Engineers have already 'snatched' 40 million tonnes of water released from reservoirs upriver to slake the thirst of residents in Zhuhai and Macau and see them through the dry season, but the operation, which drove back saltwater tides so that pumps could draw freshwater, will have to be repeated later this year. An engineering feat that poured 800 million tonnes of precious freshwater into the sea when it was first tested last year, has this year been fine-tuned so that the entire 'water snatching' operation, the first in the world, lasted six days and saved about 300 million tonnes of water. Engineers were able to predict to the hour when to start the pumps, and by opening and closing sluice gates as dictated by tide times, they reduced water wastage and avoided dropping water levels in upstream reservoirs so low that they would be unable to generate electricity. Still, water experts are worrying about so-called salinity intrusions for a fourth consecutive dry season this year, because emergency water-provision measures won't be in place before 2007. Some experts are advocating an integrated plan to manage the mainland's scarce water resources. Salinity intrusions were particularly severe in the dry season of 2004-05, when saltwater tides reached as far upriver as Panyu and threatened Zhongshan as well as Zhuhai, which supplies Macau's water. Although they now affect only the drinking-water supply, at their worst salinity intrusions can cause the soil to become salinised and useless for agriculture, as has happened in Shandong where salt has crystallised on the ground. While the phenomenon had not occurred in the Pearl River Delta and rainfall in the Pearl River basin was heavy enough to prevent that process, 'we can never say never', warned the State Oceanic Administration's South China Sea director Feng Weizhong. Salinity intrusions are not new. They occurred in the 1970s and 1990s, except that back then the dry season was not as pronounced and the population was less dense, so no one paid any attention to a problem that was easily resolved by drawing water from wells. Experts say drought was the main cause of salinity intrusions but man-made interference also contributed, along with population densities, the economic boom, the complex makeup of the Pearl River Delta and, indirectly, dams. The dry season, from September to March, has been drier than usual in the past two to three years and that has affected the river's flow to the extent that it's not strong enough to stop sea tides from pushing up the river mouth. In 2004, the entire river basin's above-ground water volume was 39 billion cubic metres, 12 per cent less than in 2003, and 25 per cent down from the average in recent years. Cui Weizhong, vice-chairman of the Pearl River Water Resources Commission, said the problem was so severe that sodium content rose to 800mg per litre in Zhuhai on January 4, well above the permissible 250mg. Consumption of water with such high sodium content could lead to high blood pressure and heart diseases. But salinity intrusions are not a problem unique to the Pearl River. The Yangtze River is also affected, but it is a bigger and more powerful river than the Pearl River. In South America's mighty Amazon River, saltwater intrudes further inland but the riverine population is too sparse to raise a hue and cry. Chinese University of Hong Kong water expert Chen Yongqin said that dredging of the river bed had possibly affected channel slopes and caused the river's flow to become uneven. Changes may have occurred at the entrance to the sea that led to the intrusions. The Pearl River also enters the sea in eight places, unlike the Yangtze River, which basically disgorges water into the sea at a single entrance, and so has more force to keep the sea from rushing in. Dams might also have an impact on river flow that would cause salinity intrusions but the extent of the impact hadn't been studied, said Tang Wanlin, Dam Issues co-ordinator of the World Wide Fund for Nature (China). He said dam builders weren't required to meet minimum environmental standards to ensure there was enough river flow to reduce impact on the environment and ecology. 'They are aware of this but they only pay lip service [to environmental protection]. There is no legal framework to enforce it,' Mr Tang said. With the problem predicted to get worse - the Guangzhou meteorological bureau forecast a 20-year drought cycle has just started - experts are pushing for more long-term solutions than opening floodgates upstream. Experts also advocate integrated river basin management - as Germany did to clear pollution of the River Rhine - and promoting conservation consciousness. Ma Jun , the author of China's Water Crisis, said there should be a study done on the connection between dams and saltwater tides. 'We should study how much water should be held back for consumption upstream and the minimum needed to prevent saltwater tides,' he said. 'We need to strike a balance. The situation will get worse if we don't pay attention, because more dams will be built and water consumption will rise. 'Dams are planned for Nanpan River, Beipan River and Hongshui River. If there is no proper co-ordination, the problem will become very grave.' Cui Weizhong said there were pros and cons to dam construction, but there was a need to build a big enough dam on the Pearl River to regulate water flow. 'We need a Three Gorges Dam to regulate river flow. If we have a dam at Dateng Gorge in Guangxi , it will take only three days to flush out the saltwater tides compared to one week now,' he said. A proposal to build the dam was submitted to the central government in the 1980s, but a decision was never made on the project. Mr Cui said a draft integrated water resources management plan would be ready to be presented to provinces in the Pearl River basin, but he foresaw obstacles negotiating water rights with provincial governments. In the meantime, the central government approved an emergency pumping station, costing several hundred million yuan, for Zhuhai in October. The station, which can draw one million tonnes of water daily, will be built at Pinggang near Doumen , where saltwater tides would affect water intake only for nine days during the dry season, compared with 40 days at Modaumen. It will be completed next year. There are also plans to deepen reservoirs to enlarge their capacity and to divert water from west of Modaumen, where it is less densely populated, to the more heavily populated east. Mr Feng also suggested drawing water as far upstream as possible, for example in Shunde , to avoid saltwater tides, as in the cases of Shanghai and Shenzhen. 'Shenzhen is also affected by salinity intrusions but they don't complain about salty water because they draw water near Huizhou . They took saline tides and pollution into consideration when building the Dongshen water plant,' he said. Salinity intrusions on the East River, from which Hong Kong's water is drawn, have reached as far upstream as Xintang.