PRIEST RIVER 



IDAHO 



Figure 1 — General locations of sampling for thie 

 foliar litter studies. 



Laboratory Procedures 



Three temperature-controlled cabinets were used to 

 condition litter samples. Two cabinets were used to es- 

 tablish high and low MC starting points, and the third 

 one maintained a constant atmospheric humidity. The 

 high-humidity condition was established over water 



at the test temperature in the first cabinet and the low- 

 humidity condition of 2 to 3.5 percent relative humidity 

 (RH) was set in the second cabinet, with ambient air 

 heated to 322 °K (120 °F). This procedure was similar 

 to Van Wagner's (1972) and provided comparability. 

 Although the possibility of a decrease in hygroscopicity 

 existed because of influences like internal and external 

 physical stresses, the results indicated that this was not 

 a major influence. Within each cabinet, RH was moni- 

 tored with the aid of a dew point detector. Samples of a 

 selected litter were weighed, placed in cabinets 1 and 2, 

 weighed periodically until the weight change had stabi- 

 lized, and then placed in cabinet 3 so the EMC was ap- 

 proached with desorbing and adsorping litter samples. 

 Again, samples were weighed periodically until the 

 weight change had stabilized. This period of time was 

 from 1 to 2 weeks, depending on the foliar litter being 

 tested. When the samples had reached their final values, 

 they were processed with xylene distillation to determine 

 the final MC. Saturated salt solutions were used to main- 

 tain the relative humidity at each of the test tempera- 

 tures used. Table 1 shows the humidities achieved at 

 the various temperatures. 



An initial test series was done at 295 °K (71 °F) with 

 litter samples considered to be recently cast. The results 

 indicated the samples were often a mixture of recently 

 cast and weathered materials. A second series was tested 

 at 300 °K (80 °F) using litter samples collected more care- 

 fully. Four of these fine fuels, ponderosa pine and 

 Douglas-fir needles, cheatgrass, and quaking aspen 

 leaves, were also tested at 278 °K (40 °F) and 322 °K 

 (120 °F) to investigate the change in EMC with tempera- 

 ture. Fuel and air temperatures were the same in these 

 tests, and two or more replications were done for each 

 sample at each temperature and relative humidity. 



Results from these tests were organized by litter type, 

 recently cast or weathered condition, adsorption or 

 desorption, and the temperature-humidity combinations. 



Data Analysis 



In addition to studying the differences in EMC values 

 for the various foliar litter fuels, a method of estimating 

 EMC at different temperatures and humidities was inves- 

 tigated. The most promising approach was the Gibbs fi-ee 



Table 1 — These saturated salt solutions maintained the relative humidities shown for the test temperatures 



Salt 



Temp 



RH 



Temp 



RH 



Temp 



RH 



Temp 



RH 





°K 



Pet 



°K 



Pet 



°K 



Pet 



°K 



Pet 



Li CI 



278 



17 



295 



13.5 



300 



11.8 



322 



12.7 



MgCI^ 



278 





295 



34.0 



300 



30.0 



322 





Mg(N03)^ 



278 



61 



295 



53.0 



300 



53.0 



322 



48.0 



NaCi 



278 





295 



74.0 



300 



70.5 



322 





KNO3 



278 



82 



295 



85.0 



300 



78.0 



322 



88.0 



°C 



5 





22 





27 





49 





°F 



41 





72 





80 





120 





2 



