In using this guide, these factors can guide users:
Once several alternative fuel models have been selected as possibilities, evaluation of their fire behavior outputs (rate of spread and flame length) with typical or reference inputs is important. However, making several good fuel model selections is only a preliminary step in the calibration process.
As suggested here, when comparing modeled and observed fire behavior, it may be helpful to think of spread rates and flame lengths in ranges or Fire Behavior Classes. If fireline personnel can effectively report observed fire behavior in these terms, differentiating what they see through the burn period and as environmental inputs change, the analysis will be improved dramatically.
Testing the range of a fuel model’s characteristic fire behavior requires analysis of several environmental inputs. Consider these. BehavePlus, as a sensitivity tool, only allow consideration of two variables at a time. However, there are generally at least 3 significant environmental factors that govern the day-to-day variation in fire behavior; wind, slope, and fuel moisture. Fortunately, the Rothermel fire spread model depicts the effect of slope as an equivalent windspeed. If the calibration analysis represents the windspeed as a range of effective windspeed, slope should be at least generally incorporated. In some cases, it may still be necessary to consider its effect separately.
This guide integrates the original 13 models with the 40 standard models added in June of 2005. Though the developers of the 40 standard models intended that they stand alone, all 53 models are available to the user in current versions of all the fire modeling systems that are designed to use them. And though the original 13 models were grouped into only 4 carrier types, they can be effectively distributed into the 6 types defined with the newer set.
Consider the objectives that guided the development of these two sets.
The original 13 (Anderson, 1982) were designed to support analysis of:
On the other hand, the newer 40 standard fuel models (Scott and Burgan, 2005) were developed to:
The most important benefit of integrating fuel model sets in this guide may be the context the original 13 provide for users familiar with them. Consider it something of a dual language guide, facilitating translation for those users.
When selecting a fuel model, one of the first considerations should be whether fuels are expected to burn under high fuel moisture conditions. Though many modeling tools allow the user to define a burn period which can truncate fire behavior even when moisture of extinction has not been reached, humid climate fuel models (with high moisture of extinction) will express significant fire behavior even when corresponding dry climate fuels estimate no fire spread.
The example here demonstrates that GR4 exhibits no fire spread at 15% fuel moisture and at that same point, GR5 can project spread rates of as much as 50 ch/hr. Ensure that the fuel model selected accurately represents potential fire spread and intensity under the range of fuel moistures conditions that will be encountered.
To insure accuracy and precision in modeling efforts, fuel model selection needs to employ a disciplined process. With the addition of 40 fuel models representing 6 carrier fuel types, users will be more likely to find an appropriate fuel model based on fuel model parameters, resulting in reasonable ranges of fire behavior over the range of anticipated environmental conditions.
A feature that was implemented with the development of the National Fire Danger Rating System (NFDRS) recognizes that most herbaceous fuels transition between green and cured conditions over the course of a fire season. Effectively, this transfer of herbaceous fuel loads between live and dead categories redefines the fuel complex with each proportion transferred, making it a critical fuel model characteristic. The changes in output fire behavior can be dramatic, when compared to the static fuel models among the original 13.
The example here shows spread rate for dynamic fuels GR6 & GR8 with the corresponding static FB3 from the original set of 13.
In the development of the new set of fuel models, this “dynamic” (or proportional) fuel load transfer has been implemented for all fuel models that include herbaceous loads. It includes all grass, grass/shrub, two shrub (SH1 & SH9), and two timber understory (TU1 & TU3) models.
As depicted in the graph and table below, the fuel load transfer (implemented in FARSITE, FLAMMAP, and WFDSS Fire Behavior analysis tools) is dependent on the input herbaceous moisture content.
Important cautions: Between 90% and 100% input LHMC, very rapid changes in fire behavior outputs can occur. Be sure to test the sensitivity to this input. Though it is agreed that live fuels can provide a critical influence on fire behavior, serving as both the heat sink and heat source in varying combinations, the specifics are not well modeled or understood. There are findings that indicate that curing is not directly related to herbaceous moisture content. As a result, BehavePlus allows the user to input curing % separate from LHMC.