1.0 



0.5 



0.2 



0. 1 



0. 05 



0. 02 



0.01 



FOS BERG P = 0.08 G/CM', DEPTH = 4 CM 

 b 



,P = 0. 045 G/CM' , DEPTH = 2 CIVl 



I 



100 



200 



300 

 TIME (MIN) 



400 



500 



600 



Figure 9. — Comparison 

 of average moisture 

 content for theoretiodl 

 analysis of moisture 

 content profile 

 (Fosberg 1975) and 

 experimental results 

 for litter bed of 

 ponderosa pine. 



and moisture stress were imposed in both situations and the comparison shows that the 

 theoretical approach does predict the timelag to increase with each successive time 

 period {fig. 9) . This behavior is found at each bulk density when solar heating was 

 used and reflects the initial response to both the temperature and moisture changes 

 followed by the response to moisture only, which has a longer timelag. 



Moisture Diffusivity 



It has been noted earlier that the timelag did not remain constant during the 

 sorption process and the differences could be traced to the assumptions made in the 

 theory of homogeneous physical properties and the diffusivity. Fosberg (1975) discussed 

 the diffusivities involved and utilized them in the development of his theoretical 

 approach. At least three degrees of diffusivity need to be considered; the diffusivity 

 of the particles, of the voids, and the effective diffusivity of the litter layer. 



For individual particles, it has been shown that the Fourier number for moisture 

 describes the relationship of timelag, diffusivity, and particle thickness (Fosberg 

 1970; Fosberg and others 1970): 



F = v/XR' 



where 



o 



V 



R 

 1/X 



X 



(3) 



F = Fourier number, dimensionless 



= diffusivity, cm^/s 

 - thickness, cm 

 = T = timelag, s 



= timelag reciprocal or decay coefficient, s 



-1 



16 



