The Influence of Fuel, Weather and Fire Shape Variables on Fire-Spread in Grasslands

Fire-spread was measured on 121 grass fires in a 2500 ha experimental site in the Northern Territory, Australia. Selected plots were harvested to alter the height, load and bulk density of the fuel-bed. Fires were lit from a line and allowed to travel up to 400 m down-wind. Fire-spread was correlated with fuel, weather and fireshape variables using multiple regression techniques. Wind speed had most effect on fire-spread. The influence of the other variables was examined after a model for wind speed and moisture content had been fitted. Fuel load did not influence fire-spread. Fires in natural swards burnt 18% faster than fires in cut grass, but this increase could not be fully explained by changes in the height or bulk density of the fuel bed. Grass type characterised either by species group or by surface-area-to-volume ratio of the fuel particle, did not appear to significantly influence fire-spread. Differences in spread rates between the two grasses were attributed to differences in grass curing. The influence of grass curing appeared to be less than indicated by published models. Models of fire-spread in grasslands currently in use need to be revised. Ignition line length was a significant variable influencing fire-spread and this must be taken into consideration when using experimental fires to validate theoretical models or develop empirical models from field observations.