Calif. Firefighters Use Moisture to Predict Fire Danger

On a hillside off Green Meadow Avenue in Thousand Oaks, a firefighter heads into the brush with hand tools and metal sample containers.

There are no flames to knock down, and there's no smoke to chase.

Instead, he's after a number -- one that predicts fire danger.

"When you start seeing your vegetation getting down into the 80 to 100 (range) is when you start seeing some active fire behavior," said Capt. Brendan Ripley, who has led Ventura County Fire Department efforts to track moisture content in vegetation for the past six years.

Right now, the county average stands at 63 percent -- drier than the 10-year average for this time of year and just slightly above what officials call critical.

"What we're heading into right now is our seasonal drought," Ripley said.

The brush dries out and the potential for fire rises -- conditions that typically come just as winds and other hazardous weather conditions pick up in the county.

"Our most vulnerable time for fire is when we also receive our most extreme weather conditions. They come about the same time," Ripley said.

Strong winds may lead to a wind advisory and little else when vegetation is green. But when moisture content drops, those same winds may lead fire officials to have more equipment and teams at the ready. The National Weather Service also depends on fire agencies to help its experts decide when to issue red-flag fire warnings.

"It gives us one more scientific tool we can use to make strategic decisions," Bill Nash, a county fire spokesman, said of moisture content measurements.

Until six years ago, Ventura County fire officials didn't collect their own data, instead using regional numbers out of Los Angeles County. Ripley decided to create a local program to get more accurate levels, mapping five areas of the county to sample, from Simi Valley to the Santa Clara Valley.

Ripley tries to eliminate as many outside factors as possible to get good numbers that he can compare over time. The same firefighters collect the samples every two weeks along the same path, cutting from chamise, a resilient plant that grows in all five areas.

Moving from plant to plant, a firefighter grabs tiny sections, filling a tin for old growth and a second for new growth.

The tins are weighed, baked and weighed again to find out the percentage of moisture content trapped inside the plants. They are baked for 15 hours at 115 degrees Celsius, or 239 degrees Fahrenheit, inside a small oven.

In wet conditions, the numbers can climb over 100 percent because of the weight of moisture. In late summer and fall, the numbers drop, sometimes reaching critical levels under 60 percent.

Numbers this year show vegetation has dried out faster than normal -- as much as two months faster -- likely because of less rainfall last winter.

Deputy Chief Mike LaPlant said the local numbers have made a difference.

"We've been able to define it more to our county and particular areas in our county," he said.

When battling a blaze, firefighters must consider weather, topography and fuel. Weather is the most dynamic, but topography stays mostly static and fuel conditions typically change gradually, he said. Now, the fire department has cut a lot of the guesswork out of assessing conditions, he said.

It allows officials to get ready early, when conditions are drying out faster than normal, or judge whether blowing embers will cause problems out ahead of a fire.

"We have a scientific measure," LaPlant said. "It cuts down the uncertainty."

Copyright 2012 - Ventura County Star, Calif.

McClatchy-Tribune News Service