Calibration of a Frequency Domain Reflectometry Sensor for Humid Tropical Soils of Volcanic Origin
2000
Veldkamp, Edzo | O'Brien, Joseph J.
Recently, a frequency domain reflectometry (FDR) was developed for measuring soil water content. It has a multivibrator that sends electromagnetic waves along its probes, and it measures the frequency of the reflected wave, which varies with water content. This FDR sensor has several advantages over time domain reflectometry (TDR); it is less expensive, has a lower power consumption, and continuous monitoring of soil moisture at several remote locations is easily automated using dataloggers. Our goal was to derive a calibration function for the FDR sensor with the following criteria: it should be applicable to soils with high clay and organic matter contents and with bulk densities between 0.7 and 1.1 g cm⁻³ We used undisturbed soil samples to account for the natural heterogeneity in soils. Our results show that the calibration functions derived from a three-phase mixing model performed better than the manufacturer's empirically derived function for the soil volumetric content (θ) range of 0.45 to 0.70 m³ m⁻³ Separate values of the geometry parameter (α) and of the specific output period for soil matrix (Perₛ) were established both for the topsoil (0–0.5 m depth) and for the subsoil (>0.5 m depth). The manufacturer's calibration function underestimated the soil water content by up to 0.15 m³ m⁻³ The three-phase mixing model uses a physical basis for the derivation of the calibration function in that the soil porosity is used for volumetric partitioning among soil components. This physical basis renders the calibration function widely adaptable.
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