By Tim Hirsch BBC environment correspondent in Kyoto

The greatest challenge of the 21st Century - fresh water Dr Mahmoud Abu-Zeid The World Water Council president opens the forum. Read more

The third World Water Forum has brought together about 10,000 delegates from 150 countries to debate solutions to the crisis facing more than one billion people without access to clean water.

Inevitably, overshadowed by the Iraq crisis, the organisers of this conference say its discussions over the next week will have far more impact on mankind for the 21st Century than current events in the Middle East.

Japan's Crown Prince Naruhito opened the week-long forum with a warning that the world faced a water crisis, with shortages, pollution and floods spreading all over the world.

But on Sunday, United Nations experts told delegates that better harvesting of rainwater would have a major role to play in easing the worldwide shortage of fresh water.

Controversy

Former Japanese Prime Minister Ryutaro Hashimoto said it was important to raise awareness of the issue.

THE WATER STRUGGLE Global water use has more than doubled since 1950 One person in six has no regular access to safe supplies Contaminated water gives 200m people a year water-related diseases Agriculture uses about 75% of global water consumption and industry 20%; much of it wasted

"Already a goal has been set to reduce by half the number of people with no access to safe water and basic sanitation facilities by 2015," he was quoted by French news agency AFP as saying.

"It is now time to follow up and to ensure those goals are achieved. That's what the Third World Water Forum is for."

But while there is agreement here on the need to take action, the solutions are mired in controversy which will surface in the coming days.

Some pressure groups say the forum is dominated by private corporations who favour grand projects such as dams and major water diversion schemes, instead of simpler technology which could be used to conserve water more effectively for the world's poor and the environment.

Using ancient technology

UN experts on Sunday urged other countries to follow China's example over rainwater harvesting.

China has introduced new water systems to improve irrigation

The country recently built rain storage tanks which supply drinking water to around 15 million people.

Initiated in China's Gansu province, the simple, ancient technology had been neglected in favour of modern supply networks.

In most cities, the drainage systems are designed to remove water as quickly as possible, wasting fresh water which could be stored for the future.

But in Gansu the project helped farmers to collect rain in their fields and in tanks and has now been extended to 17 Chinese provinces, providing drinking water and extra irrigation for vast areas.

"With increasing urbanisation, particularly in Asia Pacific, to meet the needs of people for water would be a major challenge," Director of the UN's environmental technology centre, Steve Halls, said.

"Rainwater, harvesting and other fresh water augmentation approaches will be of paramount importance in meeting that need."