UNCERTAINTY EFFECTS ON SOIL ELECTRICAL CONDUCTIVITY AND PERMITTIVITY SPECTRA

Abstract

Soil permittivity and electrical conductivity values are usually reported without error bars, which limits comparisons among different studies using different equipment. Uncertainty in measured permittivity spectra could be due to the measurement system (hardware) or sample-to-sample variation. There is also uncertainty in parameters fitted from the spectra. The objective of this study was to determine the measurement uncertainty on permittivity and electrical conductivity spectra. Uncertainty related to fitting parameters, instrument, aggregate size density, and soil were considered. Different aggregate sizes of three soils were packed into a truncated coaxial cell. Electrical conductivity and permittivity spectra were calculated from reflection scattering parameter, S 11 , measured by a vector network analyzer. Measurement uncertainty was around 10 times greater than fitting uncertainty, and one-third to three-fourths that of sample-to-sample uncertainty. The square root of the apparent permittivity significantly related most strongly with soil water content but also with soil differences (bound water) and bulk density (partial R 2 of 0.71-0.74, 0.09, and 0.03-0.04). Derived direct current electrical conductivity was also significantly related to soil water content, bound water content, and bulk density, but also to aggregate size (partial R 2 of 0.53-0.54, 0.05, 0.03, and 0.02). Derived direct current electrical conductivity leveled off at low water contents, thus providing a water content. The cutoff water content was significantly related to bound water and aggregate size. The effect of aggregate size on electrical conductivity spectra and derived factors, although significant was small compared with water content effects. The greatest variation was sample to sample within a soil and among soils.