Spatial interpolation of McArthur's Forest Fire Danger Index across Australia: Observational study

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Highlights

  • The spatial distribution of a Fire Weather Index for Australia was calculated.

  • The McArthur Forest Fire Danger Index (FFDI) was utilised.

  • Fire Weather Danger was quantified by calculating return periods of FFDI.

  • The FFDI distribution was found by using an advanced interpolation algorithm.

  • Maps of 50 and 100-year return periods of FFDI were presented.

Abstract

Fire danger indices are used by fire management agencies to assess fire weather conditions and issue public warnings. The most widely used fire danger indices in Australia are the McArthur Fire Forest Danger Index and the Grassland Fire Danger Index. These indices are calculated at weather stations using measurements of weather variables and fuel information. For a vast country like Australia when assessing the risk of severe fire weather events, it is also important to calculate the spatial distribution of these indices considering the extreme tail of the distribution. The spatial distribution of one of the fire weather danger indices regularly used in Australia is presented in this paper. In particular, we present the spatial distribution of the long-term tendency of extreme values of the McArthur Forest Fire Danger Index (FFDI). This indicator of fire weather conditions was assessed by calculating the return period of its extreme values by fitting extreme value distributions to data sets of FFDI at 78 recording stations around Australia. The spatial distribution of these return periods was obtained by using spatial interpolation algorithms with the recording stations measurements. Two conventional and two new algorithms based on machine-learning techniques were tested. This study shows that the best interpolation results for the FFDI can be obtained by using a combination of random forest and inverse distance weighting interpolation algorithms. The spatial distribution of the seasonal FFDI return period shows that the highest FFDI over large parts of southern Australia occurs during the summer months whilst in northern Australia it occurs in spring. The results also show that the FFDI in eastern Australia, the most populated region of the country, is higher inland than in the coastal areas particularly during spring and summer.

Introduction

Wildfires are one of the major natural hazards facing the Australian continent (Cheney, 1976, Harris et al., 2012). Chen (2004) rated wildfires as the third largest cause of building damage in Australia during the 20th century. Most of this damage was due to a few extreme wildfire events. The worst of these extreme events, and one of the worst natural disasters in Australian history, occurred in February 2009 when 173 people died and over 2000 houses were destroyed in the Victorian wildfires known as ‘Black Saturday’ (Teague et al., 2010, Cruz et al., 2012).

Detecting regions of high fire weather danger would allow planning and emergency authorities to warn the population of the elevated risk in the areas concerned and also help to mitigate the consequences of a wildfire if such an event were to occur. One of the most widely used indicators of fire weather danger in Australia at any given time is the McArthur Forest Fire Danger Index (FFDI) (Dowdy et al., 2009, Matthews, 2009). The FFDI is a function of drought factor and varies exponentially with temperature, humidity and wind speed (Noble et al., 1980).

Lucas (2010) developed historical fire weather data sets for Australia including calculation of FFDI. These data sets allow analysts to better understand the climatic conditions in which wildfires can develop. For a large country such as Australia, with a sparse observing network of meteorological sites, it is necessary to also consider the spatial distribution of this weather-based fire danger indicator. In this study we attempt to close the gap between station-based and spatial analysis by calculating observationally-based return period of extreme FFDI and presenting the spatial distribution of these return periods over the Australian continent using an advanced spatial interpolation technique. Return period calculated by employing extreme value distributions allow analysts to make inferences about the magnitude and frequency of extreme events beyond the range of years available in the observational data.

The study also shows that the best interpolation results for this fire-weather indicator can be obtained by using an algorithm which combines new interpolation techniques such as random forest with more conventional techniques such as inverse distance weighting. The interpolated maps using this combination showed physically plausible results and gave the smallest prediction errors as discussed in Section 5.3.

This study is important in the process of understanding the fire weather danger (utilising FFDI) faced by communities that are located some distance from the nearest meteorological observing stations. It is possible to undertake empirical calculations of the actual risk to these communities by comparing the FFDI with house loss information as discussed in Blanchi et al. (2010).

