GRASS AND SAGEBRUSH 



The second method, evaluating percent cover, was used for the mixture of grass 

 and sagebrush because this mixture can be separated according to load breakdown by 

 size class and depth, resulting in two components. Grass is predominantly made up 

 of single sized particles. The bulk density, load divided by depth, varies from 

 0.028 to 0.085 Ib/ft^ (0.45 to 1.36 kg/m^) with a mean of 0.05 Ib/ft^ (0.80 kg/m^) 

 according to data Sneeuwjagt (1974) collected from the Soil Conservation Service 

 on Western United States grasses. A knowledge of either the load or the depth is 

 sufficient to quantify the amount and arrangement of the grass fuel if the bulk 

 density is known. For this example, we have considered all of the grass fuel to 

 be dead. 



Studies by Rittenhouse and Sneva (1977) and Brown (1976) allowed the evalua- 

 tion of fuel loads for sagebrush according to height and largest planform diameter. 

 Ihe first study related the mass to the crown dimensions of the plant. The second 

 study related the mass of single stems (including branches and foliage) to the 

 basal diameter of the stem. But the second study (Brown 19 76) went a step further 

 to provide a breakdown of the mass by size class. Assuming that the sagebrush 

 plant originated from a single stem. We were able to compare the two methods and 

 obtain the load (mass per planform area) related to the crown dimensions of the 

 plant . ^ 



The following dimensions were chosen to represent sagebrush; an average height 

 of 3 feet (91 cm) and an average diameter of 2.5 feet (76 cm). Table 1 lists the 

 fuel parameters associated with these bulk dimensions. Fuels larger than 1/4 inch 

 (0.64 cm) have been ignored in this analysis because they have only a minor impact 

 on the spread rate and fireline intensity; these larger fuels are not consumed in 

 the initial combustion process and therefore do not contribute to the spread process. 



Table 1.--FvbI parameters for grass and sagebrush 



: Surf ace/ volume : Dry load : • i ^ w • ^ j-? 

 J — ■ 1 ^ — Moisture^ Moisture of^ 



Fuel description : ft : cm : lb/ft : kg/m : content : extinction 



Grass (fuel depth = 1. 



.00 ft (0. 



30 m)) 























3000 



98, 



.4 



0, 



,0275 



0, 



, 134 



0. 



,05 



0. 



,15 



Sagebrush (fuel depth 



= 1.4 8 ft 



(0.45 



m)) 



















L"" 'e foliage 



1500 



49, 



.2 



0. 



,0538 



0. 



,263 



1, 



,00 



2. 



,00 



Live Ih 



677 



22, 



.2 



0. 



.0941 



0, 



,459 



0, 



,50 



2. 



,00 



Dead Ih 



677 



22 



.2 



0, 



.0235 



0. 



, 115 



0, 



.09 



0. 



,20 



The values of fuel parameters common to all fuels discussed are: 



Low heat value 8000.0 BTU/lb (18,595 kj/kg) 



Particle density 32.0 Ib/ft^ (0.51 g/cm^) 



Fractional-' mineral content 0.06 



Fractional-' effective mineral content 0.01 



•'Fraction of dryweight. 



^A parameter involved in the computation of the moisture damping coefficient 

 (Rothermel 1972) . Higher values allow the fire to spread at higher moisture contents 



Data on file at the Northern Forest Fire Laboratory, Missoula, Montana. 



