Calibration of a frequency-domain reflectometer for determining soil-water content in a clay loam soil

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:37-4