General Model Development 



Rothermel ^ provided the empirical data used for development and initial testing 

 of the live fuel moisture model. He used the xylene distillation technique of determin- 

 ing moisture content to construct profiles of herbaceous and woody plants near Missoula, 

 Mont., during the 1975 and 1976 fire seasons. The live fuel model was initially devel- 

 oped to emulate these moisture profiles, then adjusted to produce reasonable moisture 

 profiles for the remainder of the United States. 



Herbaceous Fuel Moisture 



Moisture Computations - The live herbaceous fuel moisture (HFM) is a function of 

 a modified 1000-h TL fuel moisture called the XIOOO value, which is a function of the 

 daily change in the 1000-h TL fuel moisture. The original effort to relate herbaceous 

 fuel moisture directly to the 1000-h TL fuel moisture proved reasonable during periods 

 of drying, but it produced excessive herbaceous fuel moisture recovery during periods 

 of precipitation. Therefore, the XIOOO value was designed to decrease at the same rate 

 as the lOOO-h TL fuel moisture, but have a slower rate of increase. 



During the growing season the XIOOO value is calculated as: 



XIOOO = YXIOOO + KiK2(AMC1000) 



where: YXIOOO = yesterday's XIOOO fuel moisture 



Kj = drying or wetting factor 



K2 = temperature factor 



AMCIOOO = 24 hour change in the lOOO-h TL fuel moisture 



If AMCIOOO <0(drying) Kj = 1 



If AMCIOOO >^0 (wetting) Ki = 0.0333X + 0.1675 subject to 



If average temperature >50°F (10°C] = 1.0 

 If average temperature <50°F (10°C) K2 = 0.6 



Kj limits the increase in herbaceous fuel moisture due to precipitation. It is 

 scaled to allow the XIOOO value to respond the same as the 1000-h TL fuel moisture when 

 the 1000-h TL fuel moisture is 25 percent or more. 



K2 reduces the response of the XIOOO value to compensate for slower physiological 

 processes in plants during cool weather. Figure 3 compares the lOOO-h TL moistures with 

 the XIOOO value. 



Prior to spring greenup, the herbaceous fuel is assumed to be completely cured, so 

 the herbaceous fuel moisture (HFM) is equal to the 1-h TL fuel moisture. 



7 



