Fuel moisture

1
Fuel moisture Importance of fuel
moisture Moisture affects every stage of the
combustion process ignition, spread, intensity,
smoke, fuel consumption, fire effects. With
sufficient moisture, fire doesnt burn (moisture
of extinction). When moisture is absent,
intense fire behavior can be supported. The
concept of available fuel depends strongly on
moisture relatively more or less of the same
fuel complex will be available depending on
moisture content. Fuel moisture is central to
the concept of fire danger rating (to be
discussed later).
2
Plants are designed to hold water the major
physiological function of xylem is water
transport. Almost all living plant tissues
contain aqueous solutions. Plant materials are
hygroscopic, capable of attracting and holding
water.
http//www.nsci.plu.edu/jmain/b359web/pages/xylem
.htm
3
Fuel moisture is measured as a percentage of dry
weight. Dead fuels can have very low moisture
contentbelow 10, as low as 2-3 (in deserts).
Fiber saturationthe amount of water that
saturates the cells of dead plant fuelsis
30-35. Additional water in sound wood occurs in
cracks, pores, spaces between cells. Rotten wood
has open spaces, can soak up water up to 300.
Live fuel moisture ranges from 50 to 300, can
be up to 1000. Moisture changes with time,
depending on fuel characteristics, weather, and
season. Fuels of different sizes
(surface-area-to-volume ratio) and composition
(sound, rotten wood) respond differently to
wetting and drying. Moisture varies with space
and position, depending on shade, arrangement on
the forest floor (depth, exposure to wind).
4
Dead fuel moisture Moisture content is dynamic,
as atmospheric moisture is constantly changing.
Fuels gain moisture from liquid water (rain,
snowmelt) or water vapor (humidity) they lose
moisture through evaporation.
5

• Factors affecting dead fuel moisture
• Weather
• Amount, form, timing of precipitation ? annual
  precip. patterns, rain/snow, soaking rain vs.
  showers.
• Wind usually drying (enhances evaporation) but
  wet winds can be wetting, cooling effect of wind
  can counteract solar heating.
• Solar radiation affects temperature,
  evaporation. Sunlight varies by time of day,
  time of year, slope, aspect, latitude, haziness
  of atmosphere.
• Air temp. (cold/warm fronts).
• Fuel characteristics size, chemistry, duff
  depth, soundness, presence of bark or wax.
• Fuel arrangement exposure to sun/shade, aspect,
  vertical/horizontal position, fuels attached to
  plants vs. fuels on ground.
• Topography elevation, aspect.

6
Relationships among these influences are shown
in the comparison of logs. Generally, sound wood
has less moisture than rotten. Elevated example
(air can reach all parts, center is most moist),
ground example (most moisture on bottom surface,
hardest to evaporate), rotten ground (higher
moisture, actually highest in center).
7
Equilibrium moisture content is the moisture
content that a fuel would reach if exposed to a
constant external moisture level (atmospheric
temperature humidity). Concept of chemical
equilibrium.
Fuels reach different EMCs depending on whether
they are wetting (adsorption) or drying
(desorption). Thus there is a range of
humidities associated with a given fuel moisture.
Note that maximum moisture content for these
fuels (needles, leaves) is around the fiber
saturation point (? 35), at over 90 humidity.
Note that atmospheric moisture is always
changing, so fuels dont reach EMC.
8
Timelag is the time needed for a fuel to adjust
to moisture change the rate at which a fuel
approaches EMC. Timelag interval is the time
required to lose 63 of the difference between
initial moisture content and EMC at constant T
and RH. Why not use 100 for the
timelag? Timelag classification 1 hr 0-1/4
diameter (1 hr is midpoint of 0-2 hr class) 10
hr ¼-1 100 hr 1-3 1000 hr 3, duff
9
Are these timelag values accurate? Nousually a
simplification often far wrong. For
instance, Anderson (1990) found Grasses,
mosses, lichens 2-4 hrs Weathered conifer
litter 2-14 hrs Fresh conifer litter 5-34
hrs Use timelag concepts as a general guide,
look at relative differences. Fine fuels
respond to diurnal RH shifts, big differences
between nighttime and daytime fire behavior since
fine fuels carry fire. If RH fails to rise,
active nighttime burning continues (Yellowstone
example). Remember there is a lag, so rains
wont immediately moisten heavy fuels, for
example.
10
Live fuel moisture is tied more to seasonal
patterns than short-term weather changes. High
moisture associated with fresh foliage, declines
over season, drops upon curing or frost.
Deciduous foliage stays relatively moist, but
retained old needles on conifers (up to 80 of
total foliage volume) are lower in moisture (lt
100). Many cool-season (C3) grasses cure
earlythis is why cheatgrass (Bromus tectorum) is
such a fire threat. Frost kill can lead to
rapid moisture drops. Drying from a surface
fire can lower live fuel moisture.
11
Adenostoma fasciculatum Chamise  Greasewood
http//www.mindbird.com/adenostoma_fasciculatum.ht
m
12
http//www.icess.ucsb.edu/resac/fm.html
13
Drought affects available fuels live fuels are
drier, live plants may die, heavy fuels may
become very dry. Drought doesnt affect the fine
dead fuels that ordinarily carry firethey can
get just as dry in non-drought fire seasonsbut
drought can add extra fuel.
14
ENSO (El Niño/Southern Oscillation) is a
recurring weather pattern characterized by warm
temperatures in the eastern South Pacific. "El
Niño" refers to the Christmas-time appearance of
warm currents off Peru "Southern Oscillation"
refers to fluctuating barometric pressure. El
Niño is associated with lower barometric pressure
southerly path of jetstream, generating storms
in a region that provides much moisture to the SW
US. Peak flows of the Pecos and Gila rivers (NM
and AZ) coincided with El Niño events over the
historical record (20th century) (Molles Dahm
1990). La Niña is the alternate extreme, when
cooler temperatures and higher pressure prevail,
leading to dry southwestern weather. Note that
northern US is wet while we are dry (jetstream
moves N) S Mexico was very dry during 1998 El
Niño.
15

• El Niño affects fire summary of 1998 fires (Food
  Agriculture Organization, fao.org)
• Fires in Mexico and Central America burned a
  reported 1.5 million ha. These generated large
  quantities of smoke which blanketed the region
  and spread into the USA as far as Chicago.
• From January to June 1998, about 13,000 fires
  burned in Mexico alone, covering 500,000 ha, and
  killing more than 70 fire-fighters and local
  residents.
• Between December 1997 and April 1998, more than
  13,000 fires burned in Nicaragua on more than
  800,000 ha of land. There were over 11,000 fires
  in the month of April 1998 alone.
• Severe fires in Florida in southeast USA in 1998
  burned a reported 200,000 ha of forest by May
  1998
• More than 150,000 hectares of coniferous forest
  and farmland were burned in various parts of
  Greece in August 1998, including the black pine
  forest on Mount Taygetos, site of more than 160
  endemic species and 36 endangered species of
  fauna.
• In July 1998, devastating forest fires affected
  more than 100,000 ha in the Far Eastern Russia.
  Coniferous forest burned in more than 150 places
  around Vladivostok, Sakhalin and Kamchatka
  Peninsula.

16
Measurement of Fuel Moisture Direct collect
samples, weigh, dry, weigh. Accurate but tedious
depends on sampling scheme. Can use timber
probes, but they have a limited range of moisture
values. Fuel moisture sticks (1/2 ponderosa
pine dowels) can be weighed and compared to known
dry weight. Inference can judge dryness from
color (curing grass), ability to break needles or
snap twigs. Calculation (mathematical models)
models based on correlations between fuel
moisture and weather variables.