The effect of fuel moisture content on the spread rate of forest fires in the absence of wind or slope

Most studies on the effect of fuel moisture content (FMC) on forest fire behaviour focus on dead fuel moisture; mechanisms of fire spread in live vegetation are considered to remain unexplained by current theory and modelling. In this work, an empirical model for quantifying the effect of FMC on the ratio between spread rate and fuel bed height of fires in the absence of wind or slope was proposed. The model was fitted using data from laboratory experiments, carried out in fuel beds representative of natural litter and shrubland fuel complexes in a wide range of FMC (6-179%), and tested against data from field experiments and wildfires. The pattern of spread rate variation with FMC, namely its reduced rate for values above similar to 80%, was explained by the ratio between fuel low heat content and energy required for ignition.