Determining landscape fine fuel moisture content of the Kilmore East ‘Black Saturday’ wildfire using spatially-extended point-based models

Abstract

Fuel moisture is the most dynamic component of bushfire fuels. It varies rapidly both spatially and temporally and plays a significant role in determining the behaviour and spread of bushfires, particularly through combustibility and ease of ignition of dead fine fuels (i.e. particle diameter <6 mm). The Kilmore East fire in Victoria, Australia, on 7 February, 2009 (“Black Saturday”) was the most destructive bushfire in Australia's history. Its behaviour was characterised by mass spotting (the launch, transport and landing of burning firebrands that ignited new fires downwind of the main fire), an indication of low fine surface fuel moisture content (FMC). However, no direct measurements of FMC were taken during the run of the fire.Existing point-based models of dead fine FMC of grass and forest litter fuels were used in conjunction with historical weather, topographic and fuel data for the fire area to investigate the likely spatial and temporal variation in landscape FMC during the event for the purpose of reconstructing the fire's propagation. To assess model performance, output was compared with the only suitable fuel moisture data collected in the region of the fire, measured at two locations ten days after the event. The model was found to be within 1% under adsorption conditions at one location and within 33% under desorption conditions at the other. The model was also used to study the effect of an intensive five day heatwave in the week prior to the fire event on FMC. In the absence of the heatwave, predicted FMC was 2.5 times higher than with the heatwave, however the effect of increased FMC was short-lived and would not have had a direct impact on the fine FMC during the fire event a week later, although it may have affected live fuels or larger dead fuels.