Table I. --Fuel and environmental inputs that determine values for the terms in 



the rate-of -fire- spread equation 



Input variables 





Terms 



in the 



spread equation 





: ir 



K : 





*s : Pb :- ^ 



: Qig 



I nadi na 1h /ft ^ 



Y 

 A 



Y 

 A 



Y 

 A 



Y Y 

 A A 





Heat content , 













R t 11 /1h 



Y 



A 











Fuel particle density. 













lb. /ft. 3 



X 



X 



X 



X 





Fuel particle surface- 













to-volume ratio, 













ft.2/ft.3 



X 



X 



X 



X 





Depth of fuel, ft. 



X 



X 



X 



X X 





Fuel moisture content. 













fraction of dry wt. 



X 









X 



Total salt content, 



fraction of dry wt . X 

 Silica-free salt 



content, fraction 



of dry wt . X 

 Wind velocity, 



ft./min. X 

 Angle of slope X 



The model was developed from data gathered using controlled laboratory combustion 

 facilities, fuel beds comprised of uniformly distributed dead fuel particles of a single 

 size, and quasi-steady-state fire. Conditions were varied to show the effect of fuel 

 depth, loading, particle size, windspeed, and slope. The effect of fuel moisture and 

 mineral content was taken from other work. 



Predictions of fire spread and flame front intensity using the model should apply 

 most accurately to surface fires in uniform distributions of such fuels as needle lit- 

 ter, grass, logging slash, and perhaps crowning fires in brush. Less accurate predic- 

 tions would be expected for patchy distributions of fuel. 



The model pertains to fire spread by a flaming fire front. Fire propagation from 

 spotting, fire whirls, and blowup conditions is not a part of the model. The model 

 was designed for fuel appraisal, fire-danger rating, and presuppression planning where 

 relative fuel and fire behavior evaluations are useful. Accurate fire behavior predic- 

 tions for specific fires will require further refinement of the model and understanding 

 of fuels. 



Application of the model to heterogeneous fuels--fuels containing more than one 

 size of fuel--requires recognition of the contribution to fire spread made by each kind 

 and size class of particle. This is accomplished by entering the input variables as 

 mean values weighted by surface area of each kind and size class of fuel particle. A 

 complete explanation of the model and its development is given by Rothermel (1972) . 



4 



