A global gathering of policymakers, corporations and specialists unfolds in this southern French city from Monday to ponder the future of water, fast emerging as one of the century's crunch challenges.

Staged every three years, the World Water Forum will look at the darkening shadow of water scarcity but also the opportunities this offers to make money from "blue gold."

As many as 20,000 participants from 140 countries are expected for the six-day event, including scores of ministers for the environment and water.

Gerard Payen, who advises UN Secretary General Ban Ki-moon on water, said the Marseille event could create valuable momentum.

"When governments agree at the Forum, they have to turn words into acts at the United Nations -- and there is a great opportunity for doing this at the Rio Summit in June," he told AFP.

Already, more than 2.5 billion people are in need of decent sanitation and nearly one in 10 has yet to gain access to "improved" drinking water, as defined under the UN's 2015 development goals.

The coming decades will be even more challenging.

The world will have to feed and house a population set to rise from seven billion to nine billion by mid-century. Prosperity in emerging countries will fuel a meat-eating, car-driving lifestyle that is water-gluttonous.

Totting up the figures, the Organisation for Economic Cooperation and Development (OECD) reckons demand for water will rise by 55 percent by 2050.

This surge will occur just when global warming will bite deeply, especially in a parched belt stretching across North Africa, the Middle East and South Asia, say climate scientists.

According to a study published last month in the journal PLoS One, water scarcity already affects at least 2.7 billion people in 201 river basins for at least one month each year.

"Freshwater is a scarce resource. Its annual availability is limited and demand is growing," said Arjen Hoekstra, a professor in water management at the University of Twente, the Netherlands.

"There are many places in the world where serious water depletion takes place -- rivers running dry and dropping lake and groundwater levels."

Intensifying demand is stoking tension within countries over water rights, and friction between nations over rivers and aquifers that straddle borders. One country in seven is more than 50-percent dependent on water from outside national boundaries.

Options for tackling the water crisis focus in particular on curbing waste, for which realistic pricing is essential, says the OECD.

For city consumers, use of "grey" water (for instance, soapy water from washing machines) to flush toilets, rather than treated water, is another futuristic scenario.

But the biggest and earliest gains could be in the countryside, which accounts for 92 percent of humanity's water footprint.

Farmers use some 200 million litres (50 million gallons) of water per second, a figure that experts say could be easily reduced by investment in smarter irrigation.

As for access to water, green groups warn about the temptation to build ever more reservoirs and dams.

The International Union for Conservation of Nature (IUCN) is pleading the case for wetlands, which act both as natural reservoirs and filters as well as havens for wildlife.

"Nature is a much more efficient and cost-effective solution. It provides the natural infrastructure that we need to store, move and filter water," said Julia Marton-Lefevre, the IUCN's director general.

Ministers attending the Forum will issue a non-binding statement on Tuesday affirming their awareness of the problems and intent to fix them.

The Forum is shunned by some environmentalists or development activists, who deride it as a trade fair lacking democracy and transparency. But there are others who choose to attend, saying it is useful for discussing a worsening problem.

An Alternative World Water Forum is being staged by 2,000 members of civil society from Europe, the United States, Latin America and Africa.

They are holding workshops and demonstrations to demand public ownership of water resources and distribution and ensure infrastructure projects respect the environment.