By Wendy Langhans
“It hasn’t burned in 60 years.” My heart sank when I heard that on the news reports, because I knew fire requires fuel and a lot of fuel can accumulate over 60 years. But as anyone who has ever built a campfire can tell you, not all fuel burns equally well. Firefighters refer to three characteristics that affect fuel flammability: moisture level, size of fuel pieces and vertical arrangement.
(1) Moisture level. This is affected by three things – is the fuel live or dead? How much rain did we get last winter? What is the humidity level? When building a campfire, you don’t use green, freshly cut wood. You choose dead wood that has sat around for awhile, so that the residual moisture has time to evaporate. The amount of winter rains determines the amount of moisture available at the beginning of summer. The humidity levels determine how fast the moisture evaporates.
In California, firefighters calculate moisture levels using “live fuel” measurements. Samples of a common shrub, Chamise, are weighed, dried, and weighed a second time. A 100% reading means that water comprises half of the weight of the plant. A reading below 60% generally means you are approaching critical level. Here’s an example of local readings.
View of Castaic Lake with white-tipped Chamise in foreground.
They also measure “100 hour fuel moisture” levels. This measurement is a sampling of 1-3″ diameter dead fuels, which take several days to react to changes in humidity. “It can also be used as a very rough estimate of the average moisture content of the forest floor from three-fourths inch to 4 inches below the surface.” Right now it’s at 5%.
(2) Size of fuel pieces (as measured by diameter). When you light a campfire, you apply the match to the fuel with the smallest diameter – the tinder. That’s because the smaller diameter fuel, with it’s larger ratio of surface area to mass, dries out more quickly, is exposed to more oxygen and reaches ignition temperature sooner.
(3) Vertical arrangement. Heat is transferred by conduction (next to), radiation (across and up) and convection (up). When you build a campfire, you create a “teepee” of logs sitting on top of a pile kindling which sits on top of a pile of tinder, in order to take advantage of all three types of heat transfer. Just so, in a wildland fire, the surface fuels (duff and leaf litter) lie below the understory of tall shrubs and small trees (ladder fuels) which lie below the overstory canopy of tall trees (crown). And when you factor in mountainous slopes, you increase this vertical arrangement even more.
Here’s an example of the vertical arrangement of fuel on a slope.
So, when I heard that the area hadn’t burned in 60 years, I knew the area would contain large amounts of fuel, with significant amounts of dead wood. I also knew that fuel moisture levels were already low because of low winter rainfall and low humidity levels. And I knew it was a mountainous region. So I knew that most of the factors were in place for a large fire, which to date has now burned over 140,000 acres.
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