DIGITALNA ARHIVA ŠUMARSKOG LISTA
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| ŠUMARSKI LIST 11-12/2015 str. 67 <-- 67 --> PDF |
fine fuel. Fuel heights of litter and shrubs were measured as the vertical distances from the bottom of each layer to the apparent average top of the layer. Dead fuel components were separated by size classes defined by their diameter, respectively 0 to 0.6 cm (fine fuels), 0.6 to 2.5 cm (medium branches) and 2.5 to 7.5 cm (coarse branches), corresponding to the 1h, 10h, and 100h timelags (Rothermel, 1972; Deeming et al. 1972). Fuel moisture contents were determined by sampling dead and woody fine fuels immediately before each fire. Samples were weighed, taken to the laboratory and oven-dried to a constant weight at 105 ºC for at least 24 h. Moisture contents were calculated on a dry weight percentage basis. All experimental fires were conducted from late July through early September; the fire season generally lasts from late June until mid October. Weather conditions and fuel moisture contents are thus of crucial importance for wildfires at this period in the study areas. Except for low maquis, all fires were carried out in flat areas. The preferred time of ignition was mid-afternoon to coincide with daily peak burning conditions. All fires were ignited with a drip torch to rapidly establish a fire line along the windward edge of the plot. Fires were allowed to propagate with wind down the length of the plot in order to simulate a free burning fire. In all experimental fires, the 2-m open wind speed was recorded at 15 second intervals by an automatic weather station set up on the site. The wind measurements were averaged over the period of fire spread. Fire behavior was monitored during each fire, starting from the time the ignition line was lit. Head fire rates of spread were determined by timing when the fire front arrived preplaced poles. In addition, the progress of the fires was video-recorded from two sides and photographic records were taken for later evaluation. Frontal fireline intensity was calculated for each plot as per Byram (1959): I= H×W×R where I is the fireline intensity (kW m–1), H is the heat yield of the fuel (kJ kg–1), W is the weight of fuel consumed per unit area in the active flaming zone (kg m–2), and R is the rate of spread (m s–1). Development of custom fuel models – Izrada uobičajenih modela goriva The Behave Fire Behavior Prediction and Fuel Modeling System was among the early computer systems developed for wildland fire management. It has been updated and expanded and is now called the BehavePlus Fire Modeling System to reflect its expanded scope. BehavePlus provides a means of modeling fire behavior characteristics (such as rate of spread and flame length), fire effects (such as scorch height and tree mortality), and the fire environment (such as fuel moisture and wind adjustment factor) (Andrews et al. 2005). We used BehavePlus 4.0 software for both developing the custom fuel models and estimating potential fire behavior. For each fuel complex, an initial fuel model was developed in BehavePlus by entering the average measured fuel characteristics. The variables surface area to volume ratio and heat content of the particles, as well as dead fuel moisture of extinction, are also required to run the surface fire spread model of Rothermel (1972). Representative values were taken from the literature (Dimitrakapoulos, 2001; Cohen et al. 2003; Fernandes, 2009). The resulting fuel model was then parameterized to match the experimental fire behavior characteristics, especially through the adjustment of fuel depth. The measured 2-m wind speed was assumed to equate the midflame height wind speed required by BehavePlus. RESULTS AND DISCUSSION Rezultati i rasprava Table 1 contains the parameters for the fuel models developed in this study. Fuel loads are isimilar between low and tall maquis indicating that tall maquis is a more aerated fuel complex. This should be an outcome of the different dominant species – Quercus coccifera L. in low maquis, Arbutus andrachne L. and Pistacia lentiscus L. in tall maquis – and because biomass increase in tall shrubs is largely caused by increases in larger live branches that do not necessarily contribute to fire behavior. Also, low maquis is richer in fine dead fuels, because the species dominating tall maquis do not carry dead fuels in the canopy, and because litter fuels were incipient in tall maquis. The descriptive statistics of the fuel and fire behavior characteristics are given in Table 2. Oven-dry fuel biomass (1h, 10h, 100h) ranged from 2.30 to 49 t ha–1, 0 to 19.50 t ha–1, and 0 to 0.60 t ha–1. Dead fine fuel moisture (Md) content ranged from 7%–25%. Average Md was 12,6%. Fuel depth changed from 2.3 cm to 300 cm. In this study, all experimental fires were done under the mid wind conditions. Mean wind speed was 8.8 km h–1. Rate of fire spread and fire line intensity are important fire behavior characteristics. In this study, Rate of fire spread ranged from 0.30 m min–1 to 6.10 m min–1, and fire line intensity ranged from 22 kW m–1 to 4241 kW m–1. Table 3 displays the observed and predicted fire behavior characteristics for the developed fuel models. The main criteria to adjust the fuel models was rate of fire spread, and it was not always possible to predict adequately both rate of spread and energy release (Burgan & Rothermel, 1984). Consequently, a stronger agreement exists between observed and predicted rate of spread than between observed and predicted flame length or fireline intensity. A one order of |