Measuring duff moisture content in the field using a portable meter sensitive to dielectric permittivity

Duff water content is an important consideration for fire managers when determining favourable timing for prescribed fire ignition. The duff consumption during burning depends largely on the duff water content at the time of ignition. A portable duff moisture meter was developed for real-time water content measurements of non-homogenous material such as forest duff. Using circuitry developed from time and frequency domain reflectometry (TDR and FDR) technologies, this sensor measures a change in frequency that is responsive to the dielectric permittivity of the duff material placed in a sample chamber and compressed. Duff samples from four forest cover types—Douglas fir, larch, lodgepole pine and spruce/alpine fir—were used to calibrate the frequency output to volumetric water content. A second-order polynomial (R2 = 0.97) provides the best fit of the data to volumetric water content. The accuracy of the duff moisture meter is ±1.5% at 30% volumetric water content and ±4% at 60% volumetric water content. The volumetric water content can readily be converted to gravimetric water content, which is used more frequently by fire managers and as an input to predictive models of duff consumption.