Fuels Analysis

How do I determine an appropriate fuel model?

Of the thirteen standard fuel models used nationally, abbreviated as the National Forest Fire Laboratory (NFFL) models, only three have stated applicability to chaparral (shrub) fuel beds (Anderson, 1982). Although the NFFL grass and riparian fuel models are suitable for use in the Santa Monica Mountains, the range of variability of shrub and subshrub fuels experienced in the field far exceeds what is available from the national system. Consequently, the United States Forest Service (USFS) has developed five additional custom fuel models for use in chaparral and coastal sage scrub fuels. In addition to the USFS custom models, the University of California, Santa Barbara has developed a fuel model suitable for use in wildland urban interface areas, referred to as UCSB 20.

The custom fuel models can be accomodated into existing fire spread prediction models such as BEHAVE and FARSITE, and employ the properties shown in the table on the Measuring Fuels page. Developing custom fuel models is more of an art based on trial and error comparison between fuel model input parameters and fire spread predictions rather than science. In other words, direct estimates of fuel properties from ground-based sampling are used to guide values supplied as inputs to new models, but some "fudging" of these estimates is still required. In the links to the USFS and UCSB custom fuel models below, graphs of predicted rate of fire spread (meters/sec) for each fuel model are shown relative to two reference fuel models, NFFL 1 (grass) and NFFL 4 (shrub). Predictions are stratified into three variables: moderate and extreme fire regime; terrain slope; and windspeed. The values used for fire regime are shown in the table below and are taken from Weise and Regelbrugge, 1997. By examining the performance of each of the models against these variables you will gain some insight as to the fire behavior that can be expected for a particular area. Predictions for the two reference models are included in every graph to enable comparison between the custom models and the existing standard models. The spatial datasets and input files which use these models are available from the Download page.

Chaparral Environmental Regimes Used In Fire Spread Predictions

Property	ModerateConditions	ExtremeConditions
Dead 1 Hour Fuel Moisture	0.08	0.02
Dead 10 Hour Fuel Moisture	0.09	0.04
Dead 100 Hour Fuel Moisture	0.11	0.05
Live Herbaceous Fuel Moisture	1.50	0.90
Live Woody Fuel Moisture	1.10	0.70
Source:
Weise and Regelbrugge, 1997
Notes:
Fuel Moisture content values are given on a dry weight basis.
Wind magnitude (speed) was varied from 0 m/s to 10 m/s. Magnitudes in the upper range are meant to mimic Santa Ana conditions.
Slope was varied from 0% to 100%. At 100% slope there is a 1 meter change in vertical elevation for every 1 meter of horizontal distance.
Wind magnitude (speed) in slope trials was set to 0 m/s.