DISCUSSION 



Timelag is a parameter characteristic of all physical and chemical processes 

 that occur at an exponentially decreasing rate.^ The question arises as to how 

 closely the process of either adsorption or desorption of moisture in natural fuels 

 approximates an exponential function. Our tests on reindeer lichen showed that response 

 times varied widely for the five timelag periods; time increased over the second and 

 third periods, then decreased. Nelson stated that a plot of E versus time on semiloga- 

 rithmic graph paper should be linear if the following timelag equation is obeyed: 



m - m 



^ = E = Ke ^ . • 



m - m T 

 o e 



where: 



m = average moisture content of fuel 



m^ = equilibrium moisture content 



m = initial moisture content 

 o 



E = fraction of total evaporable moisture remaining 

 in the fuel at time t 



K = a dimensionless constant for any given fuel shape 



e = base of natural logaritlims , 2.718 



t = time 



T = timelag 



Plots of E versus time for adsorption and desorption runs of reindeer lichen are 

 shown in figure 7. As Nelson^ found with sawdust, wood, and paper samples, our 

 experimental curves for reindeer lichen were curvilinear rather than strictly linear 

 as predicted by the timelag equation. Where Nelson's semi logarithmic plots fell into 

 three distinct regions, reindeer lichen data plotted into two straight-line portions 

 with differing slopes. 



Although the duration of the timelag periods varied, the variances do not negate 

 the practical application of the timelag concept. Observed deviations in timelag 

 do not appear to be significant in terms of general National Fire-Danger Rating 

 applications . 



The average desorption timelag for reindeer lichen was 1.7 times faster than the 

 adsorption timelag. Simard,° in comparing the average rates of wetting and drying 

 twig samples above fiber saturation, found that drying was about 1.4 times as fast as 

 wetting. This difference he laid to the added diffusion potential of evaporation 

 during the drying process. 



^Ralph M. Nelson, Jr. Some factors affecting the moisture timelag of woody 

 materials. USUA Forest Serv. Res. Pap. SE-44, 16 p., illus. 1969. 

 ^Ibid. 



^A. J. Simard. The moisture content of forest fuels. II: Comparison of moisture 

 content variations above the fibre saturation point between a number of fuel types. 

 Forest Fire Res. Inst. Inform. Rep. FF-X-14, 47 p., Ottawa, Ontario. 1968. 



6 



