* Rethink vital to meet cross-sector, cross-border claims
* Tripling of groundwater use raises reserve fears
* 6th World Water Forum starts in Marseille on Monday
By Gus Trompiz
PARIS, March 12 (Reuters) - The world's water supply is being strained by climate change and the growing food, energy and sanitary needs of a fast-growing population, according to a United Nations study that calls for a radical rethink of policies to manage competing claims.
"Freshwater is not being used sustainably," UNESCO Director-General Irina Bokova said in a statement. "Accurate information remains disparate, and management is fragmented ... the future is increasingly uncertain and risks are set to deepen."
It says that demand from agriculture, which already sucks up around 70 percent of freshwater used globally, is likely to rise by at least 19 percent by 2050 as the world's population swells an estimated 2 billion people to 9 billion.
Farmers will need to grow 70 percent more food by that time as rising living standards mean individuals demand more food, and meat in particular.
The report will be debated at the World Water Forum, which starts in the French city of Marseille on Monday.
A "silent revolution" has taken place underground, the report warns, as the amount of water sucked from below the surface has tripled in the past 50 years, removing a buffer against drought.
And just as demand increases, supply in many regions is likely to shrink because of changed rainfall patterns, greater droughts, melting glaciers and altered river flows, it says.
"Climate change will drastically affect food production in South Asia and Southern Africa between now and 2030," the report says. "By 2070, water stress will also be felt in central and southern Europe."
Asia is home to 60 percent of the world's population but only around a third of water resources, it points out.
A separate water study by the Organisation for Economic Cooperation and Development (OECD) released last week forecast world water demand would rise by 55 percent by 2050, with more than 40 percent of the global population likely to live in water basins facing water stress.
The report says with limited supply, policymakers will have to better manage the competing demands of farmers, energy producers and humans demanding drinking water and sanitation.
"The lack of interaction between the diverse communities of users, decision makers and isolated water managers has caused serious degradation of the water resource," it says.
The World Health Organisation said last week the U.N. target to raise the proportion of people with access to safe drinking water by 2015 had actually been reached at the end of 2010.
However the figure was contested by French charity Solidarites International, which said 1.9 billion people remained without safe drinking water, not the 783 million estimated by the United Nations.
The charity is among groups planning to challenge official messages at the Forum, with some associations holding an alternative event in Marseille. (Additional reporting by Jean-Francois Rosnoblet; Editing by Ben Harding)