Allowance for specific fine fuel types may be necessary, particularly in areas 

 where an index, such as the Ignition Index, is used administratively and is sensitive 

 to fine fuel moisture content. For example, in an area where conditions can reach 80° F 

 (27° C) and 20 percent relative humidity on a cloudy day, the NFDRS estimates nonliving 

 fine fuel moisture content to be 4 percent. This may be 3 to 4 percent below the actual 

 moisture content. This difference changes the Ignition Component from 65 to 38, which 

 may be significant in a control unit's action plan. This type of sensitivity to fine 

 fuel moisture content suggests that the measurement techniques and predictions of fine 

 fuel moisture content be as accurate as possible. 



Timelag 



Ponderosa pine needles and litter beds exhibit similar timelag variability, but the 

 similarity is less evident in litter bed test results (fig. 3) . The variation in the 

 timelag period may be due to the various factors influencing the diffusivity of the 

 material. Byram^ discussed control of moisture flow and thought that a forcing function 

 of moisture content and saturation vapor pressure could effectively describe the change. 

 This concept was expanded upon by Nelson (1969) and Fosberg (1970) . Nelson showed that 

 the diffusion theory could describe certain changes in moisture response of fine fuels, 

 but such things as initial moisture content and relative humidity effects are not accu- 

 rately predicted. Fosberg (1975) has developed a generalized approach to the theoretical 

 solution using a timelag definition dependent upon physical properties of the material. 

 He used boundary conditions that were difference approximations to the continuous change 

 in temperature and relative humidity and noted that a different response time should be 

 expected for each different set of initial conditions. The timelag has been associated 

 with standard drying conditions of 80° F (27° C) and 20 percent relative humidity as noted 

 by Nelson (1969) for NFDRS use. For constant conditions, timelag is expressed as a 

 function of material thickness, moisture diffusivity, and a dimensionless number, 

 defined as a Fourier number for moisture by Byram^'. For our tests, we are starting 

 from the standard set of conditions to establish a stable method of evaluating timelag 

 and to indicate the effect of bed bulk density on timelag. 



Previous work and our test results show the timelag response is curvilinear, 

 yielding shorter times for the fourth and fifth time periods. The latter periods 

 involve small changes in moisture content, so small errors in moisture measurement 

 result in large variations in the time measurement. For these reasons, and because 

 the first three periods account for 95 percent of the total change and exhibit a near 

 linear response on semilog graph paper (fig. 3; that is, a constant log drying rate) 

 we determined the average timelag from the time to achieve a 95 percent change in 

 moisture content. The time to achieve a 95 percent change represents three time 

 periods, so one-third of that total time is the average timelag for a run. The mean 

 timelag for each litter bed and sorption condition are given in table 2, along with the 

 physical properties of each litter bed test condition. 



Comparison of the results obtained using five time periods, three time periods, 

 or the time for a 95 percent change of each run to compute an average and standard 

 deviation showed that less variability occurred with the time to 95 percent change. 

 For the litter beds with a bulk density of 0.015 g/cc, the standard deviation de- 

 creased from 125 min for five time periods to 69 min for three time periods to 48 min 

 for a 95 percent change. Although greater stability is indicated for the latter 

 method, the standard deviation indicates considerable variability in the vegetative 

 material and litter beds; this indicates a number of runs are needed to obtain a 

 reliable mean. 



12 



