The forum, held only every three years, will address growing water scarcity, the risk of conflict as countries squabble over rivers, lakes and aquifers, and how to provide clean water and sanitation to billions.

Loic Fauchon, president of the World Water Council, staging the conference in Istanbul, said humanity was called to take action against waste and abuse of the liquid that kept it alive.

"We are responsible," he said. "Responsible for the aggressions perpetrated against water, responsible for the current climate changes which come on top of the global changes, responsible for the tensions which reduce the availibility of freshwater masses so indispensable to the survival of humanity."

He added: "At this very time in the history of water, we are faced with a major challenge to use more water resources but at the same time to protect, enhance the value of and even reuse these waters."

The world's population, currently more than 6.5 billion, is expected to rise to nine billion by mid-century, placing further massive demands on water supplies that are already under strain.

According to the Organisation for Economic Cooperation and Development (OECD), the number of people living under severe water stress is expected to rise to 3.9 billion by 2030, amounting to nearly half the world's population. Most of these will live in China and South Asia.

That tally does not include the impacts of climate change. Global warming may already be affecting weather patterns, changing the time and place where rain and snow fall, say some experts.

Around 2.5 billion people today do not have access to decent sanitation, defying one of the targets of the UN's Millennium Development Goals.

Hydrologists say the crisis is rooted in excessive irrigation, leakage of urban water supplies, pollution of river water and unbridled extraction of water from nearly every type of source.

The Water Forum, running in Istanbul until Sunday, begins with a mini-summit of a small number of heads of state and government, invited by host Turkey. It concludes with a large ministerial gathering aimed at crafting guidelines for smarter management of water and resolution of water conflicts.

"In many regions, water scarcity and pollution are increasingly putting human wellbeing at risk," said Mark Smith of the green group the International Union for Conservation of Nature (IUCN).

"We have to organise ourselves to use water more sustainably. We need systems for governing water based on a balance of pollicy and good water law."

Outside the political dimension, the conference is also a gathering place for companies involving in the multi-billion water industry.

Between 92.4 billion and 148 billion dollars are needed annually to build and maintain water supply systems, sanitation and irrigation, according to a major document, the third World Water Development Report, that was issued in the runup to the Istanbul forum.

China and developed countries in Asia alone face financial needs of 38.2-51.4 billion dollars each year.

How this investment is mustered -- and the accountability of corporations in water, a key ingredient for life -- are among the big questions facing the conference.

"Those are are dealing with corporate control of water's manifold downsides -- water takings, water shutoff, price hikes, shortcuts on water treatment -- are people deeply affected by the water crisis and corporate actions," said Mark Hayes of activist group Corporate Accountability International.

"Yet these people who are going thirsty don't have a true voice at these meetings."

More than 27,000 people are expected to attend the conference, beating the record of the previous four events, the organisers said.

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