Abstract
A soil-water frequency domain reflectometry sensor, the ThetaProbe, was evaluated for its ability to measure the apparent soil dielectric constant and subsequent estimation of soil-water content. The soil-water content of a clay-loam soil, determined using factory-supplied parameters for the sensor and soil-estimated parameters, was compared to the soil-water content determined in the laboratory. The range in soil-water content was from 0.20 to 0.42 m3·m-3. A total of 78 soil samples from the 0 to 600 mm depth of a clay loam soil were used for these comparisons. There was a good correlation between sensor soil-water content determined using the factory-supplied parameters and the gravimetric soil-water content. Use of both the factory-supplied and the soil-estimated parameters resulted in more than 20% overestimation of soil-water content compared to the gravimetric soil-water content. However, using a recalibration process, the adjusted soil-water content was within 0.02 m3·m-3 for both the factory-supplied and the soil-estimated calibration constants. Soil bulk density, clay content and temperature had negligible influence on sensor soil-water contents. Water SA Vol 32(1)pp:37-42
Highlights
Knowledge of soil-water content is important for water management and hydrological studies and for calibration and validation of soil-water balance models
The frequency-domain reflectometer (FDR) method used in the present study makes use of radio frequencies and the electrical capacitance of a capacitor for determining the dielectric constant and the soil water content
Where: ao is the square root of the dielectric constant of dry soil (√ε0) where ε0 is the dielectric constant for dry soil) calculated using the corresponding measured voltage output and Eq (1) a1 is the difference between the square root of the dielectric constant of saturated (√εw) and that for dry soil divided by soil-water content at saturation: a1 = (√εw - √ε0 - )/θvs
Summary
Knowledge of soil-water content is important for water management and hydrological studies and for calibration and validation of soil-water balance models. The laboratory methods for determining soil-water content gravimetrically and pressure plate soil-water potential fail to satisfy this requirement, they are still used for calibration purposes. Dielectric-based soil-water content techniques are influenced by factors that affect the dielectric constant of the soil other than water. The frequency-domain reflectometer (FDR) method used in the present study makes use of radio frequencies and the electrical capacitance of a capacitor (formed by using the soil and embedded rods as a dielectric) for determining the dielectric constant and the soil water content. In the case of capacitance-type sensors, such as that used by Grooves and Rose (2004), the charge time of a capacitor is used to determine the soil-water content. Profile-probe versions using FDR and capacitance methods are commercially available (Whalley et al, 2004; Czarnomski et al, 2005; Mwale et al, 2005)
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