineral soil by Miller & Gaskin (1996) and that obtained by White et al. (1994).
