UNIT V--FUEL MOISTURE
The fuel, moisture content in natural fuels is such an important factor to fuels availability for fire
ignition and combustion that we have devoted an entire unit to the subject. Most fuel complexes
contain a combination of dead and live fuels; thus, a wide range of moisture contents occur
within these fuels. Since all fuels may not be involved in a flaming front or be consumed, by fire,
our analysis of fuel complexes must determine which fuels will be responsible for the
propagation of fire.
The purpose of this unit is to help you make estimations of moisture content in various dead and
live fuels, to identify those fuels which can burn, and to assess the chances of ignition from
firebrands landing in new fuels or of fire spreading by preheating fuels ahead of a flaming front.
You will also recognize that fuel moisture content is a very important input toward making fire
behavior calculations and predictions.
Before starting the unit, be sure you have read the instructions to students on page 1 of your
workbook. On page 2, you will find the unit objectives on which you will be tested at the end of
this unit. Please study these objectives, then return to this text.
The first section, starting on page 3, deals with natural fuels and their moisture contents. Fuel
complexes vary greatly by areas or regions; with extremes from sparsely vegetated deserts, to
rain forests with lush vegetation, to parched timber lands. If we view each as a potential fire
environment, our immediate assessments must include fuel loadings and fuel moisture contents.
We would expect desert fuels to be dry for extended periods, but is there enough fuel to carry
fire? The rain forest has abundant fuels which are generally too wet or too green to burn, but
infrequently these areas do have fires. Extended summer drought periods occasionally make our
timber lands extremely dry, sometimes to the point of being "explosive", should fires occur.
We can generalize at this point and say that when fuel moisture content is high, fires ignite and
burn poorly, if at all; and when it is low, fires start easily, and spread and burn rapidly. These
simple deductions might satisfy some fire managers, except that fuel moisture contents are
frequently some place between the two extremes and fluctuate with changes in weather. During
normal fire seasons, these same fire managers have experienced times when rapidly spreading
fires suddenly stop, perhaps even go out, due to changes in their fuels and moisture contents.
These fuels may have been on a different aspect, had a later curing date, or experienced a sudden
change in relative humidity.
How does one measure fuel moisture content and then anticipate what changes will take place
over time and space? First of all, fire managers have agreed upon a common description and unit
of measure for fuel moisture content. This is the amount of water in a fuel, expressed as a
percent of the oven dry weight of that fuel. If there were no moisture at all in the fuels, as if dried