Exploring the Combination of Fire Danger Indices and Their Persistence in Predicting Favorable Conditions for Forest Fires

: Wildfires are one of the major natural hazards in various regions of the globe, such as California in the United States [1], Australia, and in the Euro-Mediterranean region [2]. They have devastating impacts not only on local economy, agroforestry systems and environment, public health but also due to the loss of human lives. Portugal has the highest density and relative burned area of all European countries and in 2017 more than 100 people have died on 17 June and 15 October; therefore, it is highly relevant to invest in its prevention. The FWI (Fire Weather Index) index has been used since the 1980s in Portugal to predict the danger of forest fire, and more recently the CHI index (Continuous Haines index) has also been used. FWI translates weather conditions at the surface that are conducive to forest fires, while CHI translates weather conditions of instability or dryness in the atmosphere in a layer close to the surface. FWI values greater than 38.2 indicate a very high to maximum wildfire danger. Regarding CHI, values equal to or greater than 10 points to a risk that, in case of an occurrence of a wildfire, it will be uncontrolled and therefore very challenging to control. The persistence of these conditions increases the danger. The purpose of this study is to investigate the spatiotemporal patterns of the combination of the FWI and CHI danger indices, among other variables related to soil conditions, such as temperature in different levels, evaporation, leaf area index among others, in Portugal and to assess possible relationships with the persistence of danger conditions and the burnt area using 2003 and 2017 as control years.