Fire danger ratings were particularly sensitive to percent-green estimates. 



Although the 1972 NFDRS would respond reasonably well to properly reported changes 

 in live fuel conditions, it had limitations. 



1. Living fuels always acted as a heat sink, never a heat source. 



2. A constant moisture content was assumed for the herbaceous fuels, but the 

 live/dead ratio changed. 



3. The quantity live of woody fuels was held constant, but the moisture content 

 changed according to the growth stage assigned. 



Thus, living woody fuels and herbaceous fuels were not treated similarly. 



Improvements in the fire model used in the 1978 NFDRS made it possible to treat 

 live fuels more realistically. The result was that live fuels could act as a heat sink 

 or as a heat source. The live fuels became a heat source when their moisture content 

 became so low that they could be desiccated and ignited during combustion of the dead 

 fuels. However, if the moisture content was above some critical level, live fuels 

 would not burn, but rather would act as a heat sink (Albini 1976) . 



The live fuel moisture model was developed to provide more analytical and consist- 

 ent estimates of live fuel moisture at a time when capability to use this information 

 had improved. 



THE LiVt FULL MOiSTURL MODLLS - 1978 NFDRS 



The live fuel moisture model was developed to replace the herbaceous vegetation 

 transects used in the 1972 NFDRS. Although it is not rigorously based on principles of 

 plant physiology, the model does provide a broadscale approximation of the moisture 

 content of living herbaceous plants, leaves, and twigs of small woody shrubs. 



In the original live fuel model proposed by Rothermel , ^ plant moisture was deter- 

 mined as a function of the Keetch-Byram Drought Index. However, several empirical 

 factors required to control plant response to drying and wetting could not be derived 

 for all climates of the United States. 



During development of the 1978 NFDRS, we noticed the 1000-hour timelag (1000-h TL) 

 fuel moisture developed by Fosberg and others^ responded to wetting and drying cycles 

 similar to that expected for the live fuels. Thus, with some modifications, the 

 1000-h TL fuel moisture now serves as the basic meteorological filter for calculating 

 live fuel moistures. 



Plants adapted to different moisture regimes respond differently to rainfall anom- 

 alies. Those adapted to a moist environment will lose moisture faster during drought 

 than those from a dry environment. An essential feature of the live fuel moisture model 

 used in the 1978 NFDRS is a selection of drying rates by climate type or class. 



^Rothermel, Richard C. (n.d.) Live fuel moisture model. Manuscript in preparation. 

 Northern Forest Fire Laboratory, Missoula, Mont. 



^Fosberg, Michael A., Richard C. Rothermel , and Patricia L. Andrews, (n.d.) 

 Moisture content calculations for the 100- and 1000-hour timelag fuels in fire danger 

 rating. Manuscript in preparation. Rocky Mt. For. and Range Exp. Stn., Fort Collins, 

 Colo . 



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