World's water stocks dwindling fast

Posted: 9 January 2002

Many of the world's reservoirs, upon which billons of people depend for drinking water and food production, are suffering significant reductions in storage capacity as a result of sedimentation, warned an international conference on freshwater.

Studies presented to the International Conference on Freshwater which took place in Bonn, Germany (Dec 2001) 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 - and this exceeds the current creation of storage from new dams under construction.

The current 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 kilometres. Unless urgent action is taken, about a fifth of this, or some 1,400 cubic kilometres, will be gradually lost over the coming decades, a new book concludes.

Climate impact

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 the Bonn meeting that, "It would seem prudent and sensible for us to manage the existing stock [of dams and reservoirs] 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."

Dams in distress

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 (2001). 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.

Rodney White, author of the new publication Evacuation of Sediments from Reservoirs 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."

His report highlights that the rate of annual loss varies dramatically from region to region and country to country. China is losing over 2 per cent of its water storage capacity annually, followed by the Middle East, which is losing 1.5 per cent and Central Asia, 1 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 points to 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.

Evacuation of Sediments from Reservoirs by Rodney White is published by Thomas Telford Publishing, London, UK, at £60.