A generic fuel moisture content attenuation factor for fire spread rate empirical models

Abstract

Aim of study: To develop a fuel moisture content (FMC) attenuation factor for empirical forest fire spread rate (ROS) models in general fire propagation conditions. Methods: The development builds on the assumption that the main FMC-damping effect is a function of fuel ignition energy needs. Main results: The generic FMC attenuation factor was successfully used to derive ROS models from laboratory tests (n = 282) of fire spread in no-wind and no-slope, slope-, and wind-aided conditions. The ability to incorporate the FMC attenuation factor in existing field-based ROS models for shrubland fires and grassland wildfires (n = 123) was also positively assessed. Research highlights: Establishing a priori the FMC-effect in field fires benefits the proper assessment of the remaining variables influence, which is normally eluded by heterogeneity in fuel bed properties and correlated fuel descriptors. Additional keywords: fire behaviour; fire management; live and dead fuels; experimental fires; wildfires. Symbols used: a, b (fitted coefficients); c (specific heat, kJ kg-1 ºC-1; subscripts: f, fuel; w, water); fM (fuel moisture content attenuation factor); h (fuel bed height, m); M (fine fuel moisture content, %; subscripts: d, dead fuels; l, live fuels); Q (heat per unit mass of fuel needs, kJ kg-1; subscripts: i, fuel ignition; w, water evaporation); R (fire spread rate, m min-1; subscripts: 0, no-wind and no-slope; S, slope-driven; U, wind-driven); RH (relative humidity, %); S (slope angle, º); T (temperature, ºC; subscripts: a, air; f, fuel; i, ignition; v, vaporization); U (wind speed, km h-1; subscript indicates measurement height, m); w (oven-dry fuel load, kg m-2); ρb (fuel bed density, kg m-3).