FIELD-EVALUATION OF TIME-DOMAIN REFLECTOMETRY FOR SOIL-WATER MEASUREMENTS

In 3.5m x 5m field plots at three locations, the apparent dielectric constant was measured by time domain reflectometry (TDR) during a soil moisture drying period using probes installed vertically to depths of 15, 30 and 60 cm. Gravimetric water contents were measured for comparison. The three soils investigated were a Jyndevad coarse sand (coarse sandy mixed, mesic Orthic Haplohumod), an Askov sandy loam (coarse loamy to fine loamy mixed, mesic Typic Hapludalf) and a Ronhave sandy loam (loamy mixed, mesic calcareous Typic Agrudalf) having topsoil clay contents of 3, 11 and 14%, respectively. A root mean square error analysis showed that the laboratory calibration method of the authors (described elsewhere) performed better than the generalised calibrations of earlier workers. The new calibrations exist in versions with and without inclusion of an effect of dry bulk density. No difference was found between the two variants. For the field plots, the standard deviation was higher for water contents determined by TDR than by gravimetry (0-15 cm depth). Moreover, the standard deviation increased with increasing clay content, particularly with respect to the TDR measurements. Standard deviations for volumetric water content determined by TDR were in the range 0.005-0.023. The measuring depth (0-15, 0-30 and 0-60 cm) did not affect the variation of the TDR measurements in a consistent way.