FOREST fires could be kept in check with the help of satellites, say French researchers who used infrared images from space to calculate the risks of fire spreading in the south of France last summer.
Forest fires are a recurring problem in southern France and Corsica. For example, an average of 5800 hectares of forest have burnt each year over the past thirty years in Corsica, and 4700 hectares in the Var region, which includes resorts such as St Tropez.
The scientists, from the National Centre for Agriculture and Forestry, Engineering and Water Management (CEMAGREF) in Montpellier, took data from a weather satellite operated by the US National Oceanographic and Atmospheric Administration (NOAA) and combined the information with readings from weather stations in mainland France and Corsica. From these data the team calculated 鈥渟tress indexes鈥�, which indicate how dry the vegetation is.
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Nathalie Desbois, an agricultural engineer at CEMAGREF, says there was a strong correlation between high stress indexes in a region and fires that burnt more than a hectare of forest. 鈥淭he cause of a forest fire is independent of these stress indexes,鈥� she says. 鈥淏ut vegetation with high indexes is sufficiently dry to burn easily.鈥� This means it should be possible to identify the places where a fire would be likely to take hold.
The NOAA satellite has a resolution of about 1 square kilometre. Between July and September, Desbois and colleagues examined daily images from the satellite鈥檚 infrared detectors. These showed the surface temperature of the vegetation in each 1-kilometre square.
Plants are normally kept cool by the process of transpiration, as water evaporates from the surface of the leaves. But if water is in short supply, transpiration and evaporation slow down, and plant temperature increases 鈥� a change that can show up in the satellite images. Lack of water is not the only factor at work, however. Plant stress also depends on air temperature, humidity and wind speed.
With readings for these, plus the satellite data, Desbois and her team calculated stress indexes for the vegetation in each square kilometre of 11 forests. They also checked the number of fires that broke out in these areas last summer. The satellite images revealed that in many areas where forest fires broke out, the surface temperature of the vegetation had been particularly high the preceding day. The indexes calculated for the days leading up to the fires showed 鈥渋ncreasing levels of vegetation stress鈥�, says Desbois.
This summer CEMAGREF will check the accuracy of its indexes by measuring the water content of trees and bushes in areas with high stress indexes. M茅t茅o France, the national weather service, is also trying to improve its monitoring so that it can provide data on temperature, humidity and winds at the same resolution as the satellite鈥檚.
Desbois hopes that by 1996, regional fire departments will be using the indexes to pinpoint the areas where fire is most likely to spread. Ground surveillance could then be stepped up in these areas, or the public kept out until the fire risk begins to fall.