History of the National Fire Danger Rating System

In 1954 there were eight different fire-danger rating systems in use across the country. Fire control conferences called by the United States Forest Service highlighted need for a nationwide system. Improved communication and transportation systems made mutual assistance agreements between fire control agencies more practical than in the past. State compacts, and in the case of the Federal government, interagency and interregional agreements were bringing fire control teams together from widely separated areas of the country. The increased cooperation required a national system for estimating fire danger and fire behavior to improve and simplify communications among all people concerned with wildland fires.

Work on a national rating system began in 1959. By 1961, the basic structure for a four-phase rating system (ignition, risk, fuel energy, and fire spread) had been outlined and the fire spread phase was ready for field testing. Adaptations, interpretations, and additions to the fire spread phase quickly followed, making it obvious that the fire spread phase was not uniformly applicable across the country.

A new research project was initiated in 1965 with the goal of developing a complete, comprehensive, National Fire-Danger Rating System by 1972. In 1970, a preliminary version of the system was field tested in Arizona and New Mexico. In 1971, an improved version of the system was used operationally in the Southwest. Field trials were also conducted elsewhere across the country at stations from Maine to California and from Florida to Alaska. The National Fire Danger Rating System (NFDRS) was implemented in 1973, and then revised during a three year project from 1975-1978 and reissued as the 1978 NFDRS (NOAA 2003).

The 1978 NFDRS

1. The system only considered “initiating fire” which is fire that is not behaving erratically and does not have spotting or crowning.

2. Ratings of difficulty of containment were based solely on fire behavior. Other factors affecting difficulty of containment (accessibility, soil and resistance to line construction, and so on) were to be assessed by other means.

3. Fire behavior, defined in part by flame length at the fire front, was assumed to be directly related to the difficulty of containment.

4. Fire danger rating were based on the “worst” fire conditions by using meteorological measurements when fire behavior is normally the most extreme (i.e., in early afternoon, sites in the open, and sites of southerly or westerly exposure).

5. The system provided ratings that were physically interpretable in terms of fire occurrence and behavior. These ratings could then be used alone or in combination.

6. The ratings were relative, not absolute.

• modified responses to drought, with open ended rather than closed index scales, to reflect effect of changing day length on burning conditions and fuel moisture
• separate human and lightning caused occurrence indices
• expanded ignition component and a spread component
• increased number of fuel models from 9 to 20, increased number of slope definitions from 3 to 5, and predictive models for the moisture content of live grasses, forbs, and woody shrubs.

The updated 1978 NFDRS system contained 3 fire behavior components, 5 fuel classes (3 dead, 2 live), and the following 4 indices for rating fire danger:

1. human caused fire occurrence index: this index is derived from an assessment of human caused fire sources in the rating area, and the ignition component which is the likelihood that a firebrand will cause a reportable fire.

2. lightning caused fire occurrence index: this index was derived from the ignition component and lightning risk, an indicator of thunderstorm and lightning activity.

3. burning index (BI): this index was derived from the spread component (a relative index of rate of fire spread), the energy release component (a relative index of the amount of heat released per unit area in the flaming zone of an initiating fire), the live and dead fuel moisture, and the fuel model. The BI indicates the difficulty of containment of a single fire.

4. fire load index (FLI): the BI and the expected number of fires predicted by the human caused and lightning caused fire occurrence indices combined to produce the fire load index which was a measure of the total fire containment job. The FLI was the NFDRS cumulative index integrating risk, ignition probability, and fire behavior potential.

The 1978 system was updated again in 1988 to correct for some deficiencies when applying the model in the eastern United States. These updates improved the capability of the model to respond to drought in humid environments, provided flexibility to reflect the greening and curing of live fuels, corrected the problems of overrating fire danger in the autumn or after rainfall, and adjusted the fuel models to better predict fire danger in humid climates (Burgan 1988).

Although all the original 1978 indices provide specific types of information, the main indices currently used for fire danger rating are the spread component (SC), energy release component (ERC) and burning index (BI). The KBDI has also been incorporated into the NFDRS.

By the summer of 1976 data from more than 800 fire-weather stations across the US were being processed by a central mainframe computer (AFFIRMS), and the system indices and components were calculated each day for over 400 stations. The original computer program for calculating NFDRS has been updated several times. The NFDRSPC, or national fire danger rating system for personal computers, was developed in 1988 to make adjustment for those managers in eastern states who were using the 1988 version of the 1978 NFDRS. This system was essentially a revision to the AFFIRMS computer system with a window/menu-oriented user interface (Donaldson et al. 1990). Since then, PCDANGER, a menu-driven PC- based application has replaced these programs. This new program is beneficial for managers because it provides local flexibility in computing and graphing fire-danger-rating indices and can compute fire danger indices for both the 1978 and 1988 versions of the NFDRS (Bradshaw et al. 1997).