Fuel Moisture - Wildland Fuel Fundamentals and Applications - page 73

Fig. 5.2 The equilibrium
moisture content (EMC)
of sugar maple wood at a
constant temperature of 27 °C
(80 °F), but different levels of
humidity when the humid-
ity in the air is greater than
in the dead fuel (adsorption)
and when the air is drier than
the fuel (desorption; Djolani
1970 )
assuming an exponential drying function. One hour (1-h) woody fuel particles, for
example, lose approximately two thirds of their moisture in 60 min. It takes about
four time lags to get to 95 % of the EMC, historically set at 4.5 % (Fosberg et al.
1970 ). Time lags increase logarithmically with the diameter of the woody fuel par-
ticle; 10-inch diameter particles take 10 times longer to dry (100 days) than 1-inch
particles (1 day; Fig. 5.3 ). Larger particles take longer to dry because diffusion
has to pull water vapor from the wetter areas inside the wood to the drier, outside
areas of the wood particle, which can only happen when the air is quite dry. As a
result, the inside wood of larger particles (1000 h fuels) will often be wetter than
the outside wood.
Fuel moisture dynamics are quite different for duff and litter as compared to
wood. Time lags for litter and duff are highly variable and depend on depth, type,
stage of decomposition, and a host of other biophysical factors. A 2 -inch-deep litter
layer in a ponderosa pine stand, for example, might have a time lag of 48 h or the
equivalent of a 1.4 in diameter woody fuel (Schroeder and Buck). Hydraulic proper-
ties of duff and litter mostly depend on the degree of decomposition with slightly
decomposed material having large pore spaces resulting in increased hydraulic con-
ductivity and faster drying and wetting (Plamondon et al. 1972 ). As decomposition
proceeds, the size of the organic particles decreases resulting in smaller pores with
higher moisture retention properties and slower drying. Hydraulic conductivity de-
creases significantly with the degree of decomposition. Duff bulk density is a useful
indicator of pore size distribution and hence, can represent hydraulic conductivity
and moisture retention in organic soils. Duff layers with high bulk densities, for
example, may take longer to dry.