**1. Introduction**

The role of forests—which cover approximately 31% of the global land [1]—is of grea<sup>t</sup> importance in ecological-environmental and socio-economic terms [2]. However, internationally, there has been a radical increase in the annual number of forest fire danger days and forest fire incidents, with climate change being one of the major contributors [3–10]. During the past two years, more than 45 million hectares have been burned across regions in Russia, Brazil, Canada, the United States of America, the European Union, and Australia [11–16], where forested areas cover 56% of the global forest land [1,17,18].

Several studies and reports indicate that most of the forest fire incidents are mandriven, in the form of either arson or negligence [19–24]. Nevertheless, wildfires occur mostly during periods of high temperature, intense drought, strong winds, low relative humidity, and inadequate precipitation [25–27].

Fire danger rating systems and indices are the products of systematic research both in theoretical and in empirical terms. Hence, many environmental fire danger rating systems throughout the world focus on the calculation of the condition of dead or alive fuels, such as fuel moisture codes, alongside meteorological parameters that have an impact on the source of heat as well as the ambient oxygen supply [28–31]. However, these systems ignore the human-driven ignition causes, with the latter being covered to a certain extent in the related literature [19–21,32–38]. Furthermore, only a limited number of studies adopts an

**Citation:** Zacharakis, I.; Tsihrintzis, V.A. Environmental Forest Fire Danger Rating Systems and Indices around the Globe: A Review. *Land* **2023**, *12*, 194. https://doi.org/ 10.3390/land12010194

Academic Editors: Matej Vojtek, Andrea Petroselli and Raffaele Pelorosso

Received: 8 December 2022 Revised: 30 December 2022 Accepted: 31 December 2022 Published: 6 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

integrated and/or holistic approach combining natural and human-driven causing factors as well as weather indices and conditions [39–41].

The objective of this review article is to report, analyze, compare, and evaluate the most applied and well-established environmental fire danger rating systems and indices around the world, aiming at the development of an integrated fire danger rating system for Greece.

#### **2. Materials and Methods**

Adopting the approach by Chuvieco et al. [34], fire risk assessment consists of two pillars: danger and vulnerability. Fire danger—also reported as fire hazard—is related to the conditions that favor the fire outbreak and its spread, while vulnerability is related to the possible outcome of a fire event as far as effects and value loss are concerned [34,42,43].

The present article focuses mainly on systems and indices that estimate fire danger ignition probability related to environmental factors as proposed by Cardille et al. [33]. The present review follows the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 guides for systematic reviewing; thus, the following selection criteria for the considered references were set [44]: (1) studies must be papers published in scientific journals, manuals that are operationally in use, or technical documents supporting fire agency policies; (2) studies must contain systems or indices that focus on fire ignition probability; (3) studies must include systems indices that are not strictly geographically bound; and (4) the indices and systems must include environmental input parameters (such as weather, vegetation, hydrology, and others). Although there are already several research and review articles related to fire danger rating systems [45–50], to the best of our knowledge, the present study is the most complete as far as the number, the geographical scale, and the analysis of the computational procedure of systems and indices are concerned.

The research commenced with the examination of fire danger rating systems currently in use in countries with significant fire history and forest land, such as the USA, Canada, Russia, and Brazil, in the official websites of the respective ministries or agencies. Since the original publications, which describe the systems of the mentioned countries, were gathered, further research was held in the cited literature of the above publications. In addition, online search engines such as Scopus, Google Scholar, WorldWideScience, ScienceDirect, and ResearchGate were used, with the use of the following keywords: "fire danger", "fire danger rating systems", "fire danger indices", "fire ignition probability", "fire danger and remote sensing", "fire danger and drought", and "forest fire danger rating systems", among others. The research was conducted during an eight-month period lasting until July 2022, while the consulted sources were scrutinized according to the following steps: (1) the titles of the studies were compared with the above keywords; (2) those that matched were examined by their respective abstract; and (3) those whose abstracts fulfilled the selection criteria mentioned above were included. Moreover, filters such as "natural caused fires", "risk assessment", and "year of publication" were used.

Systems and indices included in the current study were divided into two groups: (1) the ad hoc fire systems indices; and (2) the indirect indicators. The first one consists of all the systems developed exclusively for fire danger estimation gathered based on geographical criteria, while the second one contains indices that have been proven to be to a certain extent related to fire danger estimation and are divided into drought or moisture presence and into remote sensing indices. All systems and indices involve three major procedures: (1) the collection of the input data; (2) the computational procedure; and (3) the outcome categorization in danger classes. These three procedures were identified and extracted from the collected studies. The first two are described in the next sections. The last one, alongside supporting material from the computational procedure in the form of tables, is included in the Supplementary Material (SM) of the present paper. Tables and sections in the Supplementary Material are cited in the manuscript with the indication "S", followed by the number of the respective table or paragraph.

All systems indices were eventually evaluated based on the cited literature. For validating the accuracy of each system index, the values of the latter were calculated for the two-month period from June–July 2022 using an ad hoc calculating package created in the Python programming language. Hence, the computed values were correlated to days with and without fire occurrences. The principles and the material gathered in the current review are expected to contribute positively to the forest fire science. All parameters not described after the presented equations are included in the Supplementary Material as Section S3. Nomenclature.
