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ŠUMARSKI LIST 11-12/2002 str. 28     <-- 28 -->        PDF

M. Vučetić: VREMENSKE PRILIKE I USPOREDBA SEZONE ZAŠTITE ŠUMA OD POŽARA 2001.
SUMMARY: A map presentation of mean long-term (1961-1990) seasonal
severity ratings (SSR) has been made for the first time for Croatia, at risk of
forest fires in the warm part of the year. The obtained mean status can be used
for comparing the success of seasonal forest fire protection measures, or the
impact of weather factors on the success/failure of protection. According to
the map, on average the most endangered area is that from Šibenik to Ploče
with the adjacent islands and the hinterland. High values also occur approximately
up to the northern boundary ofDalmatia, and inland in the wider Knin
area. It was found that the SSR values for Senj, due to the particular feature of
the wind regime, do not represent wider surroundings. Based on the justifiable
assumption that there are more such areas, further research will be made
aimed at obtaining a veritable presentation of fire severity.


The first comparison of the SSR with the long-term mean values was made
for the season 2001. The analysis shows that during the season June-September
2001, there were fundamental changes in the sense of increased SSR values
above the average, primarily in the continental part of Central Dalmatia
(Knin and Split hinterland) and in the wider surroundings of Šibenik and
Ploče. Decreased SSR values on the Central Dalmatian islands do not indicate
the expected absence of forest fires in this area. The reason for this is the
mean value for the period which is too long (June-September). Consequently,
in further analysis attention will be focused on shorter periods, the Monthly
Severity Rating.


The analysis of weather conditions during the season show that August
was the driest month and September the rainiest. Low precipitation quantities
occasionally prevented the occurrence of long precipitation-free periods. The
longest precipitation-free periods occur at the end of July and last until before
the end of August. This is the most critical period of this season, especially


just before its end, for the occurrence and spread of forest fires. The biggest
changes of wind conditions along the coast, the hinterland and islands are
seen during August. As most forest fires erupted in this month and caused the
largest burned areas, it can be concluded that one of the basic weather factors
for forest fires, along with dry periods, is the change in the wind regime.
According to the ISI (Initial Spread Index) and FWI (Fire Weather Index)
indicators, there were several critical periods, but 11 and 12 August feature
prominently. Under the conditions of ISI > 18 and FWI > 34, which were fulfilled
especially in the second half of August, forest fires could assume all the
characteristics of crown fires.


The presented analysis indicates that during the observed year of 2001
(the June-September season) the major part of the observed area (except for
Central Dalmatian islands) provided more than average favourable conditions
for the occurrence and spread of forest fires. It was in August in particular
that weather conditions were conducive to the occurrence of the worst


form of forest fires, the crown fire.
Key words: long-term mean values of seasonal severity rating, forest
fire, fire weather index, crown fire


Šumarski list br. 11-12, CXXVI (2002), 563-574