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- Info
Weather, fuel status and fire occurrence: predicting large fires
| Author: |
Viegas, D. X. |
| Date: |
1998 |
| Periodical: |
In: Moreno, J.M., ed. Large forest fires. Leiden, the Netherlands: Backhuys Publishers. |
| Abstract: |
Large forest fires are by far the most challenging problem which both, fire managers and fire researchers, have to deal with. Under normal circumstances of fire detection and suppression efforts, a fire becomes large usually because it has great potential of spreading very fast and intensely, precluding the chances of being suppressed in its initial stages. It is therefore of great practical importance the understanding of the natural conditions that help the occurrence of large fires and the particular conditions of their spread. When dealing with the problem of forest fires we immediately recognize that the size, measured in terms of burned surface, is a parameter that varies in several orders of magnitude, typically from 0.01 ha to 10^4 ha, at least in Southern Europe. It is well known that usually a small number of very large fires causes more destruction of the forested surface than all the remaining fires put together in a certain region. Therefore, large fires play a very important role in the entire problem of forest fire management, not only due to the prevention measures required to avoid them, or to the fighting efforts to suppress their spread, but mainly due to the economical and ecological effects that they produce, some of them with an irreversible nature. Before proceeding we must define the concept of a large fire. Usually we associate this concept to the burned surface of the fire. Alternatively we could also consider the economical or ecological value of the area, but this is quite difficult to assess in general terms. Even the concept of size associated to a ‘large fire’ is somewhat relative. For example a ‘large fire’ in North America is certainly bigger than in Western Europe. Even in Europe this concept changes from one country to another. We could propose the value of 1000 ha to be quite acceptable as a general limit between medium and large size fires, but we prefer to retain in this chapter the concept of large fire associated to one that has a potential of becoming very large, due to its
propagating conditions, even if it did not burn actually a very large area. As will be clear soon this concept is based more on physical conditions of fire start and of fire propagation, which play a very important role in the entire problem, than on human intervention and fire suppression activities. Looking at the statistics of fire occurrence for any country of Southern Europe one realizes that even if the number of fires seems to be increasing every year, the burned area varies in a wide range, in spite of the generalized improvement of fire prevention and fire suppression structures. In Figure 1 such data for Portugal are given. Bearing in mind that the total burned area is made essentially by the large and very large fires we must then try to understand why the figures are so different from one year to another. |
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