where 



m = 



average moisture content of needles or litter at time. 



t 



= 



e 



equilibrium moisture content 









initial moisture content 





E = 



fraction of total evaporable moisture remaining in the 



fuel at time, t 



K = 



dimensionless shape factor, assumed equal to 1.0 





t = 



time, minutes 





T = 



response time, time constant, or timelag, minutes. 





The descriptor, t, represents the time required to proceed 63.2 percent, or 

 (1 - 1/e) , of the way to the total expected change. The time to achieve the total 

 change can be broken in time periods. Each time period is the time to proceed 63.2 

 percent of the remaining portion of the total change and is tabulated in appendix 

 table 5 for needles and table 6 for the adsorption and desorption tests without solar 

 heating. The general trend shows an initial increase in time followed by a decrease 

 as displayed in figure 4. This type of response has been reported by Nelson (1969), 

 Mutch and Gastineau (1970), and Fosberg and others (1970). 



The desorption tests with solar heating yielded shorter response times because 

 solar heating at the 0.6 solar constant level increased needle surface temperature 23° F 

 (-5° C). This caused a temperature gradient to be imposed on the vapor pressure gradient 

 and shortened the response time. The adsorption runs following solar heating display 

 a response not presently understood. The data for these runs are presented in table 7 

 of the appendix. 



NO SOLAR ADSORPTION 

 500 r- 



400 



E 300 



200 



100 



J 



NO SOLAR DESORPTION 

 500 I- 



400 



300 



200 



100 







1 2 3 4 5 



TIMELAG PERIODS 



1 2 3 4 5 



Figure 4. — Adsorption and desorption timelag periods for ponderosa pine needle beds 

 at bulk density of 0.94 Ib/ft^ (0.015 g/cc) . 



7 



