Evaluation of a system for automatic dead fine fuel moisture measurements

Abstract

Dead fine fuel moisture content is a key parameter for wildfire ignition and behaviour: the higher the fine fuel moisture content, the more activation energy has to be spent on the evaporation of this moisture before the fuels can ignite and release their energy of combustion. Thus, high fuel moisture leads to a lower probability of ignition and a more moderate fire behavior. These facts have been recognized for a long time and fine fuel moisture has become an essential part of several fire danger rating systems (e.g. 1- and 10-hour dead fuel moisture in the National Fire Danger Rating System (NFDRS) and Fine Fuel Moisture Code in the Canadian Forest Fire Danger Rating System (CFFDRS)). Additionally, dedicated fine fuel moisture models are available as well. Nevertheless, direct information on dead fine fuel moisture can be valuable for monitoring, scientific research as well as during exceptional conditions, e.g. extreme fire danger with exceptional temporal or spatial distribution of fire danger, for the management of large wildfires. However, dead fine fuel moisture measurements are hard to obtain since common methods involve manual sampling in the field and potentially oven-drying, adding a time delay before the results become available. In this study, a thorough evaluation of the Campbell Scientific, Inc. instrumentation for automated 10-hour dead fuel moisture (CS506-L fuel moisture sensor) has been carried out. Ten fuel moisture sensors and associated Ponderosa pine dowels were obtained and subjected to tests in a constant climate chamber as well as at two field sites in Central Europe for one fire season. The comparability of the values determined from these ten different dowels as well as a potential influence of weathering were investigated along with correlations to manual-gravimetric fuel moisture. Results show that dowel-to-dowel comparability is relatively good but may be further increased by additional calibration. In the field, there was an excellent correlation between 10-hour fuel moisture as measured by the CS506-L sensors and fast-drying dead fuels such as needle litter and dead grass (R² 0.81 and 0.82, respectively). Weathering effects during the study period had no major influence on sensor performance.