DISCUSSIOIM 



Equilibrium Moisture Content 



A variety of studies of individual fuel types have been conducted to determine the 

 EMC's of wildland fuels (Dunlap;^ King and Linton 1963; Blackmarr 1971; Van Wagner 1972; 

 Britton and others 1973). However, Gisborne (1928) noted that, tests by Dunlap^ on woody 

 samples of different species yielded results so nearly alike that a common EMC could 

 be assigned to given relative humidities. Van Wagner (1972) found that leaf litter had 

 higher EMC's than common woods by about 3 percent moisture content. Differences between 

 leaf litter types were felt to have little practical importance for predicting fine fuel 

 moisture content. However, Blackmarr (1971) and Van Wagner (1972) did observe an in- 

 crease in EMC with weathering and aging which possibly is associated with the leaching 

 of waxes and oils from the needles. The newly cast needles collected in September and 

 October that we tested achieved EMC levels slightly lower than those found by Van Wagner 

 for red pine needles. 



The EMC data for ponderosa pine needles given in table 1 are plotted in figure 5 

 for adsorption and desorption. The curves displayed are from prediction equations 

 generated from the data, according to the approach presented by Van Wagner (1972) . The 

 general form of the prediction equation is: 



EMC = aH^ + c • EXP [ (H - 100) /d] + m(T^ - T) (2) 



where 



EMC = equilibrium moisture content, percent ovendry weight 

 H = relative humidity, percent 

 T^ = reference temperature 



T = temperature 



a, b, c, d, m = coefficients dependent upon species, as noted by Van Wagner. 



The last term adjusts for temperature which we did not test, but comparisons will be 

 made to results obtained for ponderosa pine needles in other studies (Anderson 1964; 

 Rothermel and Anderson 1966; Anderson 1969) . These studies provide data on the adsorp- 

 tion equilibrium moisture content at 90° F (32° C) as ponderosa pine needles were con- 

 ditioned for burning tests. 



The coefficients of the terms in the equation were determined by first calculating 

 a least squares fit for a power function, aH^, up to 60 percent relative humidity for 

 the first term. The calculated values of EMC were subtracted from the observed EMC 



2 Dunlap, M. E. Forest Products Laboratory, Madison, Wisconsin; work cited by 

 Gisborne (1928), p. 29. 



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