Environmental Engineering Reference
In-Depth Information
wildfires is by lightning, and analyses of the global lightning strike and fire
frequency data suggests that ignition is a limiting variable for wildfires in
only a few areas on the planet (Krawchuck et al., 2009). Once fire is initi-
ated, the extent of a fire burn depends on fuel distribution and moisture
content, wind, and topography.
1
Hence, vegetation type, weather, and the
antecedent weather history (climate) are the main environmental parameters
that determine the frequency and extent of wildfire.
Comprehensive spatial and temporal records of wildfire and of climate
variables are available for the western United States. Analyses of these data
have led to a conceptual model that links wildfires to climate variability
and vegetation type. For example, Westerling and Bryant (2008) found that
in California and surrounding states, wildfires in “wet regions” (defined as
when climatological soil moisture exceeded 28% of capacity) tended to be
reported as forest fires, while wildfires in dry regions tended to be reported as
grassland and shrub fires. They further found that “wet” regions were much
more prone to fire in months when the maximum temperature exceeded
23ºC. Hence, they called these wet regions “energy limited”—positing the
probably of fire increases with increasing temperature because the moisture
content of the fuel also decreases with increasing temperature. They noted
that wildfires in dry regions were
positively
correlated with the
previous
year's precipitation—a result previously known from the study of Swetnam
and Betancourt (1998)—and posited that increased moisture in the previous
year allowed for an increase in biomass production in grassland environ-
ments that was then available to burn in the following summer. Hence, they
called these dry regions “moisture limited”—although in the sense of mois-
ture limiting the mass of fuel available to burn, rather than the incidence of
fire. Mid-elevation regions in the Sierra were classified as “energy limited,”
while southern California was classified as “moisture limited.”
Littell et al. (2009) develop empirical models for wildfire area burned
throughout the western United States. For predictors, they use seasonal
and antecedent climate variables including temperature and precipitation,
and the Palmer Drought Severity Index (PDSI) as a proxy for soil moisture.
Littell et al. formulate empirical models for zones of similar vegetation and
1
Fire suppression by management is found in many studies to explain much less variance
in total area burned than does the variability in weather and climate. For example, climate
variables account for more than 80% of the variance in annual area burned in the boreal for-
ests of western North America between 1960-2002 (Balshi et al., 2009); climate variables and
vegetation type account for two-thirds of the variance in area burned in the western United
States (Littell et al., 2009).
