FANG HAIXIN, a subsistence farmer near the village of Huopu in western Guizhou province, never used to fret about not having enough water. True, there was the odd dry year. But rainfall was usually abundant, and harvests adequate. Yet like many in south-west China over the past three years, Ms Fang has learnt to grapple with the sort of drought conditions that, until now, were more commonly a feature of China's dry northern plains. Accustomed to finding water close to the surface, Ms Fang and her neighbours now have to dig wells 30 metres (100 feet) deep. Potato yields on her small hilltop plot, she says, are down by two-thirds. Corn harvests are somewhat better, but also down sharply. In the past year alone, say officials, levels in nine lakes on the Yunnan-Guizhou plateau have dropped by 70cm, marking a total loss of 300m cubic metres of water. The regional drought is now in its third year, and in that time 270-odd rivers and 410 small reservoirs have dried up in Yunnan alone. Residents at lower elevations are better able to tap into groundwater, and have so far been able to carry on as before. Indeed, just 10km downhill from Ms Fang struggling with her potato crop, this correspondent stopped at a car wash and was assured by a man with a gushing hose that water supply was not a problem. Even so, not only hilltop farmers are affected. Residents in Yunnan's capital, Kunming, suffer periodic stoppages in supplies of tap water. West of the city, dry conditions are causing forest fires. The region's growers of such valuable crops as tea and medicinal herbs have suffered, causing prices of those commodities to soar. Hydropower production has also fallen with water levels. Officials report that last year nationwide output rose by 9% year-on-year, but in the southern provinces of Yunnan, Guizhou, Guangdong, Guangxi and Hainan drought caused a 47% fall in reserve power capacity.

The repercussions threaten to affect an ambitious central-government project, imperial in scale, to transfer vast amounts of water from the south to the parched north of the country. China has already invested 137 billion yuan ($22 billion) in what it calls the South-North Water Diversion Project, and is set to invest another 64 billion yuan this year. The idea is to pump water from the Yangzi river northward through pipes and canals along three separate routes. Priority is given to the eastern route, which by 2014 is expected to bring 1 billion cubic metres of water a year to Beijing—a quarter of the capital's annual supply.

From its inception, the vast scheme has suffered both delays and criticism. One concern has been the cost, both of the initial construction and of the expense of pumping so much water over such distances. The project has also required that at least 330,000 residents along its course accept being relocated.

The river systems of Yunnan and Guizhou figure only modestly in the planned supply chain of the South-North Water Diversion Project. But if the causes of the drought in these provinces have to do with changing global climate patterns, the main assumption underlying the project—that of permanent water abundance in the south—may not hold up.

Liu Xiaokang of the Yunnan Green Environment Development Foundation, an NGO in Kunming, believes the causes are mixed. Global climate may be affecting patterns of precipitation, he says. But his group also notes that the parts of Yunnan that are hardest hit are those where development has been fastest and deforestation most extensive.

Marco Gemmer of China's National Climate Centre says droughts across southern China are linked to changing patterns in other parts of the world, such as anomalies in the Arctic oscillation or in the ocean current La Niña. Short-term droughts have occurred in the region for all of recorded history, but they may now occur with far greater frequency.

His colleague, Professor Jiang Tong, warns of problems with the transfer project if both south and north suffer drought at the same time. The situation, he adds, is complicated by the Yangzi's Three Gorges dam. It provides massive amounts of hydroelectricity to the Yangzi basin. He is concerned about what will happen should Shanghai need more power at the same time that Beijing needs more water.

On her hilltop plot, Ms Fang has a different view. “The government says they will help. But I don't know what they can do if it doesn't rain.”