Moisture diffusivity and response time in fine forest fuels

Abstract

Fine forest fuels, such as grasses, hardwood leaves, and conifer needles, vary greatly in response times and mean moisture diffusion coefficients when exposed to desorption and adsorption conditions. Results are reported for tests made with recently dead and weathered dead fine forest fuels and small woody samples. Test conditions were 26.7 °C (80°F) with changes in relative humidity from 90 to 20% and back, in an environmental chamber. Moisture diffusivities of fine forest fuels were found to be smaller than diffusivities of woody samples. The diffusivities of the foliage and grass fuels tested ranged from near 1.0 × 10−10 to 1.0 × 10−8 cm2/s, whereas the woody fuels ranged from 1.5 × 10−7 to3.0 × 10−5 cm2/s. Weathered fine fuels had faster response times and higher diffusivities than recently cast materials. Adsorption response times were longer and diffusivities lower than for fuels in desorption. Response times of various recently dead fine fuels ranged from 0.2 to 37 h and weathered fuels from 0.5 to 10 h. Therefore, specific fuel types need to be tested to assign more precise response times. Under the drying conditions of 26.7 °C and 20% relative humidity, fine forest fuels had lower diffusivities and longer response times than anticipated in the United States National Fire Danger Rating System. As a result, predicted fire danger during or after a weather change may be overestimated because fuels are responding more slowly than anticipated. Equations are presented for making first estimates of response time and (or) diffusivity if certain physical properties are known: surface area-to-volume ratio, packing ratio, and bed depth.