Table A . --Comparison of diffusivLty calculated for ponderosa pine litter beds 

 under desorption conditions 





Fosberg 



(1975) 



: Anderson, 



Schuette , 



and Mutch 



Physical properties : 



1 : 



2 



: 1 : 



2 



: 3 



Fuel depth, H, (cm) 



3 



4 



2 



2 



2 



Bulk density, P^, (g/cc) 



0.08 



0.08 



0.005 



0.015 



0.045 



Needle timelag, t, (s) 



4,608 



4,608 



15,084 



15,084 



15,084 



Free air diffusivity, v , 

 (cm2/s) 



0.313 



0.313 



0.29? , 



0.292 



0.292 



Porosity, (}> , (dimension less) 



0.830 



0.830 



0.990 



0.971 



0.912 



Tortuosity factor, (J)^^, 

 (dimension less) 



0.748 



0. 748 



0.983 



0.950 



0.861 



Void diffusivity, v, (cm^/s) 



0.234 



0.234 



0.287 



0.278 



0.251 



Litter bed timelag, t, (s) 



4,248 



7,560 



18,588 



21,024 



19,968 



Litter bed diffusivity, v, 

 (cm2/s) 1 



.25x10"'+ 1 



25x10-"+ 



1.27x10-5 1 



. 12x10-5 



1. 18xlO"5 



The free air diffusivity was reduced by the above factor and the results for the litter 

 beds compared to the predicted response of weathered ponderosa pine litter beds present- 

 ed by Fosberg (1975) in his figures 2 through 5, table 4. Although the void diffusiv- 

 ities are not greatly different, the bed dif fusivities differed by a factor of 10 when 

 computed by equation (6) . This is primarily a result of the longer particle and bed 

 timelags existing in our experiments. 



According to figure 9 of Fosberg's paper (1975), our experimental conditions appear 

 to be at the limits of the theoretical considerations because our bed depth or thickness 

 of 2 cm is in the zone where response time or timelag decreases as bulk density increases. 

 As Fosberg notes, the timelag should increase as the bulk density increases and bed 

 porosity decreases. Our results do not show a strong relationship between timelag and 

 bulk density for the fuel loadings and depth we used. 



Discussion of empirical refinements to improve the description of moisture diffusiv- 

 ity by Bramhall (1973) has proposed the inclusion of diffusivity as a linear function 

 of moisture content. However, for litter beds and needles of ponderosa pine, the 

 diffusivity appears to remain constant except for early in the sorption change. The 

 same response of diffusivity is obtained using equation (6), as indicated in figure 10, 

 for ponderosa pine heartwood dowels (Fosberg and others 1970). The value of 2.3 x 10 ^ 

 cm^/s is higher than the value cited in the above work but is comparable to values 

 cited by Stamm (1964) for various woods. The variation in diffusivity does appear to 

 be nearly constant by species of material as long as the physical properties do not 

 change. Then equation 3, 6, or equation 30 presented by Fosberg (1975) could be used 

 to estimate the response time for a given fuel situation. If diffusivity is nearly 

 constant and the thickness of material is known, response time can be readily calculated. 

 For a given weather change in temperature and humidity, the time response can be con- 

 sidered with the EMC equations to estimate the moisture content of the litter and as 

 related to f lammability. 



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