Factors Regulating the Moisture Content of Dead Fuels

Dead fuels absorb moisture through physical contact with water (such as rain and dew) and adsorb water vapor from the atmosphere. The drying of dead fuels is accomplished by evaporation. These drying and wetting processes of dead fuels are such that the moisture content of these fuels is strongly affected by fuel sizes, weather, topography, decay classes, fuel composition, surface coatings, fuel compactness and arrangement:

Particle size

Small fuels can gain and lose moisture faster than large fuels. Based on this principle, dead woody fuels are divided into 4 diameter classes, also called timelag classes: < 1/4, 1/4-1, 1–3, and > 3 in. See Timelag classes.

The Wildland Fire Assessment System produces daily maps of dead fuel moisture across the U.S. based on time-lag classes: Map of estimated 10- hour fuels, Map of estimated 100-hour fuels, and Map of estimated 1000-hour fuels.

Weather

Short-term and seasonal/annual patterns in weather determine fuel moisture changes, and thus fuel consumption. Primary weather factors that determine fuel moisture changes are sun, wind, precipitation, relative humidity, and air temperature (Schroeder and Buck 1970). Sunny skies, elevated wind speeds, lack of precipitation, low humidity, and warm temperatures all act to dry fuels. Cloudy or hazy skies, still winds, precipitation, elevated humidity, and cool temperatures either act to increase fuel moistures or prolong their present moisture state. Long-term events like droughts affect 100- and 1000-hour fuels significantly. For more information see: Effects of weather and topography on fuel moisture.

Topography

Topography affects fuel moisture indirectly by influencing microclimate. Fuel moisture content is generally higher on north facing slopes than south slopes because there is less direct exposure to sun. Fuels are also moister at high elevations due to lower temperatures and higher relative humidity (Pyne et al. 1996). For more information see: Effects of weather and topography on fuel moisture.

Decay class

Moisture can be gained and lost more rapidly in decayed wood because particle size is reduced, surface area-to-volume ratio is increased, and moisture-holding capacity is lost. Dry decayed wood combusts and is consumed rapidly. Decay may also remove flammable volatile compounds from dead fuels.

Fuel composition

The major dead fuel categories are dead standing herbaceous material, leaf litter, cones, fallen twigs and branches, fallen logs, and standing snags. Fuel moisture response varies among dead woody fuels, deciduous leaf litter, grass litter, and coniferous needle litter (see Timelag classes). The moisture content of the forest floor complex also has unique characteristics based on its thickness, composition, arrangement, and compactness. Slash (logging debris) presents a special fuel complex.

Surface covering

A covering of organic material, such as bark, waxes, or cutin, can slow the movement of moisture in or out of dead woody fuels. For example, dead woody fuel with bark can gain and lose moisture at 2/3 the rate of fuels without bark (Simard et al. 1984).