"Fire in the Top End"

The use of NOAA-AVHRR for fire management In Australia’s tropical savannas

Richard Smith, Ron Craig, Mike Steber, Jackie Marsden, Carolyn McMillan and John Adams

Wildfires are a significant influence on most tropical savanna ecosystems. It is certainly true in Northern Australia. However, the fire regime in the Top End appears to have changed dramatically since European settlement in the last 100 years, and this is having a significant impact on the environment. Routine monitoring of fires by satellite has emerged as a viable method of trying to understand this process.

Traditionally, aborigines used fire as a way of gathering food. They began burning early in the dry season, ensuring that fire stayed contained and food resources were protected. Modern burning patterns are significantly different. Fire regimes now appear to be occurring later and with greater intensity. This seems to be a consequence of indigenous populations moving to a more settled existence and pastoralism becoming established as the major form of land use.

By destroying infrastructure, stock, feed and station assets, uncontrolled late season wildfires can devastate remote communities and the economic livelihood of pastoralists. They can also contribute to environmental degradation by depleting vegetation cover, which causes increased wind and water erosion.

Recurrent late season burning can also reduce ecological diversity by restricting the distribution and abundance of plant species. Many areas of remnant rainforest are under threat as a result. Also, native mammals can be put at risk when there is an inadequate mix of burnt areas, which give an enhanced food supply, and unburnt areas, which give protection from predators.

Biomass burning also contributes significant amounts of greenhouse to the atmosphere. Australia's biomass burning emissions may be under-estimated by around 60%. This would make it equivalent to industrial sources of carbon dioxide.

Routine monitoring of wild fires across Northern Australia is needed for sustainable management methods to develop. Observation from satellite is the only practical way to do this. Ground-based and aircraft observations have been tried, but the high cost means that only areas near population centres or under commercial airline routes are monitored.

Therefore Satellite Remote Sensing Services, the Bush Fire Service (WA) and NT Bushfires Council (NTBFC), have developed a set of near real-time information products from satellite to assist land managers with tactical and strategic fire management decisions.

The area of Northern Australian covered is approximately from 11 to 21 degrees South and 120 to 140 degree East. It has a wet season from December to March and a dry season for the remaining months. Broadly, the vegetation is low open woodland, commonly called tropical savanna, in which the grasses grow vigorously during the wet season and then cure during the dry season to provide a highly inflammable fuel source.

Pastoralism dominates the land use with relatively small areas of intensive agriculture. Large areas are vacant crown land, aboriginal and nature reserves, which are often remote and inaccessible. This combination of vegetation, land use and climatic conditions produce an environment that is highly susceptible to wildfire. During the dry season, many fires driven by the prevailing south easterly winds burn for weeks and even months covering vast areas.

We used the Advanced Very High Resolution Radiometer (AVHRR) sensor carried on satellites operated by the US National Oceanic and Atmospheric Administration. The sensor has five spectral bands. It is suitable for detecting fires, measuring burnt areas, and detecting green vegetation at a resolution of 1 to 2 km.

The data is also free, so the main costs are data reception, R&D, processing and distribution.

NOAA satellites pass over WA approximately ten times in 24 hours and are locally acquired by the Western Australian Satellite Technology and Applications Consortium (WASTAC) via a fully automated receiving station at Curtin University of Technology, Perth WA. The data is transferred via a microwave link to SRSS on reception and processed to provide a quick-look image of the overpass.

This image is good enough to show the location of hot spots at night. Additional datasets are produced later, after the data has been archived and included with additional NOAA-AVHRR data from receiving stations in Darwin and Melbourne.

There are four of these datasets. The first is a twice monthly Normalised Difference Vegetation Index (NDVI) composite which indicates green vegetation cover. It is obtained from the afternoon passes of the AVHRR sensor on the NOAA-14 satellite. The images are geometrically, radiometrically and atmospheric corrected. A maximum value composite NDVI image is produced to provide land managers with a near real-time estimate of green vegetation cover at a resolution of 1 km.

The second dataset is a curing index, which is used as an indicator of fuel flammability. The curing index images are provided on request, via the Internet, or as hard copy images to Bush Fire Service (BFS) regional offices and other government agencies. The images are used by the BFS to assess the flammability of the grasslands and to help determine the best time and areas for controlled burning. If the burn is too early it can fail to spread, while if too late, it may become an uncontrolled wildfire.

A third product is a map of hot spots for the location of fires. This uses the night time NOAA-12 and NOAA-14 satellite data. Hot spots of possible fires are detected automatically seven days a week from night time overpasses and automatically posted on the world wide web within two hours of the overpass. The early morning overpasses from Monday to Friday are interpreted manually by panning through the image and looking for groups of dark pixels. The location of the fires are then determined and the latitude and longitude of all hot spots indicating currently burning fires are faxed and posted on the world wide web to the appropriate BFS and NTBFC regional offices. All the processing takes place within three hours of the overpass to provide land managers a near real time update of the location of currently burning fires.

Finally, we produce a history of areas burnt by fire. The images that are processed for the NDVI are reprocessed to map the fire history. A cloud free image is chosen near the repeat of the orbital cycle every 9 to 10 days. It is rewarped to a base image, any cloud and water are masked out and the fire scars are detected using the difference from the image in the previous orbital cycle. Fire scars that have occurred during the orbital cycle are identified from the difference between Channel 2 reflectance and the NDVI between the two dates. These scars are manually vectorised. Image to image co-registration is crucial, otherwise errors occur in determining the fire scars.

Maps of the burnt areas are posted on the world wide web and hard copies provided on request. At the completion of the fire season the information is put into a GIS which has fire area information from 1993. To assist the BFS and NTBFC with fire education and management, querying with the GIS is used to produce maps illustrating the location, extent and patterns of burning since 1993.

Fire in the North End: Some Results

The provision of this information as a routine service has received a favourable response from land managers. The Bush Fire Services report that negative attitudes to fire management, typified by the feeling that little can be achieved, have been changed positively when reliable, near real-time information has been made available.

Due to the positive response to the fire scar-maps, mapping was extended to the entire state of Western Australia in 1995, to the Northern Territory in 1996 and to Northern Queensland in 1997.

It is imperative that land managers learn more about fire, so that the location and spread of wild fires can be monitored, models to predict fire risk developed and the ecological impacts of different fire regimes determined. Information such as this can contribute to the development of more informed fire management decisions.

The authors are with Satellite Remote Sensing Services in the Department of Land Administration, PO Box 471, Wembley, Western Australia 6014. The email address for correspondence is richard_smith@notes.dola.wa.gov.au. See also the web site at www.rss.dola.wa.gov.au

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(This page last modified on 1 December 1998)