Resolution No. 4814 - Laserfiche WebLink

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relatively little fuel energy and tends to burn out quickly. Wildfires fueled by larger vegetation may spread more slowly, but larger vegetation contains more fuel energy and tends to burn for a longer duration. Moisture content of vegetative fuels is also a major determinant of wildland fire potential. The lower the moisture content the greater the fire potential. Moisture content at any given time depends on antecedent (before the given time) weather conditions. The moisture content of larger fuels (e.g., trees) depends on previous weather conditions over periods of several weeks or even months. The moisture content of smaller fuels (brush) depends on previous weather conditions over several days or a week or two. The moisture content of very small fuels (e.g., grasses ) depends ~argely on previous weather conditions over a few hours or a day or two. The fire hazard posed by vegetative fuel loads also depends on fuel continuity, both horizontally and vertically. Horizontal continuity, the distribution of fuels over the landscape, strongly affects the spread and containment of wildfires in a given geographic area. Vertical continuity of fuels, the linkage between fuels at ground level and tree crowns, also affects the fire potential. Forests with strong ladder fuels (understory growth between ground fuels and tree crowns) are more likely to have major fires involving tree crowns. Forests with limited ground fuels and little or no ladder fuels are much more likely to experience minor ground fires without a fire involving tree crowns. Weather also has a profound impact on wildland fire potential. Weather conditions of high temperatures, Iow humidity, and high winds may greatly accelerate the spread of a wildland fire and make containment difficult or impossible. Changes in weather conditions can greatly accelerate a fire's spreading rate. Many casualties have occurred when firefighting personnel were trapped by sudden bursts of fire spread in response to changes in wind conditions. For many larger fires, containment is possible only with a little help from mother nature via lower temperatures, reduced winds or significant rainfall. Local topography is also a major factor in the spread of wildfires. Fires burn much more quickly up slope than they do down slope. Doubling a slope approximately doubles the rate of fire spread. Canyons, gulches and other local topographic effect can act as chimneys, intensifying fires in certain areas. Fires tend to slow at ridge tops and thus ridge tops are often chosen as locations for fire breaks. Suppression of wildland fires depends on the three main factors - vegetative fuel load, weather, and topography - that, in combination, govern fire potential. High fuel loads, hot, dry, windy weather and steep slopes increase fire potential and make fire suppression much more difficult. Conversely, Iow fuel loads, cool, moist weather with Iow winds, and gentle slopes make fire suppression easier. In addition, however, fire suppression also depends on two other important factors: availability of fire suppression resources and access. Fire suppression resources include firefighting personnel, equipment and apparatus, as well as water and chemical fire suppressants. The greater the availability of fire suppression resources, the more likely it is that a given fire will be contained quickly. Fire suppression also depends on access. Fires in remote areas without ground access via roads are more difficult to Public Review Draft: August 6, 2004 9-7