A simple method for assessing fuel moisture content and fire danger rating

The flammability of wildland vegetation is strongly dependent upon the moisture content of fine dead fuels. Consequently, assessing the moisture content of these fuels to within a reasonable degree of accuracy is an important part of wildland fire management. Estimates of fine fuel moisture content can also be combined with information on wind speed, vegetation type and drought effects to provide a measure of fire potential or fire danger rating. For example, the moisture content of eucalypt litter is an important consideration in determining fire danger rating in the forests of southeastern Australia. This paper describes a simple and intuitive linear index, which provides an equivalent measure of the moisture content of eucalypt litter. Despite its simplicity, the so-called fuel moisture index, defined as FMI = 10−0.25(T−H), where T is temperature and H is relative humidity, is shown to be remarkably effective at reproducing the results of a more sophisticated model for the moisture content of eucalypt litter. In previous work the fuel moisture index was combined with information on wind speed U to provide a simple fire danger index F. Under the restrictive assumption of constant fuel availability, F was shown to compare favourably with other measures of fire danger rating that feature in the literature. In this paper we extend the definition of F to incorporate the effects of variable fuel availability. This is done very simply by multiplying F by the drought factor to obtain the index FD. The drought factor, which is determined through knowledge of antecedent rainfall, is used to describe fuel availability in forest fuel types. The utility of the index FD is tested by comparing its value with the McArthur Mark 5 Forest Fire Danger Index (FFDI) using data from two automatic weather stations. Moreover, by converting grass curing, which is used to describe fuel availability in grassland fuels, to an equivalent drought factor the index FD is also compared with the McArthur Mark 4 Grassland Fire Danger Index (GFDI). The comparisons indicate that FD provides a plausible measure of fire danger rating for forest fuels and grassland fuels in particular. The structure of the two McArthur indices with respect to the fuel moisture index is also investigated. It is shown that when viewed in terms of the (FMI, U) phase plane, the McArthur indices have a very simple geometric characterisation: constant FFDI corresponds to straight lines and constant GFDI corresponds to cubic curves in the (FMI, U) plane. This confirms that FMI is a unifying variable which permits a simpler conceptualisation of fire danger rating, at least as it is treated in the McArthur schemes. Hence, characterising fire danger rating in terms of FMI could have a pedagogical advantage over other methods and could provide fire management personnel with a simpler and more intuitive method for assessing fire potential.