Most forest fires in Switzerland occur in the southern part, which represents, with about 4000 km2, 9.8% of the total national area. Normally forest fires break out during winter (December – April), in the colline-submontane deciduous forest belt. Therefore, most forest fires are surface fires. In the past 30 years (1968 – 1997) an average of 66 forest fires occurred burning 805 ha of forests and pasture land annually. This resulted in an average of 12.2 hectares of burned area per forest fire in the last 30 years. Taking in account only the last ten years (1988-1997), the number of forest fires came down to an average of 56 forest fires and 490 ha of burned area annually, resulting in an average of 8.8 ha per forest fire.
Fig.1. Evolution of area burned in southern Switzerland 1900-1997. Data of 1997 are provisional and reported until end of October. Source: Forest Fire Database FNP SdA.
Fig.2. Burned area per month in southern Switzerland 1900-1997. Dates of 1997 are provisional and reported until end of October. Source: Forest Fire Database FNP SdA.
Fig.3. Statistical data of the forest fire season in southern Switzerland (January-April 1997). Source of meteo data: Swiss Meteorological Institute. Source of forest fire data: Forest Fire Database FNP SdA. probability of forest fire occurrence according to Mandallaz and Ye (1997).
After six years where the number of forest fires and the burned area dropped below the average values (Fig.1), the southern part of Switzerland experienced a relatively bad forest fire season with some dramatic fire events in 1997. Four large forest fires (> 100 ha) were recorded in March and April, of which two broke out on the same day. Up until the end of October 102 forest fires were reported to have burnt a total of about 1450 ha of forests and 250 ha of pasture land. The average of more than 16 ha burned area per forest fire is the highest since 1981. The burned forest area of 1997 represents nearly 1% of the total forested area of southern Switzerland. About 20% of these burned forests have a particular protection function because, as a rule, burned slopes in southern Switzerland are normally very steep (60-100%).
Of all meteorological parameters in the southern part of Switzerland it is especially precipitation that varies very strongly. Periods of dryness alternate with periods of very heavy precipitation. Periods of dryness are normally between December and March/April and shorter ones also between October and November. On an average of every four years there is a dry period of 30 to 40 days while every ten years there is one of more than 40 days without any rainfall. Dry periods of more than 60 days with less than 10 l/m2 of precipitation happen every 10 to 20 years.
Due to the dryness periods, which are normally most severe between January and April, the greatest part of the area is burned within these four months (Fig.2). In 1997 more than 1650 ha of the total 1700 ha, which burned through the end of October, burned in March and April. One reason was that in 1997 the dry period lasted extremely long. Statistically, within 93 days, between January 23 and April 25, there was only 5.9 l/m2 of rainfall (Fig.3-a). During this period, the temperature for each month was over the average values of the normal period 1961-1990 – in January 1.2 ° C, in February 2.4 ° C, in March 4.5 ° C and in April 1.5 ° C (Fig.3-b). This was caused by subtropic air masses (especially in March) as well as north foehn winds (warm winds which causes high temperatures and very low air humidity; see Fig. 3b and 3-c). Because of high air temperature and very low air humidity, litter and the upper soil layers dry up very fast and the risk of forest fire ignition increases.
Figure 3-f shows that burned areas greater than 50 hectares per day are always related to situations where there is a north foehn and the fire risk index is high (dotted area in Fig.3). The comparison of the temperature, air humidity, and the burned area show this very clearly. During such north foehn periods there is normally more than one forest fire per day reported (Fig.3-e).
In general, the forest fire index, proposed by Mandallaz and Ye (1997), shows a good fitness in fire probability rating (Fig.3-d). In future this model could also be useful in predicting fire severity, becoming in addition an important tool in estimating the ecological problems after fire events.
In conclusion the first four month of 1997 were characterized by abnormally big fires: about 1000 persons have been involved in fire fighting (fire brigades, military, police and forest service) with an aerial support of up to 10 helicopters (civil and military) per forest fire, summing in total 800 hours of aerial fire fighting.
Mandallaz, D., Ye, R., 1997: Prediction of forest fires with Poisson models. Can J. For. Res. 27, 1685-1694.
From: Marco Conedera & Peter Marxer Paolo Ambrosetti, Guido Della Bruna & Fosco Spinedi Address: Swiss Federal Institute for Forest, Snow and Landscape Research Swiss Meteorological Institute – Meteo Swiss FNP Sottostazione Sud delle Alpi Via Monti della Trinita 146 PO Box 57 CH – 6605 Locarno-Monti CH – 6504 Bellinzona-Ravecchia Fax: ++41-91-821-5239 Fax: ++41-91-753-2310 Tel: ++41-91-821-5231 Tel: ++41-91-756-2311 e-mail: email@example.com e-mail: firstname.lastname@example.org