Released March 4, 2011
STILLWATER, Okla. – It is fire season in Oklahoma. About this time every year, Oklahomans watch thousands upon thousands of acres of land burn up in wildfires.
So far, in 2011, much of the state has been on high alert, and more than 20 counties currently have burn bans. This is done, in part, because of the drought that has dried out the majority of the state.
How big of an effect does drought have on wildfire this time of year?
Not as much as people might think, according to J.D. Carlson, fire meteorologist in the department of biosystems and ag engineering at Oklahoma State University, and program director of OK-FIRE (http://okfire.mesonet.org).
“Most Oklahoma fuels this time of year consist of dead grass and respond quickly to hour-to-hour weather changes,” said Carlson. “It’s not the amount of rain that matters, it’s the hour-to-hour relative humidity and wind speeds.”
There are two types of fuels available to wildfires – dead and live. Examples of dead fuels are dead leaves, dead grasses and dead wood on the ground surface. Live fuels include any vegetation that has any degree of greenness, which is essentially limited this time of year to evergreens, such as eastern redcedar.
Dead fuels
The moisture content of dead fuels is controlled exclusively by hour-to-hour weather changes. Dead fuels are further categorized into four classes according to their diameters: 1-hour, 10-hour, 100-hour and 1,000-hour fuels.
The dead grasses and other dead fuels that now cover much of Oklahoma, falling in the 1-hour and 10-hour classes, are the state’s primary fuel before spring greenup. Carlson can confidently say that existence of a drought is not much of a factor with these fuels’ ability to burn
“The moisture content of 1-hour fuels, like dead grasses, is very responsive to any little moisture peak or dry period. So when it rains or the relative humidity rises, their moisture goes sky high, but also can quickly dry out,” Carlson said. “That’s because they are so thin and can respond to the changing weather conditions.”
While the drought will lower the moisture content of live fuels this time of year, it would have a much more drastic effect after the spring greenup period. This would cause the live grassy fuels to die and add to the dead fuel that already exists.
In addition, live woody vegetation would see its moisture content greatly reduced since its water demand is greater during the warm season.
“Drought isn’t as important to fire danger during this time of year, because the amount of dead fuel is constant and can’t increase even if the drought increases,” said Carlson. “It’s already dead.”
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Oklahoma State University, http://www.dasnr.okstate.edu/Members/trisha.gedon-40okstate.edu/dead-fuel-already-is-dead
Writer: Sean Hubbard, 405-744-4490, sean.hubbard@okstate.edu
