7th Workshop on Forest Fire Management - EARSeL, European ...

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II - VALIDATION OF RS PRODUCTS FOR FIRE MANAGEMENT
pre-fire season may therefore represent an added value for fire management
in Portugal (Gouveia et al., 2009). In fact, the Iberian Peninsula is recurrently
affected by drought episodes and a combined effect of lack of precipitation
over a certain period with other climatic anomalies, such as high
temperature, high wind and low relative humidity over a particular area may
result in reduced green vegetation cover. In fact, major fire events in Iberia
are frequently preceded by drought periods, reflected by the decrease of
vegetation activity before the fire episode. This effect is further aggravated
by a quick response of dead fuels to changing meteorological conditions,
such that even short periods of drought may lead to an exponential
increase in fire risk.
2 - Results
Burned area (BA) data covering the period 1982-2006 were obtained from
the official data supplied by the National Forest Authority (AFN). Figure 1
presents the inter-annual variability of burned area over Portugal during the
considered period. The close resemblance between the annual and the summer
curves implies that the annual fire regime in Portugal is mostly dominated
by events occurring in July and August. Vegetation stress prior to the
begining of the fire season was assessed using averaged values in May of
the Normalized Difference Vegetation Index (NDVI) as obtained from the
Global Inventory Modelling and Mapping Studies (GIMMS) database (Zhou
et al., 2001), covering the considered 25-year period. Values of NDVI in May
were then spatially averaged over the forested regions of Portugal as identified
by downgrading the Global Land Cover (GLC) for the year 2000
(Bartholome et al., 2002) to the GIMMS scale. Meteorological fire danger in
summer 1982-2006 was estimated using July + August means of the Daily
Sevirity Rating (DSR), an index that may be derived from the so-called
Canadian Fire Weather Index (FWI) system (van Wagner, 1987). DSR is a
numeric rating of the difficulty of controlling fires and reflects the expected
efforts required for fire suppression (CFS, 2007).
Regression tree analysis was used to assess how yearly amounts of BA in
the fire season could be related with vegetation stress prior to the fire season
and meteorological conditions during the fire season. Regression tree
(Breiman et al., 1984) is a nonparametric technique that is very effective
in selecting from given variables the interactions among them that are
most important in determining the outcome variable to be explained.
Figure 2 presents a scatter plot of NDVI in May versus DSR in summer 1982-
2006 (left panel) together with the recorded amounts of BA during the
same period(right panel). Figure 3 provides a schematic overview of the
obtained regression tree. It may be noted that years of weak activity are
associated to both low values of NDVI in May (i.e. low biomass) and DSR in
summer (i.e. meteorological conditions that do not favour the onset and
spreading of fire). The largest event (in 2003) is, on the contrary, associ-