Fuel Moisture - Wildland Fuel Fundamentals and Applications - page 78

requiring times that range from overnight to days depending on the fuel compo-
nent. The dried fuel is then weighed and the difference between field-measured
fuel weight and oven-dried weight is the mass of water lost in the drying process.
This water loss is usually divided by the oven-dried fuel mass to get FMC in units
of percent (Matthews 2013 ). The best results for this method occur when samples
are dried at lower temperatures (80-85 °C) for longer time periods (48-72 h), which
prevent unwanted changes to the organic structure of the fuel particle, especially
live fuels and litter.
There are some problems with using the oven-dry method for estimating FMC
(Simard 1968 ). First, fuel samples take a long time to process, making this method
difficult to apply in real-time fire management operations, such as prescribed fires
and wildfire suppression, when FMCs need to be quickly estimated. Second, care
must be taken in both the field and the laboratory to ensure accurate fuel moisture
measurements; collected samples must be immediately put into a plastic bag and
put in a cool environment to prevent adsorption and water loss before weighing
in the laboratory. Drying organic materials in a hot oven may also volatilize other
chemical constituents of the fuel particles, especially live foliage, resulting in an
unwanted loss of organic biomass and a slight overestimate of FMC. On the other
hand, if the oven temperature is too low, some water may be retained in the cell
structure and FMC estimates may be underestimated. Some woody fuel particles
are so large that it may take days to completely dry the particles, while small fuels,
such as twigs, take only hours to dry. These disparate drying times make obtaining
a comprehensive set of FMCs for all fuel components difficult in an operational
setting.
Several other field methods have been used to estimate FMCs in the field. Banks
and Frayer ( 1966 ) correlated the angle at which a pinaster pine needle broke to the
needle's FMC to create the “leaf-bending method” for quick and easy field use,
albeit only for that forest type and only for needles. The “speedy moisture meter”
used by Dexter and Williams ( 1976 ) measures the pressure of acetylene gas evolved
from mixing finely divided fuels with calcium carbide to estimate FMC, but its
use was limited because it required careful preparation of fuel particles (Marsden-
Smedley and Catchpole 2001 ). Another FMC method is to pulse an electric charge
through the fuel particle and measure electrical signals, such as resistance, imped-
ance, or capacitance. FMC is then calculated from a calibration curve that relates
the electric signal to FMC (Stamm 1927 ). The Wiltronics TH Fine Fuel Moisture
Meter, for example, estimates FMC from the electrical resistance of a fine fuel
sample (Chatto and Tolhurst 1997 ). Gravimetric sampling, however, is still the most
common and reliable method to calculate FMC (Viney 1991 ; Chatto and Tolhurst
1997 ).
Because FMC is difficult to conveniently and accurately measure in the field,
it is often estimated using indirect techniques. One indirect method is to estimate
FMC for dead woody fuels using fuel analogues or moisture indicator sticks. A set
of four wooden dowels of standard size and weight are placed at a weather sta-
tion and weighed at various time intervals to approximate fuel moisture for 10-h
woody fuel component. The current and past moistures of these dowels are related