Q+A-Is the Three Gorges Dam responsible for the Yangtze drought?

BEIJING May 27 A 200-day drought in central China has provoked a fierce debate among scientists and government researchers about the impact of big dams like the Three Gorges on local weather systems.

Government officials and experts have been forced to respond to a flurry of accusations by netizens and environmental activists that the world's biggest hydropower plant has disrupted downstream water flows and could have a long-term impact on local weather patterns.

WHY IS THE THREE GORGES BEING BLAMED?

Experts say that the 600-km (350-mile) long reservoir required to serve the 26 700-megawatt turbines at the Three Gorges hydropower plant prevents considerable volumes of water from flowing downstream.

But some environmentalists and climate specialists have also said that the reservoir acts as a giant heat reflector that affects the microclimate of the region, raising temperatures and reducing rainfall.

They also point to longer-term impact, saying that large reservoirs like the Three Gorges are net greenhouse gas producers because they submerged vast tracts of forest and farmland that would otherwise have absorbed climate-altering carbon dioxide from the atmosphere.

WHAT HAS BEEN THE OFFICIAL RESPONSE?

Government experts have put the blame for this year's drought squarely on natural phenomena.

A statement issued on the government website (www.gov.cn) on Wednesday cited meteorologists as saying that seasonal high pressure from the northwest Pacific subtropical zone was weaker and more easterly than normal, and this was making it difficult for cold fronts to reach the downstream areas of the Yangtze.

Shen Guofang, an engineer involved in the design of the Three Gorges, acknowledge that the project has altered the climate in the region, but said the effects were confined to within 2 km (one mile) of the reservoir and it could not be blamed for the drought affecting downstream regions.

HAS THE DAM HELPED RELIEVE THE DROUGHT?

One of the main arguments in support of the project during its construction was its ability to regulate the water levels of the Yangtze, easing cross-country navigation, guaranteeing irrigation for millions of downstream farmers and taming a river responsible for the most calamitous floods in China's history.

Zheng Shouren of the China Academy of Engineering, also one of the dam's chief designers, told the official Xinhua news agency that the drought would have been much worse if not for the Three Gorges, noting that water stored in the reservoir had been used to raise downstream flows and irrigatge crops.

The reservoir is in the middle of a programme aimed at releasing billions of cubic metres of water downstream. The move will eventually reduce water levels at the Three Gorges Dam to 145 metres (450 feet), well below the 156 metres required to run all its turbines effectively.

But the release of emergency water supplies could cause problems. Guan Fengjun, director of the geological department at the Ministry of Land and Resources, told China National Radio that the drastic move could increase landslide risks as the torrent overwhelms the Yangtze's fragile banks.

WHAT DOES THE DEBATE MEAN?

The issue reflects a wider divide in government about large-scale hydropower as China tries to wean itself off polluting and climate-changing fossil fuels over the next decade.

Since the Three Gorges Dam was completed in 2005, China has approved no new large-scale hydropower plants. Premier Wen Jiabao, a geologist, is said to be sceptical about the massive financial, environmental and human costs of such projects.

The cabinet, chaired by Wen, concluded last week that while Three Gorges had provided benefits, it had also increased earthquake and landslide risks in the region and disrupted downstream irrigation. [ID:nL4E7GI1VO]

But there are signs that hydropower is back in favour. Energy officials say new dams on the Yangtze upstream and elsewhere will be crucial in efforts to raise the proportion of non-fossil fuel energy to 15 percent of the total energy mix by 2020.

They say China will put another 140 gigawatts of hydropower capacity into construction before 2015, with big dam constructors eyeing Tibet, the Yangtze upstream and the previously untouched Nu River in the southwest. (Reporting by David Stanway; Editing by Ken Wills)