Environmental Engineering Reference
In-Depth Information
geography (ecoprovinces) that encompass most of the western United States
2
They develop empirical models using the interannual records for the time
period 1977-2003. The empirical models account for, on average, two-thirds
of the variance of the wildfire area burned in the 16 ecoprovinces that com-
prise the western United States (cross-validated). Littell et al. find that in most
of the forested western United States, wildfire area burned is best explained
by dry and warm conditions in the seasons immediately preceding the fire,
presumably because such climate conditions enhance evapotransporation
and reduce fuel moisture. In contrast, fire area burned in the southwest
United States and in other arid ecoprovinces is strongly dependent on the
increased precipitation or positive PDSI (moisture) in seasons preceding the
fire season, consistent with the long-standing hypothesis that extra biomass
is produced in the seasons prior to the season that experiences an unusually
large area burned in these regions.
Together, the studies by Littell et al. (2009), Westerling and Bryant
(2008), and others demonstrate that the role of climate variability in regulat-
ing area burned is mainly a complex function of the climate (temperature,
precipitation, and wind), vegetation type, and orography. Nonetheless,
the climate and fire data for the United States are sufficient in spatial and
temporal coverage to formulate sensible and remarkably skillful predictive
models or incidence of wildfire and wildfire area burned. An example is
provided in Figure 5.6 from Littell et al. (2009: Figure 1), which shows the
observed wildfire area burned for the entire historical record (1916-2003)
and that predicted by empirical models that are trained and cross-validated
using the data from 1977-2003.
The results from the aforementioned studies strongly suggest that empiri-
cal models of wildfire should provide a skillful projection of how climate
change due to increasing greenhouse gases will impact wildfire across wide
regions of the globe, including the western United States and Canada. In
these regions, much of the area burned is due to large wildfires that result
mainly from climate variability, with in-season summer temperature, pre-
cipitation, and soil moisture being the predominant controls on fire. The
probability of summer averaged temperature and humidity is relatively
straightforward to quantify (precipitation perhaps a bit less so), and empirical
and hydrologic models needed to project summer soil moisture are quite
mature.
To date, only a few studies have examined how the projected climate
2
An ecoprovince is a large-scale region that is characterized by a common vegetation dis-
tribution, orography, and landscape structure (Bailey 1995). Most of the western half of the
continental United States is covered by 16 of Bailey's ecoprovinces.
