A number relating to the potential fire incidence within a rating area. The Occurrence Index is derived from the Risk Factor (the degree to which an area will be exposed to man-caused and lightning-caused ignition sources) and the Ignition Component (the likelihood that a spreading fire will result from an ignition source). The man-caused occurrence index (MCOI) and the lightning-caused occurrence index (LCOI) give daily projections of the number of man-caused and lightning-caused fires per one million acres of protected land. Occurrence index values range from 0 to 100 and are scaled such that a value of 100 indicates an expected fire density of 10 fires/million acres

Burning Index

A number relating to the potential amount of effort needed to contain a single fire in a particular fuel type within a rating area. NFDRS uses a modified version of Bryam's equation for flame length - based on the Spread Component (SC) and the available energy (ERC) - to calcuate flame length from which the Burning Index is computed.

The equation for flame length is listed below:

where (j) is a scaling factor, (SC) is the spread component, and (ERC) is the energy release component. Consequently, the equation for the Burning Index is:

where j1 is the Burning Index scaling factor of (10/ft). Therefore, dividing the Burning Inex by 10 produces a reasonable estimate of the flame length at the head of a fire. A unique Burning Index (BI) table is required for each fuel model, similar to the one shown below.

Fire Load Index

A number relating the total amount of effort required to contain all probable fires within the rating area during a specified period of time. The difficulty of containing a single fire as indicated by the Burning Index (BI), multiplied by the probable number of fires projected by the Occurrence Index (OI), gives a measure of the potential fire containment effort on the protection unit for the day. Like the Burning Index, a unique Fire Load Index table is required for each fuel model.

The equation for the fire load index is shown below.

NOTE: The importance of the basic aspects of fire behavior - ease of ignition, rate of spread, and the rate of combustion - cannot be overemphasized since they dictate what is needed to control a wildfire. Any system or scheme which attempts to rate fire danger should provide accurate and reliable predictions of these three variables. This is the reason why the fire behavior components are the principal keys to fire-danger rating since they integrate the effects of fuel, topography, and weather into a set of numbers which fire managers can use to meet his or her needs.

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