CSIRO PUBLISHING
www.publish.csiro.au/journals/ijwf
International Journal of Wildland Fire, 2004, 13, 227–240
The post-fire measurement of fire severity and intensity in the Christmas 2001 Sydney wildfires Chris J. Chafer A , Mark NoonanB and Eloys Macnaught A,C A Sydney
Catchment Authority, PO Box 50, Appin, NSW 2502, Australia. Telephone: +61 2 4640 1018; fax: +61 2 4640 1006; email:
[email protected] B Sydney Catchment Authority, PO Box 323, Penrith, NSW 2751, Australia. Telephone: +61 2 4725 4727; fax: +61 2 4725 2523; email:
[email protected] C Present address: ESRI Australia, Level 1, 414 Kent St, Sydney 2000, Australia. Telephone: +61 2 9285 5509; fax: +61 2 9267 2099; email:
[email protected]
Abstract. Using pre- and post-fire satellite imagery from SPOT2, we examined the fire severity and intensity of the Christmas 2001 wildfires in the greater Sydney Basin, Australia. We computed a Normalised Difference Vegetation Index (NDVI) from the two satellite images captured before (November 2001) and after (January 2002) the wildfires, then subtracted the later from the former to produce a difference image (NDVIdiff ) which was subsequently classified into six fire severity classes (unburnt, low, moderate, high, very high and extreme severity). We then tested the fire severity classification on 342 sample sites within the 225 000 ha fire affected area using a qualitative visual assessment guide. We found that the NDVIdiff classification produced an accuracy of at least 88% (Khat = 0.86), with the greatest discrepancy being between the low and moderate classification. Knowledge of rate of spread over some of the affected area, coupled with a complete knowledge of fuel loads, was used to retrospectively model fire intensity, which in areas of extreme fire intensity, produced heat energy levels exceeding 70 000 kW m−1 . Importantly, we found no positive effect of topography on fire severity, in fact finding an inverse relationship between slope and fire severity and no effect due to aspect. Further analysis showed that flat to moderate slopes less than 18◦ across all aspects suffered the greatest vegetal destruction, and there was no relationship between north-westerly aspects and fire severity. We also introduce a relatively simple method for estimating fuel load biomass using a combination of satellite image and rapid field assessment. We found 79% accuracy for this method based on 125 sample sites. It is postulated that this type of analysis can greatly improve our understanding of the spatial impact of fire, how natural areas within the fire ground were impacted, and how remote sensing and GIS technologies can be efficiently used in fire management planning and post-fire analysis.
Introduction Although attempts have been made to utilise the remote sensing capabilities of satellite-derived information to monitor fire histories around the globe (Tanaka et al. 1983; Fox and Stuart 1994; White et al. 1996; Viedma et al. 1997; Sunar and Ozkan 2001; Bertolette and Spotskey 2001; Rogan and Franklin 2002), the technology has not been widely received or utilised in eastern Australia. Wildfire in particular is a common source of spatial and temporal disturbance to plant communities in south-eastern Australia (Gill 1975; Whelan 1995), and the effect of fire severity and intensity is poorly understood (Morrison 2002). The 2001 Christmas wildfires in the Sydney Basin of south-eastern Australia, 3 December 2001 to 14 January 2002 (Winter and Watts 2002; Noonan et al. 2002), provided an ideal opportunity to examine how remote sensing technologies can be utilised in Australian © IAWF 2004
conditions to study the effect of fire intensity and the spatial severity of conflagrations. The Sydney Catchment Authority (SCA) is responsible for the environmental regulation of landscape activities within a 16 000 km2 catchment supplying six primary drinking water reservoirs in south-eastern New South Wales (NSW), Australia (Fig. 1). Over 65% of the water supply catchment is vegetated with sclerophyll-dominated communities (SCA 2001), and an understanding of how wildfire may potentially impact water quality is imperative for complying with various legislative and regulatory obligations of the Authority (SWCMA Act 1998). A number of post-fire water quality studies have been instigated by the SCA and independent researchers (e.g. Shakesby et al. 2003), and critical to these studies has been knowledge on fire severity. Fire severity can be defined as the integration of the physical, chemical and 10.1071/WF03041
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Fig. 1. Location of the Sydney Basin, Australia, highlighting the Sydney drinking water supply catchments (Special Areas) and geographic locations mentioned in the text.
