Forest fire danger on the increase

LAST MONTH’S DEVASTATING WILDFIRE NEAR NELSON IS A taste of what we can expect in future, according to a leading fire expert.

Grant Pearce, fire scientist with the Scion Rural Fire Research Group, says the dry conditions that helped to fuel the big blaze will become more common and more extreme.

“The major forest fire at Pigeon Valley, near Nelson, showed what a dramatic and devastating event, wildfires can be,” says Pearce.

“The Nelson region has a history of significant forest fires, with major ones occurring in February 1981 at Hira, on the outskirts of Nelson city (which burned 1,972 hectares); in October 1997 at Harakeke, in the Moutere Hills (535 ha); in December 2004, in Irvines Forest (200 ha); and in November 2009, near Tadmor (600 ha).

“The 1981 Hira fire occurred at a similar time of year (February 5) and is closest in size to the (latest) event. Many smaller wildfires have also occurred across the region, both in plantations and other vegetation types.”

Pearce is one of New Zealand’s leading specialists in wildfire behaviour. His research has enabled the development of fire behaviour models for New Zealand fuel types, which are routinely used by fire management agencies.

He says the Nelson region’s fire climate severity isn’t as high as neighbouring Marlborough but, based on weather data for Nelson Aero, the region still experiences 9-to-10 days per year on average of very high and extreme forest fire danger.

However, in a bad fire season, as many as 16-to-17 days of very high and extreme fire danger may occur. Nelson has seen a number of days of very high and also extreme forest fire danger since the beginning of January, with the recent heatwave being a major contributor to the Pigeon Valley event.

Pearce says fire managers have reported that fire danger levels during February were the highest they’ve seen in almost 20 years and NIWA’s soil moisture maps were showing significant soil moisture deficits across the region. The level of extreme dryness can be seen in a comparison of fire danger code levels with previous years, as for example shown by the Build-up Index component, which describes the overall fuel dryness and amount of fuel available for consumption (with a value of 60 being very high, and 80 extreme).

“The very hot, dry and windy weather had a major effect on the drying of forest fuels in particular, including slash and prunings, understorey scrub vegetation, pine needle litter on the ground and organic material in the soil, which all contribute to the amount of vegetation fuel available to burn,” says Pearce.

“These dry, elevated fuel loads contribute to easy fire ignition and spread, and to high fire intensities including crown fires that are very difficult, if not impossible to control, especially in steeper terrain often favoured for forestry plantings.

“Surrounding grass fuels are also dry, but the lower fuel loads mean that fires are usually easier to control once they spread out into these grass areas.

“Modelling of potential future changes in fire danger indicates that the number of severe fire weather days is likely to increase in many parts of the country. This includes the Nelson region, where the number of days of very high and extreme forest fire danger could increase to more like 12-to-13 days per year on average, and maybe 20-to-25 days in the worst years, with climate change.

“So the Nelson region, as well as other parts of the country, are likely to see more of these types of wildfire events in future.”