Wildland Fire Susceptibility Index
As used in the Florida and Southern Fire Risk Assessment, the Wildland Fire Susceptibility Index is a value developed to represent an index related to the probability of an acre burning. The determination of an acre burning integrates the probability of an ignition and expected final fire size, the latter being affected by rates of fire spread in four weather categories and fire suppression effectiveness.
Fire Occurrence (Fire Occurrence Areas)
The first task to determine the WFSI is to determine the probability of an acre igniting. A Fire Occurrence Area (FOA) is an area where the probability of each acre igniting is the same. The historical fire ignition locations for a defined period of time are used. Pictorially, if one were to locate the point location for historic ignitions on a map of an FOA, the points would appear with randomly dispersed densities different from adjacent FOAs.
A grid illustrating the probability of a wildfire igniting was developed using ArcMap by analyzing the location of historic ignitions from 1997 to 2003. Fire occurrence rates in Fire Occurrence Areas were described as the number of fire ignitions per 1,000 acres per year. A surface grid with fires per 1,000 acres per year was generated using a spatial filtering calculation available in ArcMap. FOAs were developed to identify areas where the probability of a fire igniting was similar. Hence, within an FOA, the probability of each acre igniting is the same.
An example of a FOA map for Flagler County in Florida is shown in Figure 1.
Weather Influence Zones
To determine an estimate of fire spread upon fire ignition using a fire behavior model, environmental conditions are needed so that fuel moisture and wind speed values can be used in the fire behavior models. To determine these environmental conditions, areas of uniform weather conditions were defined and the weather conditions within each area determined. A Weather Influence Zone (WIZ) is an area where the weather on any given day is uniform. A fire weather meteorologist developed 20 Weather Influence Zones in Florida, and these are displayed in Figure 2.
Development of Percentile Weather Values
Within each WIZ, daily weather data is gathered for a defined period of time. This data was gathered from land management agency weather stations (National Fire Danger Rating System (NFDRS)) and from National Oceanographic and Atmospheric Administration (NOAA) maintained weather stations. A computer program developed by research meteorologist Dr. Scott Goodrick (Forestry Sciences Laboratory, 320 Green Street, Athens, GA 30602-2044) was used to change weather observations from NOAA stations to NFDRS standards. Another program developed by Dr. Goodrick was used to georeference the weather observations from the weather stations within a WIZ to the geographical center of the WIZ. Hence, one weather data set was developed with a weather observation for each day during the defined time period for each WIZ. From this weather data set, percentile weather was developed for each WIZ.
The weather observation data set was checked for errors and then imported into the USDA Forest Service’s FireFamilyPlus program. The NFDRS index Spread Component (SC) was calculated for each day. The fire season was set for each WIZ and the SC calculated using the NFDRS fuel model G. Fuel model G is used as it contains fuel loading in all of the dead (1-h, 10-h, 100-h and 1000-h) and live (herbaceous and woody) fuel categories. This allows for the influence in the Spread Component calculation of the fuel moisture values from all of the fuel categories. In addition, climate class 3 (subhumid/humid) and slope class most applicable to the WIZ were used.
The Spread Component was then divided into four commutative percentile categories: Low (0-15 percent), Moderate (16-90 percent), High (91-97 percent), and Extreme (98-100 percent). The median SC was determined for each category. The environmental values for 1-h, 10-h, 100-h timelag fuel moisture, live herbaceous fuel moisture, live woody fuel moisture, and the 20-foot, 10-minute average wind speed were determined as the average of the respective values on days when the SC was equal to the median SC. This allows for the determination of four percentile weather categories with the percent of occurrence of each category and with environmental values to define the weather conditions within each category.
Probability of a Fire Occurrence within Each FOA by Percentile Weather Category
We allow for the possibility that the higher percentile weather categories may be relatively more conducive to generating fire ignitions from ignition initiating sources. That is, if 15 percent of the days during the fire season are in the Low Percentile Weather Category, one cannot assume that 15 percent of the fires during the fire season will occur on the days in this percentile weather category. Four percentile weather categories were developed: Low, Moderate, High, and Extreme. The percent of days within each is 15 percent, 75 percent, 7 percent, and 3 percent, respectively.
Each fire within the fire occurrence database for all agencies within a Weather Influence Zone has a fire start date. Each historic fire was assigned a Spread Component based on the fire’s start date from the results of the FireFamily Plus runs. The four percentile weather categories were also developed using the same assumptions for SC, and the four categories have SC ranges. Hence, a correlation is made assigning each historic fire to one of the four percentile weather categories. From these assignments, the proportion of fires that occurred in each percentile weather category by WIZ was determined. For Florida, 14.2 percent, 74.1 percent, 8.1 percent, and 3.5 percent of the fires started within the Low, Moderate, High, and Extreme categories, respectively.
The probability of a fire within an FOA for each percentile weather category is the product of the total fire occurrence rate in the FOA by the proportion of fires within each percentile weather category.
Encyclopedia ID: p3502
