Yet they say it's an essential link in a vast system of water transfer projects from China's relatively abundant rivers in the south to the increasingly parched north and northwest.

Despite this year's unusually heavy rains, northern China has been prey to drought in recent decades, and underground water tables have been rapidly depleting.

President Hu Jintao, a hydro-engineer who worked for decades in western China, has backed the plan, Liu Changming, a hydrologist at the Chinese Academy of Sciences, told Reuters.

Construction could start as early as 2010, Liu said. It would involve harnessing rivers cascading from the Tibetan highlands to quench Qinghai province and other poor western areas.

Li Guoying, director of the Yellow River Water Conservancy Committee, said in Beijing on Tuesday the Yellow River, one of the north's main waterways, was shrinking because of rising demand for water.

"When the economic and social development of the northwest reaches a certain level and the potential of water-saving measures is exhausted, this project will be launched," he told a news briefing.

The so-called Western Route of China's South-North Water Transfer Project will join the Central and Eastern Routes, already under construction, that will draw water from the much larger Yangtze River to ease shortages in Beijing and elsewhere.

Two-thirds of China's roughly 600 cities suffer water shortages, including 108 with serious shortfalls, Li said.

TIBET THREAT?

The Western Route will use a 300 kilometre relay of tunnels drilled through mountains to pump water from the Yalong, Dadu and Jinsha Rivers that flow into southwest China.

"The route isn't especially long, but it's technologically challenging, and it's a matter of resolving the engineering and environmental questions," said Liu, who is advising the government on the project.

The completed project would cost 300 billion yuan (US$37.5 billion) at current prices, and the total cost of the whole South-North scheme is 500 billion yuan (US$62.5 billion), Li said.

For China's Communist Party leaders -- nearly all engineers -- the Western Route promises to fulfil promises to use rising economic and technological might to lift the less developed west.

A recent Chinese book, Tibet's Water Will Save China, details leaders' enthusiasm for diverting the region's rivers and has been widely circulated among senior officials, China's Southern Weekend newspaper reported last week.

But the Western Route promises to be among the most controversial of Beijing's efforts to reshape the country's rivers. Environmentalists and advocates of Tibetan autonomy have said the plan threatens the region's ecology and culture.

"It epitomizes this assumption that Tibet is the water tower of Asia," said Tashi Tsering, a Tibetan expert on the region's resources at the University of British Columbia in Canada.

"Tibet's water availability is actually quite limited and these rivers depend on glaciers that are receding."

Liu said planners would carefully calibrate flows to ensure the source rivers remained viable. Other Chinese researchers have said earthquakes and landslides could threaten the project.

In its first phase, the project will transfer about 4 billion cubic metres of water annually -- about the size of California's main water transfer scheme, according to Liu -- and decades later the project will divert 17 billion cubic metres a year.

In past decades, the Yellow River's yearly runoff has been about 58 billion cubic metres but is shrinking, according to the Conservancy Committee.