Recent images of forest fires, withered crops and depleted rivers are a vivid illustration of the threats posed by drought. But the parched conditions inflicted by the northern hemisphere's current heatwaves will be eclipsed by increasingly devastating water shortages in the decades ahead, a conference will hear this week.

The Stockholm Water Symposium, which opens on Monday, is aiming to grapple with the problem of water scarcity, which the United Nations believes will affect between 2bn and 7bn people by the middle of the century. Over the next 20 years, the average supply of water worldwide per person is expected to drop by a third.

The consequences will be most devastating for the world's poorest people, according to William Cosgrove, vice president of the World Water Council, a think tank. "The minimum consequences will be higher food prices and expensive food imports for water-scarce countries that are predominantly poor."

The main pressure is from population growth, coupled with agricultural and industrial development. But water supplies are also under threat from pollution and climate change.

A changing climate is expected to make rainy seasons more intense and droughts longer. "As global temperatures continue to warm due to climate change, the number and intensity of extreme events might increase," the World Meteorological Office warned last month.

The problems caused by these climatic variations "need immediate attention in order to satisfy future food demands for an exploding human population and to minimise human and economic suffering from catastrophic floods", according to the Stockholm International Water Institute, organisers of the Stockholm symposium.

The problems will be most acute for farmers, already by far the biggest consumers of water, accounting for 75 per cent of all water withdrawn from rivers, reservoirs and aquifers.

Less water-intensive crops could be part of the solution. Water-saving techniques are being developed that could save up to a quarter of the water used to grow rice, according to scientists at the Manila-based International Rice Research Institute. Researchers are also developing hardy breeds of tropical corn that could increase harvests by 40 per cent in the tough environments of the developing world.

Some arid countries, such as Morocco, Jordan, Israel and Egypt, are deliberately reducing their water needs through increasing food imports or growing higher value, less water-intensive crops such as dates, grapes and olives.

Reducing water dependence by importing food - sometimes described as "virtual water" - is an attractive solution for wealthier countries. Jordan's population survives on just 176 cubic metres of water per person a year, far below the 1,000 cubic metres defined as absolute water scarcity.

But this solution is not available to the poorest countries or those such as India and China with large populations, which believe the world market would be unable to supply their food demands in a crisis.

Instead, India and China are contemplating ambitious engineering schemes to solve their looming water crises. China, which is faced with drying river beds and a falling water table in its northern grain-producing regions, is launching a 50 year project to transfer water from the Yangtze River in the south to revive the Yellow River in the north. India's water engineers have come up with a scheme to link rivers that drain the Himalayas with those in the country's arid south and east.

These massive hydrological schemes - along with similar proposals for Spain and central Africa - have aroused intense controversy. Critics predict they will cause ecological havoc and prove excessively costly. They argue that water demand should be reduced through conservation, the introduction of water charging and, where possible, moving crop production away from arid regions.

Few experts doubt that agriculture, which is estimated to waste more than half of the water it uses, has scope for improvements. Replacing the flooding of fields and use of sprinklers with drip irrigation would produce a 25 per cent cut in demand, the World Bank estimates.

But the bank is convinced that better management of water is not enough. New, big water infrastructure projects have a vital role to play in protecting poor countries against water shortages, it says.