The Forestry Division’s Vegetation Management Unit samples vegetation and produces a bi-monthly live fuel moisture report for Los Angeles County. Fuel moisture content describes the moisture content within the vegetation/fuel. Moisture content is the single most important factor that determines how much of the total fuel is available for burning, and ultimately, how much is consumed. Fuel moisture determines if certain fuels will burn, how quickly and completely they will burn, and what phases of combustion the fuels will support. Fuels with a higher moisture content reduce the rate of energy released during a fire. This is due to the moisture in the vegetation's ability absorbing 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 the firebrand (Simard 1968).

Fuel moisture content is the percentage of the fuel 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.