Tom's Blog

Friday, March 15, 2013

Modelling oak savanna/woodlands burns

When planning for prescribed burns it is useful to use the computer program BehavePlus to analyze the various possibilities. This free program is available from a U.S. Forest Service web site. (An internet search will bring it up.) Two key parameters given as output are rate of fire spread and flame length. Weirdly, rate of spread is given in chains per hour, which shows the U.S.F.S. background. (A surveyor's chain is 66 feet. Multiply ch/h by 1.1 to get ft/min.)

The screen shot below shows how the data are input to the model. Although I only used single input parameters, you generally will want to input a range of values. If you do, then the output is a table giving the calculations for your range of values, as well as a graph of the same output data.

When people started burning oak woods one gap was what fuel model to use. Fortunately, Patrick Brose and associates at the USDA Forest Service Northeastern Research Station in Irvine, PA carried out field research to determine the appropriate model. The data showed that oak leaves burned hotter than other kinds of hardwood litter, but lots cooler than grasses. Brose determined that fuel model 9 worked best in the Behave program. The table below shows my results for a typical oak savanna burn and a comparison with two other fuel models that we often carry out.

Most of the U.S.F.S. theoretical work related to prescribed fire has been done in the west but they are slowly recognizing that things are different in the east. Obviously, fire in an eastern oak forest is quite different from that in a lodgepole pine forest. The fuel model (9 for oak litter) was determined by research in hardwood forests in Virginia. The researchers found that oak leaf litter behaves differently than that of other northern hardwoods. According to the researchers: "...oak leaves weigh more than other northern hardwood litter and burn at a hotter temperature. Also, oak leaves form a porous fuel bed and are slow to decompose, even after being buried in snow for several months." The measured temperature for oak leaf combustion was 192 C.

As my table shows, fire moves much slower through oak litter than through short grass (little bluestem), although the flame heights are not much different.

Some agency prescribed  burners are required to present their BehavePlus calculations along with their burn plans.


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