Fuel Moisture

Fuel moisture content is among the most important fuel characteristics affecting fire behavior (Byram 1959, Pyne et al. 1996). It determines how much fuel is available for burning, and ultimately, how much is consumed (Miller 1994). Moisture absorbs heat released during combustion, making less heat available to preheat fuel particles to ignition (Burgan and Rothermel 1984). By raising a fuel’s heat capacity, fuel moisture content influences ignition. At high moisture contents, the heat required to evaporate moisture in fuels is more than the amount of heat available in the firebrand (Simard 1968 in Miller 1994), and combustion can be stopped. This point is termed the moisture of extinction (also called extinction moisture content). Moisture of extinction is a function of the fuel type. For most dead fuels (forest floor duff is an exception), the moisture of extinction is between 12 and 40 percent. For live fuels the moisture of extinction generally exceeds 120 percent (fuel moisture is expressed per unit of dry fuel weight, making moisture contents >100 percent possible).

Both live and dead fuels can slow, stop, or contribute to fire spread, depending on their moisture content. The factors that regulate fuel moisture differ among live and dead fuels. The primary determinants of live fuel moisture content are: internal factors that regulate diurnal and seasonal changes, climate, site factors that affect the fuel environment, phylogenetic differences among species groups (evergreen vs. deciduous), and differences among plant tissues (leaves vs. stems). Fuel moisture in dead fuels ranges widely based on particle size, short and long-term weather changes, topography, decay class, and fuel composition (Byram 1959). How these factors regulate live fuel moisture is briefly explained in the following sections:

• Factors Regulating Moisture Content of Live Fuels

• Factors Regulating Moisture Content of Dead Fuels

Fuel moisture content is a primary variable in all fire behavior prediction models, affecting ignition, combustion, amount of available fuel, fire severity, and smoke generation. Estimating fuel moisture is therefore a critical step in planning a prescribed fire or assessing risk of fire danger. There are several ways to measure fuel moisture, including volumetric analysis, gravimetry, moisture indicator sticks, formulas and graphs, and ocular methods, explained briefly in the following section:

• Determining Fuel Moisture

Land managers can also obtain quick estimates of live and dead fuel moisture from the Wildland Fire Assessment System which produces daily maps of both live fuel moisture and dead fuel moisture across the U.S.:

• Map of estimated live fuel moisture

• Map of estimated 10- hour fuels

• Map of estimated 100-hour fuels

• Map of estimated 1000-hour fuels