In practical applications the actual fire danger potential for a region depends on the type of vegetation in the region. In some cases an FFDI would not be the most appropriate index, for instance in regions dominated by grassland. In these cases the GFDI (Grassland Fire Danger Index) would be a more appropriate index to use. In more complex vegetation a combination of both, forest and grassland indexes weighted by a factor, could be a better indicator of fire danger. In this paper we use the FFDI to illustrate our methodology although grass rather than forest is the most common fuel type in Australia (Cheney and Sullivan, 2008).

Section snippets

Data sets available for this study

The main data sets used in this study are the fire weather data sets of 78 Australian stations compiled by Lucas (Lucas, 2010). These data sets contain records of daily weather variables which influence fire weather conditions such as temperature, relative humidity, drought factor, daily rainfall and wind speed and direction. After quality assessment and cleaning of these records Lucas calculated daily maximum FFDI and GFDI. These data sets cover the temporal range of June 1972 to June 2010.

The

Return Period of FFDI (RP)

Natural hazards can be quantified by using the Average Recurrence Interval (ARI) more commonly known as the Return Period (RP) of the natural phenomenon. If a given value of the phenomenon, termed ‘return level’, is exceeded with probability ‘p’ on average once a year, the RP corresponding to this return level is 1/p years (Coles, 2001). For instance, if the average annual probability of exceeding a gust wind speed of 45 m/s at some location is 0.002, we can say that the 500-year RP (1/0.002)

Spatial interpolation algorithms

RP of FFDI for a wide range of years (5–500) was calculated for the data of Lucas (2010) using the procedure described above. This produced discrete location data for a sparse network of observing stations. However, spatially continuous environmental information is often required by the government, industry, community and various organisations for environmental management and hazard mitigation. For our intended long-term application (fire weather danger assessment), measurements were often only

Calculation of RP curves using moments and maximum likelihood

Curves of RPs of FFDI for the 78 stations were calculated. The threshold to fit the GPD was found using the automatic threshold selection procedure developed by Sanabria and Cechet (2007). For 70 observing stations the GPD curve using the method of moments was higher than the maximum likelihood method. Eight stations, however, went against this trend. The station at Mt Gambier, SA, is one of those as shown in Fig. 2. However, the difference between the two curves was small (less than 1% for the

Examining the quality of FFDI data

Clarke et al. (2012) studied the quality of the data sets used in this project. In some stations wind speed measurements were based on visual estimates prior to the installation of anemometers. Other data sets have cumulative missing observations of more than one year. In general the FFDI calculation using these stations is considered of poor quality. After the low quality stations are eliminated, only 38 stations out of the 78 remain. In order to examine the impact of the quality of FFDI on

Spatial maps of seasonal and monthly FFDI

In the previous sections we presented a model for assessment of fire weather danger over the Australian continent. In practical applications it is convenient to look at maps of RP of FFDI with more detail. It is also important to look at the seasonal characteristics of FFDI. These two issues are addressed in the next sections.

For a more detailed assessment of fire weather danger we present maps of 50-year RP using a discrete scale which is very similar to the partitions utilised in the National

Conclusions and future work

Fire danger indices calculated at widely-spaced point locations are used by fire management agencies to assess fire weather conditions and issue public warnings. For the assessment of local hazard and risk, high resolution mapped information is becoming more available to consider community exposure and impact. The most widely used fire danger index in Australia is the McArthur Fire Forest Danger Index (FFDI). We have developed an efficient technique for the spatial mapping of long-term (return

Acknowledgement

We thank our Bushfire Cooperative Research Centre (Bushfire CRC) colleagues for their reviews and encouragement. We also thank the Geoscience Australia Product Development Team for their assistance in preparing the maps. This work is co-funded as part of the Bushfire CRC Understanding Risk Theme, Fire Impact & Risk Evaluation Decision Support Tool (FIRE-DST) project.

We want to express our appreciation to the anonymous reviewers for their constructive critique and thoughtful comments on this

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