PARIS France will need plentiful rain this winter to revive groundwater levels hit by a spring drought and a dry start to the autumn, the French geological research office (BGRM) said.
Rain this summer had helped reduce stress on water tables but warm, dry conditions in September and October meant the share of water tables below normal levels was now in line with that seen after the spring drought, the BRGM said.
The autumn is a period when aquifers -- a wet underground layer of permeable rock from which water can easily be extracted -- usually start to be replenished in France.
The March-May period was the hottest in France since 1900 and the driest in 50 years, prompting restrictions on water use in much of the country and leading to millions of euros in aid to farmers.
Summer rain, particularly in July, alleviated the situation and limited losses in this year's harvest in the European Union's top grain producer, but the BRGM said Indian summer conditions since September had reinforced the water deficit.
"To sum up 2011, the summer started very early and visibly the season is lasting a long time," Ariane Blum, hydrogeologist with the BRGM's water unit, told Reuters in an interview.
"It has already happened that we see these levels," she said. "For now I wouldn't say it's alarming, but it's time it rains and that it lasts until March."
Water has become a major geopolitical issue as a booming world population has put pressure on unevenly distributed reserves.
France has relatively abundant groundwater, supported by a temperate climate. But French weather forecasters project drier conditions this century, with the hotter south most at risk.
France's farm sector, like in other countries, is locked in a fierce debate with environmental groups over its role in depleting water resources.
Almost 80 percent of water tables in France were showing lower-than-normal levels as of November 1, the BRGM said in a monthly bulletin published on Tuesday, representing a sharp deterioration from the 63 percent recorded on September 1.
Rainfall in October, which was 45 percent below average for the month according to public weather service Meteo France, had contributed to the situation, although heavy rain that has caused flood damage in southeast France, is expected to boost groundwater reserves there, Blum said.
The situation was most acute in the Paris Basin around the French capital and the Aquitaine Basin in the southwest after repeated winters of below-normal rain since 2003, she said.
It was not yet clear if this decline in rainfall was linked to climate change but the fact Meteo France's long-term models suggest a fall of up to a third in rainfall together with hotter summers in the coming decades meant there was a risk of increased water stress during peak summer use, she said.
"At the moment the overall situation is more or less okay, but if we don't do anything and continue with the same uses and pumping, at some stage, there will be less rain and in 10 or 20 years we'll be in a critical situation," she said.
ARTIFICIALLY REPLENISHING AQUIFERS
The priority in protecting French water supply would be limiting use, with major water consumers like agriculture, industry and households all having to contribute, Blum said.
She said there was scope for consuming less crops, such as maize, that require a lot of water in summer, or for reducing leaks in the drinking water system.
Drinking water represents the biggest draw on groundwater reserves in France, with 3.6 billion cubic meters pumped annually compared with about 1.3 billion for industry and 1 billion for agriculture, which derive the bulk of their pumped water from surface sources like rivers.
The flow of most rivers was close to normal as of October 1, according to the most recent update published by the environment ministry.
But Blum said it was not possible to rely on surface water instead of groundwater, with the two interdependent, notably in summer when aquifers maintain minimum river flows.
On the supply side, the BRGM is looking at options for artificially replenishing aquifers, including injecting treated waste water, which is not currently authorized in France.
The Mediterranean coast is one focus because of the potential risk of salination from sea water if pure-water aquifers decline sharply due to tourism, she added.
(Editing by Muriel Boselli)