SOIL-MOISTURE MEASUREMENT USING PORTABLE DIELECTRIC PROBES AND TIME DOMAIN REFLECTOMETRY

Portable dielectric probe (PDP) and time domain reflectometry (TDR) instruments are increasingly being used for rapid in situ determination of soil moisture contents. These techniques provide alternatives for acquiring soil moisture data in field experiments where the logistics of gravimetric sampling can restrict the number of samples obtained. The real part of the dielectric constant of soils, measured by P, L, C, and X band PDPs, was empirically related to soil water content for soils with a wide range of textures. The results were compared to TDR measurements as well. The P and L band probes and the TDR produced very comparable and accurate results, while the C and X band probes showed sensitivity to variability within a soil, the measurement technique, and soil texture. Both TDR and PDP instruments may be useful for field measurement campaigns, as they sample different depths and volumes. In general the TDR is suitable for 0-5 cm or deeper layers while the PDP can be used for measuring layers of the order of 1 cm in thickness.