Studies indicate that, on average, one per cent of the water storing capacity of the globe's reservoirs is being lost annually because of a build up of muds and silt.
The current storage capacity of reservoirs world-wide is estimated at just under 7,000 cubic kilometers. Unless urgent action is taken, a fifth of this or some 1,500 cubic kilometers, will be gradually lost over the coming decades, a new book concludes.
Experts fear that the loss could be even higher and faster if the scientific forecasts on climate change prove sound and the rates of deforestation in the developing world are not checked.
Global warming is predicted to increase the severity of storms and rains, accelerating the natural erosion rates in and around rivers that feed reservoirs. It is also likely to exaggerate the extremes in rainfall patterns making it even more vital that the storage capacity of reservoirs is maintained. Meanwhile the felling and clearing of trees for agriculture is aggravating the situation.
The levels of erosion from hillsides, planted with crops, are 150 times higher than from the same land covered with trees, studies show.
Klaus Toepfer, Executive Director of the United Nations Environment Programme (UNEP) told a press gathering at the International Conference on Freshwater taking place in Bonn, Germany: "The issue of dams can arouse strong passions on both sides. Some people are very much in favor of building dams and others are vehemently against. However what we are talking about here is the state and fate of the existing stock of dams and reservoirs on whose waters billions of people depend for not only irrigation and drinking water, but also for industry and the production of hydroelectricity".
"It would seem prudent and sensible for us to manage the existing stock in the most sustainable way possible. Otherwise we face increasing pressure on natural areas with water, such as wetlands and underground aquifers, with potentially devastating environmental consequences to wildlife and habitats," he said.
"We must act to reduce the loss of forests and to re-afforest cleared areas as part of a comprehensive strategy of watershed management of the world's river systems. We must also act to reduce the threat of global warming. However there will always be natural levels of erosion which will contribute to a loss of water storage capability. So I call on engineers to also provide technical solutions that offer environmentally friendly ways of extending the lives of the world's reservoirs," said Mr Toepfer.
He said sustainable management of reservoirs would take a central role in the work of UNEP's new Dams and Development Project (DDP), which is based in South Africa. The unit was formed in the wake of the World Commission on Dams which published its final report last year.
The unit has secured funding and pledges of over $2.5 million from the governments of Germany, the Netherlands, Sweden, Switzerland and the United Kingdom.
Jeremy Bird, Interim Coordinator of the DDP unit, said they would also be looking at how to improve the performance of reservoirs and dams across a wide range of issues from agriculture to power generation. Next week a meeting in Prague, Czech Republic, will examine, how it might be possible to raise the hydroelectric output of such schemes.
Rodney White, author of the new publication "Evacuation of Sediments from Reservoirs" (see notes to editors) and a fellow of the Institution of Civil Engineers, said:" The loss of capacity of the world's dams should be of highest concern for governments across the globe and at the moment I do not believe this issue is commanding the attention it deserves. The demand for water is rising, not falling, as the population of the planet climbs from six billion today to an estimated 10 billion by 2050. I am extremely concerned that water shortages in some of the poorer parts of the world will intensify unless we act to reduce reservoir sedimentation and conserve storage in existing dams using sound management techniques. Sediment removal should be a fundamental feature in the design of dams and their associated infrastructure".
Mr Toepfer said the issue of dwindling capacity of reservoirs was one piece in the puzzle of delivering sufficient quantities of clean water to the world's people.
Other research shows that significant amounts of the water used in irrigation is also being lost and squandered. In developing countries water consumption for agriculture is typically 70 to 80 per cent of the total water consumption. Typically 60 per cent is wasted or used inefficiently.
UNEP is calling for action in this area too. Meanwhile the distribution systems in many cities in developing countries lose 50 per cent or more of the water as a result of leaks and poor management. "Progress needs to be made here as well," said Mr Toepfer.
Dr White's report (see notes to editors) estimates that the storage capacity of the world's more than 25,000 reservoirs, based on figures from the Paris-based International Commission on Large Dams, amounts to around 6,815 cubic kilometers.
The global rate of storage loss as a result of sedimentation is approaching one per cent annually and this exceeds the current creation of storage from new dams under construction.
The report highlights that the rate of annual loss varies dramatically from region to region and country to country. China is losing over two per cent of its water storage capacity annually, followed by the Middle East, which is losing 1.5 per cent and Central Asia, 1.00 per cent.
It also highlights how clearing of forests is reducing the storage capacity of dams.
"For example data from the Ringlet reservoir in Malaysia shows clearly the dramatic effects of deforestation. The catchment has been gradually changed from forests to plantations and holiday facilities," said Dr White.
Sedimentation rates are now eight times higher than they were in the mid-1960s, the report concludes.
The report also highlights some management techniques that can restore some of the storage capacity of reservoirs including a method known as flushing in which flood waters due to heavy rains or melt waters from mountains are used to sweep debris, mud and silt out of the reservoir downstream.
It concludes that the technique is likely to work in parts of Central America; areas in North and South America where the rivers are fed by the Rockies and Andes; parts of Central Africa from Cote D'Ivoire in the west to Sudan in the east; areas in Central Asia where the rivers are fed by the Himalayas including Pakistan, India and Nepal and parts of Asia including Cambodia, Vietnam and Thailand.
Notes to Editors: Evacuation of Sediments from Reservoirs by Rodney White is published by Thomas Telford Publishing, Thomas Telford Ltd, 1 Heron Quay, London E14 4JD, UK. Price 60 pounds Sterling.www.thomastelford.com
For More Information Please Contact: Nick Nuttall, UNEP Head of Media, on Tel; 254 2 623084 (In Bonn on Mobile: 254 733 632755), e-mail: firstname.lastname@example.org
Or Rodney White on Tel: 44 (0) 1491 822253, e-mail:email@example.com or Jeremy Bird, DDP I Cape Town, South Africa, on Tel: 27 21 426 4000, e-mail: firstname.lastname@example.org
UNEP News Release 01/116