Estimating spatially varying severity thresholds of a forest fire danger rating system using max-stable extreme-event modeling

Fire danger indices are used in many countries to estimate the potential fire danger and to issue warnings to local regions. The McArthur fire danger rating system is used in Australia. The McArthur forest fire danger index (FFDI) uses only meteorological elements. It combines information on wind speed, temperature, relative humidity, and recent rainfall to produce a weather index of fire potential. This index is converted into fire danger categories to serve as warnings to the local population and to estimate potential fire-suppression difficulty. FFDI values above the threshold of 75 are rated as extreme. The spatial behavior of large values of the FFDI is modeled to investigate whether a varying threshold across space may serve as a better guide for determining the onset of elevated fire danger. The authors modify and apply a statistical method that was recently developed for spatial extreme events, using a "max-stable" process to model FFDI data at approximately 17 000 data sites. The method that is described here produces a quantile map that can be employed as a spatially varying fire danger threshold. It is found that a spatially varying threshold may serve to more accurately represent high fire danger, and an adjustment is proposed that varies by local government area. Temporal change was also investigated, and evidence was found of a recent increase in extreme fire danger in southwestern Australia.