Multi-index’s assessment of the two 2017 major wildfires in Portugal

Forest fires are one of the most severe natural disasters that periodically affect Mediterranean countries, as well as other countries with Mediterranean-like climates, such as the southwestern regions of the USA, or in more dry climates like in Australia. Forest fires though part of a natural forest renewal process, since they reduce the existing build-up load and thus the intensity of future fires, when frequent and assuming large-scale proportions have detrimental impacts on biodiversity, agroforestry systems, soil erosion, and economy. Portugal (PT) has a high inter-annual fire season variability and endures extreme forest fires, with a record of burned area in 2017 around 500 kha. These extreme wildfire events (EWE) concentrated in few days but with high burned areas, are linked to severe fire weather conditions.

In this study a comparison between several fire danger indices is performed for a reference period 2001‒2021 and 2017 (May‒October) for the Fire Weather Index (FWI), Continuous Haines Index (CHI), Keetch-Byram drought index (KBDI), Burning Index (BI) and Fire Danger index (FDI). A daily analysis for the so-called Pedrogão Grande wildfire (June 17^th) and the October major fires (October 15^th) included the Spread Component (SP) and Ignition Component (IC). Results revealed high above average values for all indices for 2017 in comparison with 2001‒2021 particularly, for October. High statistically significant monthly correlations between FWI, FDI and BI were found, along with lower between FWI and CHI as expected. These correlations are depicted in the spatial patterns between FWI and FDI for the two EWE. Furthermore, the spatial distribution of FDI has the best performance in capturing the locations of the occurrence of the two EWEs’. Though the remaining indices are informative, they lack some accuracy that can be achieved with a calibration procedure.

Overall, the implementation of a multi-index’s methodology might be a highly relevant tool for PT, whose complex orography and land cover, along with the projected increase in temperatures and intensification of drought conditions will lead to an increase in conditions prone to the occurrence of EWE. The outcomes allowed to conclude, that since fire danger depends on several factors a multi-index’s diagnosis is highly relevant, though calibration and scale adjustment are needed for some indices. The implementation of a Multi-index’s Prediction System should be able to further enhance the ability of tracking and forecast unique EWE, since the shortcomings of some indices are compensated by the information retrieved by others. Overall, a new forecast system can help ensuring the development of appropriate spatial preparedness plans, proactive responses by the civil protection regarding firefighter’s management, suppression efforts to minimize the detrimental impacts of wildfires in Portugal.

Funding: This research is supported by National Funds by FCT—Portuguese Foundation for Science and Technology, under the project UIDB/04033/2020.