Water, water everywhere, but...

The shortage of fresh water in the developing world is reaching critical levels. And a new dam in Brazil only serves to highlight the environmental problem. Steve Connor reports

Later this year, the construction of the world's latest dam is due to be finished. As dams go, the one being built at Castanhão, in Brazil's arid north-eastern state of Ceará, is not the biggest nor the most controversial, and, in many respects, its completion will go largely unnoticed by the millions of Brazilians who stand to benefit from it. Yet the fact that the Castanhão dam needs to be built at all is testament to the growing worldwide crisis in the supply of fresh water.

The latest study by the United Nations Environment Programme, which was published last month in its Global Environment Outlook 3 report, identifies water shortages as one of the most pressing problems facing the developing world. The report points out that one-third of the world's population is currently living in countries of moderate-to-high water shortages. Within the next 25 years, this is due to rise to two-thirds of the human population living in "water-stressed" regions.

By 2020, the demand for water is expected to increase by 40 per cent, and 17 per cent more water will be needed to irrigate the crops that will have to be cultivated to feed a growing population. Yet already in the world today, nearly 20 per cent of the world's population do not have ready access to drinking water, while 40 per cent lack adequate sanitation. This is despite the attempts to fulfil one of the main goals of the 1992 Earth Summit in Rio de Janeiro, which identified a long-term aim of guaranteeing access to clean water and sanitation for everyone.

Water, and the lack of it, is also on the agenda of the forthcoming World Summit on Sustainable Development in Johannesburg this August. In addition to improved sanitation and pollution control, the summit will inevitably have to confront the need to control even more rivers, using dams such as the one at Castanhão.

In many respects, the Castanhão dam exemplifies how a dam should be built. It involved detailed planning, and extensive consultation with the people whose homes in the nearby city of Jaguaribara were to be flooded. The planning also involved an assessment of the dam's environmental impact. Preliminary studies were carried out in the early 1980s, and the work itself began at the end of 1995, with the help of funding from the World Bank.

The new city of Jaguaribara was built to replace the old one that was flooded. The street plan of Jaguaribara "Nova" precisely matches the layout of the old, flooded city, even down to the position of the local churches. Each of the 12,000 residents was consulted about their new home, and they were given the opportunity to choose whether they would like to live next to their old neighbours – most said they would. Even the dead at the local cemetery were exhumed and reburied in new graves to match the ones that were to be flooded.

An ecological park has been set up adjacent to the flood site in order to preserve the native plants and animals, while three seismological stations will monitor any seismic movements related to the build-up of the 4.5 billion cubic metres of water behind the dam's concrete walls. Engineers say that the local rock and soil conditions will ensure that the dam will not silt up in the way that has affected other dams, such as the Aswan dam in Egypt. They insist that every effort has been made in its construction to minimise the dam's environmental impact.

Although much of Brazil benefits from heavy rainfall, the state of Ceará in the north-east suffers badly from drought. Flying west by helicopter from Fortaleza, Ceará's capital city on the Atlantic coast, the effects of the drought quickly become apparent. Although the coastal region is relatively green and lush, the land quickly dries out as you leave the climatic influence of the ocean. After about 15 minutes of flying, you cross the line in the vegetation that marks the point at which the aridity of the hinterland becomes clearly visible.

From here, hundreds of miles inland, the ground is brown and parched. After another hour or so of flying and the arid landscape is broken up by the glistening lake that is already building up behind the new dam built at Castanhão.

Local Brazilians view the dam as vital to the irrigation of vast tracts of potentially fertile farmland in the state of Ceará. Brazilian engineers estimate that the dam will be able to irrigate 43,000 hectares of crops, as well as supply the needs of the two million inhabitants of Fortaleza, with its important tourism industry. The dam will also control the flooding that regularly plagues Ceará's river basins – about half of the annual rainfall of the state falls in just two months, often in torrential downpours that can sweep away crops and buildings.

If Ceará needs anything, say Brazilian officials, it is a regular and reliable water supply. The state typifies the problem with water – the planet's most abundant substance is often not where you need it most. Even when water does arrive in the form of rain, it frequently comes suddenly, causing widespread and destructive flooding.

It is somewhat ironic that Brazil, famous for its rainforest, also sits on one of the largest underground water sources on Earth. But the Guarani aquifer system, covering some 1.2 million square kilometres and holding a stupendous 48,000 cubic kilometres of fresh water, is in the south-east of the country, many hundreds of miles from Ceará in the dry north-east. Just extracting 20 per cent of the amount of water that drains into the Guarani aquifer each year would be enough to supply 300 litres of fresh water per day to 360 million people – if only it could be distributed across this vast, continent-sized country.

Argentina, Brazil, Paraguay and Uruguay are working together on an integrated approach to developing a sustainable method of extracting water from the Guarani aquifer. The project is being closely watched by various international bodies, including the World Bank, which is helping to fund the initiative. "Success would be an important step towards ensuring long-term availability of freshwater and aquifer resources for people in these countries," says the GEO-3 report of UNEP.

The Castanhão dam and the Guarani aquifer system, along with the truly giant Three Gorges Dam in China, represent the traditional ways of meeting the worldwide water crisis caused by population growth, industrial development and expanding agriculture. But UNEP believes that other approaches to water management will have to be considered in the future.

"Planners have always assumed that growing demand would be met by taming more of the hydrological cycle through building more infrastructure," says the UNEP report. "This infrastructure has provided important benefits in the form, for example, of increased food production and hydroelectricity. There have also been major costs. Over the past 50 years, dams have transformed the world's rivers, displacing some 40-80 million people in different parts of the world and causing irreversible changes in many of the ecosystems closely associated with them."

As important as dams have been in the past, planners and politicians are now having to think of other ways to meet the problem of water shortages. "Policy makers," the report says, "have now shifted from entirely supply solutions to demand management, highlighting the importance of using a combination of measures to ensure adequate supplies of water for different sectors." Greater water efficiency and better controls on water pollution are two obvious improvements that could result in real benefits.

The poor management of water resources – such as over-irrigation – has already resulted in the salinity levels of about 20 per cent of irrigated land rising to a point where agriculture becomes difficult or impossible to sustain. Over the past 30 years, the pollution of groundwater sources has become a significant problem in many parts of the world. Many rivers are now suffering from high nitrate levels, caused by the use of agricultural fertilisers. And even some once-pristine rivers, such as the Amazon and Orinoco, are seeing rising levels of artificial nitrates.

For a planet that is mostly water, it may seem ironic that water shortages are becoming such a limiting factor in human development. Yet only 2.5 per cent of the Earth's water is fresh water, and less than 1 per cent of this can actually be used for drinking. Even this limited natural resource is dwindling quickly, as encroaching human settlements contaminate and overexploit newly discovered sources of water. The point may soon come when, even on such a watery planet as Earth, there is water everywhere but not a drop to drink.