What is Fuel?

A fuel is anything that fire can and will burn. Fuel is the combustible matter used to sustain fire. Typically this is the most prevalent type of vegetation in a given area. However after the disastrous wildfires that raged through an increasing urban wildland interface in Washington and California, a new fuel model probably should be considered that includes "houses" and subdivisions.

The National Fire Danger Rating System (NFDRS) and Fire Behavior Prediction System (FBPS) are mathematical models that require mathematical descriptions of fuel models and their respective fuel properties as input. This input combined with meteorological and topographic parameters is used for calculations of fire danger and/or fire behavior.

A fuel model is a mathematical representation of various fuel types used in mathematical fire spread models. Fuel models are expressed numerically 1-13 or 1-20 (depending upon which mathematical fire prediction system is being used) that describe fuels in terms that spread models understand (surface area to volume ratio, fuel load, size, shape, compactness, horizontal and vertical continuity, moisture of extinction, etc.). Simply put, fuel models are tools that help land management agencies realistically estimate fire behavior or fire danger.

Fuel models are divided into four vegetation groups that account for fuels composed mainly of GRASS - SHRUB - TIMBER and SLASH. In the NFDRS these four vegetation types are further subdivided into 20 separate and distinct fuel models. The FBPS divides the four vegetation types into 13 fuel models that are more simply structured. The icons below link to summaries of the four main fuel types with pictures of each.

NFDRS and FBPS fuel models are related, but there are some significant differences. Figure 1 shows how these fuel models can be cross referenced, based on which fuel is controlling fire spread in severe burning conditions (drought/wind).

Criteria for choosing a fuel model include the fact that the fire burns in the fuel stratum best conditioned to support the fire. This means situations will occur where one fuel model represents a rate of spread most accurately while another fuel model BEST depicts fire intensity. In some situations two fuel conditions exist, so that the spread of fire across an area must be weighted by the fraction of the area occupied by each fuel.

The NFDRS is used to calculate fire behavior characteristics based upon fuel, weather and topography. A flowchart of the NFDRS illustrates the interaction of those elements that lead to various output. A "Fire characteristics Chart" for the Ninemile District shows ADJECTIVE CLASS RATINGS applicable to fuel model G (closed conifer timber with a lot of litter beneath). These ratings assist in land management resource allocation for a fire incident. These ratings are directly proportional to Burning Index values (unitless), which are the by-product of energy release component and spread component. REMEMBER these values vary depending upon fuel model type.

In Southern California only, criteria to issue RED FLAG WARNINGS depend upon BI values for the predominant fuel model that will support and carry fire - a value that changes seasonally. In spring finer fuels such as grass are used. From June until January chamise and chaparral fuel models are used for elevations below 4000 ft - and timber above 4000 ft. If BI's are low and critical weather conditions exist, NO RED FLAG is issued.

Fire behavior differences among these four groups are basically related to the FUEL LOAD and its DISTRIBUTION among the fuel particle size classes. See figure2. Each fuel model is described by the fuel load and the ratio of surface area to volume for each size class; the depth of the fuel bed involved in the fire front; and fuel moisture and/or moisture of extinction. Additionally, fuel loading varies with fuel depth - and the horizontal or vertical orientation of the fuel bed.

A second term associated with fuels is Ladder fuels. These are a continuous arrangement of fuels from the surface to the canopy that carry a fire from the surface up into the canopy. When the fire is carried from one fuel group into another, the fire likely will behave differently as changes occur in fuel models, fuel moistures and meteorological conditions. Often new fire behavior calculations are required if the initial calculations did not account for this change.