POLITICIANS, corporate executives, engineers and greens will gather in Turkey today for a week-long arena aimed at tackling the planet's fast-growing water crisis.

About 20,000 people are expected for the Fifth World Water Forum in the Turkish city of Istanbul, where a charged agenda awaits them.

Access to clean water and sanitation, river pollution, madcap extraction of aquifers, jockeying for water rights and the impact of climate change have turned the stuff of life into a fiercely contentious issue.

The forum, held only every three years, has been foreshadowed by a report issued by a constellation of UN agencies.

In 348 pages, their document, published last Thursday, warned of a triple whammy in which supplies of freshwater were being viciously squeezed by demographic pressure, waste and drought.

It spoke of a "global water crisis" with plenty of potential for instability and conflict.

Loic Fauchon, head of the World Water Council which is organising the Istanbul meeting, said the facts amounted to a glaring message that times have changed.

"The era of easy water is over. We have to embark on policies for regulating demand," Fauchon said in Paris last week.

"Over the last 50 years, water policies around the world have focused on providing ever more water. Absolutely no thought was given to water consumption, which has reached shameless proportions in some countries."

He added: "All of us around the world have to ask questions about our relationship with water and work to use less of it."

Hydrologists point to some notorious acts of water vandalism over the last century.

They include the desiccation of the Aral Sea, once the world's fourth largest inland lake, by Soviet-era plans to grow cotton in the central Asian desert.

Less visible, but also massively destructive, is over-irrigation, in which water is used to grow thirsty crops in scorching climates and soils that are naturally parched. California's Imperial Valley and Australia's Murray Darling river system are often cited for such waste.

Then there is the damming of rivers for hydro-electric projects, which affects flows downstream, and the frenzied extraction of "fossil water" - underground aquifers that took hundreds of thousands of years to build up.

Amplifying the problem is climate change, affecting patterns of rainfall and snowfall.

Water scarcity has the potential to stoke unrest, frictions within countries and conflicts between states, according to the UN document, the Third World Water Development Report.

"Conflicts about water can occur at all scales," the report warned.

"Hydrologic shocks that may occur through climate change increase the risk of major national and international security threats, especially in unstable areas."

One objective of the Istanbul meeting is to develop ways of avoiding these feared "water wars" by encouraging agreements on sharing the use of rivers, lakes and aquifers that straddle boundaries.

Sources close to the conference expect the announcement of a three-way deal between Turkey, Iraq and Syria over the sharing of waters of the Tigris and Euphrates, the two rivers that reputedly fed the Garden of Eden.

The conference will kick off with a mini-summit, hosted by Turkey, gathering around 14 countries, before splitting up into debates on six themes on water management and conservation.

This will culminate in a three-day ministerial-level meeting, gathering 107 countries, that will issue a non-binding statement of recommendations on March 22.

On the sidelines of the political meeting is a major fair gathering actors in the water business from large corporations dealing in drinking water and sanitation to inventors peddling rainwater harvesting.

AAP