biological changes at a site as a result of fire disturbance (White et al. 1996; Rogan and Yool 2001). In this paper we illustrate fire severity as a function of vegetal change, and discuss methodologies for assessing this using remote sensing and GIS technologies. Since the late 1800s, SCA water supply catchments have been managed for the supply of drinking water to Sydney and the surrounding urban centres, and since the late 1960s active fuel mitigation works have been undertaken (Chafer et al. 2001). Within surrounding National Park and State Forest estates, similar periodic fuel reduction methods were employed (Williams and Gill 1995; Jasper 1999; Bean and Jones 2001; Bean 2002). Despite the use of fuel reduction works it is reasonably well known that sclerophyllous vegetation can actively carry a fire after as little as 3 years’ regrowth (Bradstock et al. 1998). One of the main reasons for this in Sydney sandstone woodland and forest communities is the ladder effect of vegetation fuels. Fundamentally this provides a continuous vertical fuel gradient of terrestrial herbs, forbs and grasses, a substratum of dense shrubs up to 4 m and a canopy dominated by Eucalyptus spp. and Angophora spp. Thus during high to extreme fire danger periods, it is relatively common for fires to rapidly climb from ground level into the canopy and become uncontrollable by direct fire fighting attack. Fires have been known to advance at rates up to 12 km per hour in extreme conditions (Sanders 1999; Speer et al. 1999).
Questions that are often asked by land managers after high intensity fires are: How extreme was the fire across the landscape? Did wildfire burn homogenously through the environment? and What intensities were reached during such conflagrations? To help address some of these issues we: (i) Describe a method for rapidly assessing the spatial impact of post-fire severity using SPOT satellite imagery; (ii) Describe a method for rapid field assessment of fire severity; (iii) Describe a method for remote fuel assessment in sandstone communities; (iv) Discuss the estimated intensities reached in the Woronora catchment; and (v) Explore the relationship of topography and fuel in relation to fire behaviour. Study area and methods The study area is centred around the southern Sydney Basin of south-eastern NSW, Australia (151◦ 30 E, 34◦ S). The area is bounded by the Pacific Ocean in the east, Katoomba to the north, Jenolan Caves to the west and Robertson to the south, an area of approximately 900 000 ha. Of this
Fire severity of the 2001 Sydney forest fires
area approximately 480 000 ha (53%) is naturally vegetated environments conserved in National Park estate, State Forest estate, water supply catchment (Special Areas), Crown land and Commonwealth military reserves (Fig. 1). The study area experiences a warm temperate climate with average diurnal temperatures ranging from 12.5◦ C in July to 21.5◦ C in December and average annual rainfall varies from 850 to 700 mm, with heavier averages being recorded along the eastern section of the study area. Elevation varies between 50 m at Sydney to 750 m around Robertson, 1200 m around Jenolan Caves and 1100 m near Katoomba (Fig. 1). In the 12 months preceding the fire period, most of south-eastern Australia had been officially declared as being in drought. During November and December 2001, diurnal temperatures were above average and hot dry north-westerly winds were a regular occurrence. Within the study area we focused on the detail of fire effects associated with the Woronora and Metropolitan water supply catchments (Fig. 1). These catchments are Special Areas jointly managed by the SCA and NSW National Parks and Wildlife Service (NPWS), encompassing approximately 100 000 ha of mostly undeveloped land stretching 30–120 km south-west of Sydney (Fig. 1). The catchments are dominated by dry sclerophyll sandstone woodland communities comprising a Eucalyptus/Angophora canopy with an often dense shrubby understorey up to 4 m high, dominated by Banksia, Acacia, Hakea and Leptospermum species. Sandstone gully forest is slightly moister with a eucalypt canopy and an open shrubby understorey often supplemented by ferns. Tall eucalypt forest is dominated by trees up to 40 m high with a variable, though often dense, understorey (Fairley and Moore 1989; NPWS/SCA 2002).The general study area environment is moderately rugged, with relatively flat plateaux dissected with intervening narrow gorges. In summer, this combination of topography and sclerophyllous vegetation provides ideal conditions for wildfire to spread rapidly once ignited (Luke and McArthur 1978; Macnaught 2000). Satellite image processing of post-fire severity SPOT 2 imagery was obtained for the study area for 1 November 2001 and 7 January 2002. Images were supplied fully orthorectified by SPOT Australia and were both captured at around 11.00 a.m. (Eastern Standard Time) with view angles of
