By Michael Bristow
BBC News, Beijing
Villagers have used a number of methods to encourage the rains
A multi-billion-dollar project to divert water from southern China to the arid north is already four years behind schedule.
The news comes as parts of northern and central China struggle to cope with severe drought.
Officials recently admitted that water would not flow along the project's central route - a total of three are planned - until 2014.
But there appears to be a difference of opinion about what is actually causing the delay.
One official said it was because of environmental concerns, another said it was taking longer than expected to resettle affected farmers.
Whatever the reason, the entire scheme is unlikely to solve northern China's dire water shortage, even when it is finished.
To solve that problem, experts say the region must conserve what little water it has.
China first started considering building the South-to-North Water Diversion Project in the 1950s.
The need is obvious. An area along three major rivers in northern China has 35% of the country's population, but only 7% of its water resources.
A recent severe drought is a reminder of just how dry some parts of China can be.
Nearly four million people are short of water. Livestock and crops are also under threat.
It is problems like this that prompted numerous studies into the water diversion scheme, which finally gained the go-ahead in 2001.
The $62bn (£42bn) project includes eastern, central and western routes that will divert water from China's Yangtze River to the parched north.
Some parts of the eastern and central routes have already been completed, although work has yet to start on the western route.
Three separate routes would bring water from south to north
China has shown in the past that it is unafraid to tackle massive engineering projects, such as the Three Gorges Dam. This scheme is no different.
Engineers will have to tunnel under the Yellow River in two places to send the water north.
The recently announced delay has occurred on the central route, which is nearly 1,300km (800 miles) long and stretches from Hubei Province to Beijing.
One project official, Wang Fangyu, told a conference that environmental concerns were holding up the scheme.
The main problem appears to involve the Danjiangkou Reservoir, which is being enlarged as part of the central water route.
The reservoir's dam is also being heightened.
Mr Wang said enlarging the reservoir would have a "profound" influence on the area's natural environment, according to a report of the conference in the Yangtze Business News.
He said the extended dam would prevent flooding downstream from the reservoir on the Han River, a tributary of the Yangtze.
But he also told the conference: "When the project is finished, the Han River's ability to clean itself will be reduced.
"This means that in the lower reaches of the river in Hubei we will need to build even more pollution treatment facilities."
This is why the central route is being delayed by four years, said Mr Wang.
But the minister in charge of the diversion project, Zheng Jiyao, recently denied that environmental problems were the cause of the delay.
He blamed it on the need to relocate 300,000 people to make way for the reservoir's expansion.
This is proving a challenge because the area is already densely populated and there is little land for migrants.
There are other controversies too - not least whether the western route is even viable.
It will be built on the Qinghai-Tibet Plateau, and there is a debate about what impact this will have on the local environment.
And there is a bigger problem.
Experts say the billions of tonnes of water that will be sent north will still not satisfy northern China's water demands, even when the project is completely finished.
Water will be diverted from the Yangtze River to the dry north
Chinese water expert Ma Jun said: "It is an emergency project because certain cities in the north are seeing dire water shortages.
"But in the long term this is not the final answer because the water being transferred is simply not enough."
Mr Ma said that water supplies in the north cannot expand any further and so the government needs to encourage water conservation if it is to find a permanent solution.
"There is huge potential, but it hasn't yet been fully tapped," added the author of China's Water Crisis.
A recent World Bank report made a similar call for improved water conservation, and recommended increasing its price to reflect its scarcity.
The report gives a grim account of the various water problems facing some parts of China, which for years have suffered from shortages, pollution and flooding.
"There is no doubt that China is facing a major challenge in managing its scarce water resources to sustain economic growth in the years ahead," said David Dollar, head of the World Bank in China.
Even if everything goes according to plan, China's south-north diversion project will only solve some of those problems.