Their waders raised to the hips will not protect them higher than the ankles: the scale graduated up to two meters and fixed under a bridge displays for the river Le Chevré, a depth of 98 mm, a level already observed but " not before September" according to Fabien Simon, agent of the Regional Direction of the Environment of Brittany (DREAL), the eye riveted on a sensor of the small station of hydrometry.
His colleague David Danet continues: "in July 2022, we carried out 132 gaugings (in Brittany, editor's note), against 39 last year", a sign that the water situation is critical in Brittany.
Gauging, understanding the control of the height and the flow of a watercourse, this is the mission of these two hydrometers among the eight that counts the DREAL.
“It all starts with us,” says Mr. Danet, who says he is “well equipped to do the job”.
Their tools? A radar installed on the deck to measure the air draft, a pressure sensor for the draft, a rod equipped with a propeller at the cost of a new car for the flow, floating boards equipped with Doppler and an aquatic drone whose cost exceeds 100,000 euros.
With 200 colleagues in France, David and Fabien carry out surveys sent to the French Office for Biodiversity (OFB), which are then used to issue prefectural orders restricting the use of water.
- Seven months in deficit -
David Danet refuses any alarmism and warns that it will take "years" before being able to analyze the 2022 drought. But the Ille-et-Vilaine prefecture is worried.
Contacted by AFP, she assures that "seven months generally lacking in rain at the regional level" have been observed since October 2021.
And the month of July was catastrophic: the rain gauges of Dinard and Rennes recorded "an accumulation of precipitation of 3% compared to the average monthly precipitation".
This rainwater usually runs off to the dams which constitute 75% of the production of drinking water in Brittany, and whose resources "for supplying public networks is the most worrying", according to the prefecture.
Looking lost in the diminished Chevré, David Danet recognizes "a small risk that it will stop", like a dry river near Dol-de-Bretagne, the Guyoult, already dried up in 2003 or 2016.
Unfortunately, it is not enough for the water to return for the fauna and flora to follow: "the return to a normal ecosystem after a drought takes months, even years" warns Fabien Simon.
Guyoult is in fact one of the seven lands of the department at the beginning of August.
Without rain by the end of the summer, "it is possible" that we see dry conditions observed only "during the historic drought of 1976", according to the prefecture.
They will be detected by automatic stations like that of Chevré. There are 170 in Brittany, feeding a database of water levels every 6 minutes.
Pairs of hydrometers criss-cross this network to check the flow, inspect the installations and maintain them.
With risks related to traffic (they often work on the side of the road) and to the water itself.
During the floods in particular, in which they train, concludes Fabien Simon: "we participate in whitewater courses, we are thrown into the water and we have to get out of it on our own".
Provided you have water above your ankles.