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

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 (theta) in permeable media, but material-specific calibration functions should be developed. This study aimed at developing relationships between the bulk dielectric permittivity (root epsilon(b)) and theta in woodchips considering their particle size distribution (PSD), temperature (T) and the dielectric permittivity of the dry material plus air (root epsilon(sa)). The root epsilon(b) of fine, medium and coarse PSDs were measured with CS616-L water content reflectometers up to theta of 0.83 cm(3)/cm(3) and at T (degrees C) of 10 degrees, 24 degrees, 37 degrees, 55 degrees and 70 degrees. 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 (root epsilon(sa) similar to 1.09); while medium (root epsilon(sa) similar to 1.09) and coarse (root epsilon(sa) similar to 1.03) PSDs were more influenced by T and the presence of large air voids (R-2 similar to 0.92; RMSE - 0.06 cm(3)/cm(3)). A modified calibration equation independent of PSD (root epsilon(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 theta than previously published functions and they are applicable within the range of T found in field applications.