Fuel age, weather and burn probability in Portugal

Abstract

The relative influence of the factors acting upon burn probability, namely fuel and weather, is not well understood, especially in Europe. We use a digital fire atlas (1975-2008) and apply survival analysis to individual fires (1998-2008) to describe how burn probability changes with fuel age in Portugal. The typical FRI (fire return interval) and median fire-free interval vary regionally from 23 to 52 and 18 to 47 years, respectively. Increase of the hazard of burning with time is generally near linear, denoting moderate fuel-age dependency as in some other shrub-dominated Mediterranean environments. Analysis of complete fire intervals resulted in shorter FRI and higher fuel-age dependency of burn probability than findings that included censored observations. Increasingly severe weather conditions either expressed through fire size or by extreme fire danger, concurrently decreased fuel-age dependency and selected for older fuels. The results are discussed from the viewpoints of fire suppression and fuel treatments.