Fuel Array Assessment 



The geometric nature of the fire behavior model presented resulted in a fire 

 spread algorithm coupled to a hexagonal array of fuel cells. However, application 

 to the field is not possible unless we are able to fill the hexagonal array in a 

 manner that preserves the horizontal stratification of the actual fuel array. 

 Filling the hexagonal cells requires fuel array data (load, size, depth) that must 

 be acquired in a manner that fills the needs of the cell filling algorithm. In 

 general, we should look for some classifiable character to give the assessed non- 

 uniformity a recognizable distinction related to its habitat type or other compar- 

 able classification, i.e., fuel type, and age. Habitat classification according 

 to Daubenmire and Daubenmire (1968) is presently used by the USDA Forest Service 

 in Idaho and Montana to classify vegetation and its associated environment. Fuels 

 are a byproduct of the habitat type but may occur in different arrangements of 

 load, particle size distribution, and depth, and thus are classified separately. 

 Fuel models used in the U.S. National Fire Danger Rating System are obtained by 

 grouping depth, particle size, and load into a classification scheme. Data 

 gathered from sampling fuel arrays should characterize the horizontal pattern 

 of differing fuel types as well as the spatial occurrence of the basic fuel pro- 

 perties (load, size, depth) within a fuel type. Other fuel properties--heat con- 

 tent, mineral content, and particle density--have little variation within the fuel 

 type and are assumed constant. Fuel moisture is time -dependent responding to diur- 

 nal changes in humidity and temperature. However, these changes are sufficiently 

 slow so that fuel moisture can be considered constant over the duration of the 

 fire being examined. Although moisture can be introduced as a spatial variable, 

 it is held constant so that the response to nonmoisture fuel variability can be 

 emphasized. It is anticipated that experienced land managers will make adjustments 

 for moisture changes. 



Two methods of fuel assessment are presented; (1) sampling a specific size 

 area at periodic intervals along transects, and (2) if components are random, 

 evaluating the percent cover and describing the uniform fuel properties of each 

 component . 



SLASH 



The first method stated was employed in slash owing to the absence of recog- 

 nizable patterns in the spatial arrangement of the fuels. Transects were obtained 

 from a slash area composed primarily of western larch and grand fir. The area was 

 essentially a clearcut with only a few remaining trees. Trees were cut to an 

 8-inch (20 cm) unmerchantable top and the entire tree except the top skidded to 

 the landing. Nonuniformity was assessed in terms of load and depth along a 100-foot 

 (30.5 m) transect at 2-foot (0.61 m) intervals. The fuel load was estimated by size 

 class from the number of intercepts through a vertical sampling plane (Brown 1974). 

 The following size classes were assessed: Ih, lOh, and lOOh.^ Pieces greater than 

 3 inches in diameter were measured but not considered in the model because they do 

 not significantly contribute to fire spread. 



Fuel size classes are characterized by the time lag constant related to their 

 ability to respond to humidity by absorbing or desorbing moisture (Fosberg 1970) . 

 The 0-1/4 inch (0-0.63 cm) size class is called Ih , 1/4-1 inch (0.64-2.54 cm) the 

 lOh, and the 1-3 inch (2.55-7.62 cm) the lOOh. 



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