The National Forestry Office fights against fires: the essential calculation of the humidity level of plants in Occitania

Des prélèvements des végétaux effectués chaque semaine et comparés tout l'été. Midi Libre – MICHAEL ESDOURRUBAILH

Par son travail de collecte et de pesée des végétaux, l'ONF calcule la teneur en eau pour, en lien avec Météo-France, permettre aux pompiers de mieux évaluer les risques de feux de forêt.

Secateurs in hand, kneeling in the low scrubland of Cournonterral (Hérault), Gervaise de Chazeau carefully takes pieces of Kermes oak and places them in collectors.

"We only collect the shoots of the year, samples, approximately 15 to 20 g in each of the five boxes&quot ;, specifies this work manager within the National Forestry Office (ONF) and Gard-Hérault referent for the water network.

This field work, carried out every week throughout the Mediterranean, from Perpignan to Monaco, via Corsica, is an essential tool for fighting fires and, above all, preventing their spread. How ?

Provide data to Météo France

“We assess the water content of plants, which provides data to Météo France, which will integrate them into the calculation of forest fire weather danger. This is one of the parameters”, explains Fabien Brocherio, head of the Forest Fire Defense Unit (DFCI) for Gard, Hérault and Lozère.

In these three departments, samples are taken on Tuesdays, at three sites, ensuring a coherent network across the territory (*). “We make sure to distribute them to have a global vision of the entire southern zone and take into account the disparity in the state of dryness of the plants”, continues Fabien Brocheiro.

Dried in the oven and weighed

The ONF has been acting in this way empirically for around twenty years, but with a role that has never been as important as today, in terms of global warming and the risk of mega-fires.

Two species are collected – in Cournonterral, the kermes oak, which reacts gently to drought, and the cottony cistus which loses its water much more quickly – and the boxes are weighed before the harvest is put in the oven for 24 hours; at 60 °.

"We can then compare the fresh weight and dry weight and determine the water content of the plant."

This shadow work of sampling plants, which also includes temperature readings and weekly precipitation, is accompanied for the first time by the contribution of ;a drone allowing the state of the vegetation to be further refined.

"If it doesn't rain a lot, the shrubs will turn yellow, this is the indicator of sensitivity to fire", says Fabien Brocherio .

This Tuesday at the beginning of July, Gervaise de Chazeau recorded 27° in the Gard as in Hérault and the plants still have 45% water. The risk is currently low. Not that of fire, because low and already dry grass can catch fire, but the risk of propagation, facilitated by the species collected. Hence the importance of comparing them from one week to the next.

"We also know the proportion of this dead vegetation to further refine the forest fire risk", finally indicates the DFCI manager.

(*) Cournonterral, Montblanc and Le Bousquet-d’Orb for Hérault, Conquérac, La Grand-Combe, Marguerittes for Gard, Saint-Etienne-Vallée-Française, Mende and Saint-André-de- Capcize for Lozère. I subscribe to read more