Live Fuel Moisture

Live Fuel moisture content describes the moisture content within living vegetation ("fuel").  Moisture content is the single most important factor determining the amount of fuel available to burn, and how much fuel might be consumed during a wildfire.  Fuel moisture determines if certain vegetation will burn, how quickly and completely it might burn, and what phases of combustion the fuels will support.  Fuels with a higher moisture content generally reduce the rate of energy released during a fire.  This is due to the moisture in the vegetation's ability to absorb heat, making it less available to preheat fuel particles to ignition temperature (Burgan and Rothermel 1984).  Ignition will not occur if the heat required to evaporate the moisture in the fuels is more than the amount available in a firebrand (Simard 1968).

Fuel moisture content is the percentage of a given fuel's weight, represented by water, based on the dry weight of the fuel.  In a word equation, it is: Percent Moisture Content = Weight of Water / Oven-dry Weight of Fuel x 100. Moisture content can be greater than 100 percent because the water in a fuel particle may weigh considerably more than the dry fuel itself. For example, a green leaf may contain three times as much water as there is dry material, leading to a moisture content of 300 percent.  Moisture content of duff and organic soil can be over 100 percent.

Moisture Content (%)

Stage of Vegetative Development

300

Fresh foliage, annuals developing early in the growing cycle

200

Maturing foliage, still developing, with full turgor

100

Mature foliage, new growth complete and comparable to older perennial foliage

50

Entering dormancy, coloration starting, some leaves may have dropped from stem; also indicative of drought conditions

<30

Completely cured (treat as dead fuels)

NOTE: Moisture content represents the weight of the fuel compared to its oven-dry weight.
Source: Fireline Handbook, Appendix B, Fire Behavior Supplement, NFES 2165 (PMS 410-1), 1989, page B-29.

View California Live Fuel Moisture data online.