Anticipating potential wildfire size from fine fuel moisture content

An empirical relationship is developed to estimate potential wildfire size (SWF), i.e., the maximum SWF a fire could attain for a given set of weather and fuels conditions. We compiled a database of all large wildfires (>500 ha) in mainland Portugal in the 2001-2024 period (n = 935) and modelled potential SWF from the fine fuel moisture content (Mw) of shrubs, as most surface fuel complexes in the country are shrub-dominated. Mw is a weighted average of dead (Md) and live fine fuel moisture content (Ml). We tested the performance of this relationship using predictors other than Mw, namely Md and Ml independently, wind speed, wildfire duration, and the six components of the Canadian Forest Fire Weather Index (FWI) System. Mw accounted for 78% of variability in potential SWF, which was higher than obtained by using any of the other variables, although Ml performed similarly well while, perhaps counterintuitively, Md performed poorly. Hence, we found that potential SWF is driven mainly by Ml. Mw can be easily assessed from measurements or wall-to-wall estimates; thus, albeit simple, the model can be useful in assisting fire management operations.