Fuel Moisture Content

Severe wildfires have been burning more wildland and more houses in recent years. Many factors play a role in both the increasing intensity of the wildfires and the resulting destruction of home.  Some assert that climate change is at least partly to blame; others claim that the increasing number of homes in and near the forest (the

wildland-urban interface) is a major cause. However, most observers agree that wildfire suppression and historic land management practices have led to unnaturally high accumulations of fuels in many wildland-urban interface areas. While high intensity conflagrations (wildfires that burn the forest canopy) occur naturally in some forests (crown-fire or stand-replacement fire ecosystems), abnormally high fuels littering the forest can lead to conflagrations in wildland-urban interface areas when such crown fires were rare in the past.

Fuel moisture content is critical, but is generally a function of weather patterns over hours, days, and weeks.  This year's weather patterns have produced extremely low fuel moisture content.  Fuel size is also important—fine and small fuels (e.g., needles, grasses, leaves, small twigs) are key to fire spread, while larger fuels (e.g., twigs larger than pencil-diameter, branches, and logs) contribute primarily to fire intensity; both are important to minimizing fire damages. Fuel distribution can also affect damages. Relatively continuous fuels improve burning, and vertically continuous fuels—fuel ladders—can lead a surface fire into the canopy, causing a conflagration.  Lastly, total fuel accumulations (fuel loads) also contribute to fire intensity and damage.

Fuel moisture content may be the most important single property controlling ignition and spread of fire in the wildland-urban interface. 

This is because moisture in live or dead fuels requires heat to evaporate the moisture so the fuel can burn. The moisture content - measured as the amount of water (by weight) as a percent of the weight of oven-dried wood, grass or other fuel - of live, green foliage fluctuates widely, from less than 100% (i.e. more oven-dry fuel than water) to more than 200% (i.e., more than twice as much water as oven-dry fuel), depending largely on weather in the preceding days and weeks.  Dead fuels also contains water, but at much lower levels, typically ranging from near 100% to as little as 10%. Fire ignitions depend on an energy source or spark (e.g., from a match, cigarette, chainsaw, or from lightning) and fuel with a moisture content below approximately 20-30%.  However, once a fire is started, fuels with a moisture content of 100% can burn, especially if the fire is driven by high winds as are commonly seen along the Front Range of Colorado.

Potential wildfire fuels are generally described by the time (in hours) that it takes for the moisture content of the fuels to decline by about two-thirds.  During a fire the fuels must first lose moisture before ignition. 

The smallest diameter fuels, also called fine fuels or flash fuels, are the 1-hour time lag fuels—needles, leaves, grass, etc.—both on the surface and in the tree crowns that dry out (lose two-thirds of their moisture content) in about an hour. The next size class is 10-hour time lag fuels—woody twigs and branches, up to a quarterinch in diameter (about the diameter of a pencil). The larger size classes are the 100-hour time lag fuels (up to 3 inches in diameter) and 1000-hour time lag fuels (more than 3 inches in diameter).

Currently the 1000-hour fuels along the Front Range have a fuel moisture content of 6-16%.  For comparison the timber which you purchase at the lumber yard has been dried in a kiln to a moisture content of approximately 12%.  This means that many of the large fuels are as dry as kiln dried lumber.  The lighter, flashy fuels can dry out in an hour and support lighting of the heavier fuels.

Please exercise caution with all ingition sources.  If you see or smell smoke you can call 911.  ICFR firefighters will respond and check out any possible smoke or fires.