Dielectric permittivity-water content relationships in woodchips: Particle size and temperature effects

Abstract Information on the amount of water held by woodchips in the field is needed to assess environmental risks when they are kept in stockpiles or for inferring their efficiency as denitrifying bioreactors. Reflectometry is a widely used method for estimating volumetric water content ( θ ) in permeable media, but material-specific calibration functions should be developed. This study aimed at developing relationships between the bulk dielectric permittivity ( e b ) and θ in woodchips considering their particle size distribution (PSD), temperature ( T ) and the dielectric permittivity of the dry material plus air ( e sa ). The e b of fine, medium and coarse PSDs were measured with CS616-L water content reflectometers up to θ of 0.83 cm 3 /cm 3 and at T (°C) of 10°, 24°, 37°, 55° and 70°. Calibration equations were developed using a multivariate power law function fitted to the data with a hierarchical Bayesian inference procedure. The best fit ( R 2  = 0.96; RMSE =0.04 cm 3 /cm 3 ) was obtained for the fine PSD ( e sa  ∼ 1.09); while medium ( e sa  ∼ 1.09) and coarse ( e sa  ∼ 1.03) PSDs were more influenced by T and the presence of large air voids ( R 2  ∼ 0.92; RMSE ∼0.06 cm 3 /cm 3 ). A modified calibration equation independent of PSD ( e sa  = 1) reduced only slightly the goodness-of-fit ( R 2  = 0.90; RMSE =0.07 cm 3 /cm 3 ). The equations developed in this study are well suited for field applications as they cover a wider range of θ than previously published functions and they are applicable within the range of T found in field applications.