With much of the eastern side of the United States affected by a severe drought, there is a warning that it could prove the worst in living memory.

The drought is already the third most severe this century. But the Worldwatch Institute, based in Washington DC, says that if it lasts until the winter it could outstrip that of 1966 and prove worse even than the devastation of 1929.

Maryland has had the driest growing season since records began a century ago.

Seven states, and the District of Columbia, have issued drought advisories, warnings or emergencies. Throughout the mid-Atlantic region, more than three-quarters of all streams and rivers have registered record or near-record low flows.

Rising salinity and low oxygen levels have killed large numbers of fish, in one of which 200.000 fish died in Maryland's waterways.

Wells running dry

Worldwatch says wells are running dry in the worst-hit areas, causing landowners to drill deeper for water.

A fifth of all irrigated land in the US gets its water from a huge underground reserve, the Ogallala, which extends beneath parts of eight states, from South Dakota to Texas.

The Institute says the Ogallala's southern reaches get very little rainfall to replenish the water pumped out of the aquifer.

It says the amount of water depleted from the Ogallala so far equals the water that flows down the Colorado river in 18 years.

Worrying global trend

Although droughts eventually end, Worldwatch says the longer-term global trend is worrying.

In many key grain-producing parts of Asia, the Middle East and north Africa, groundwater is being used more quickly than it is being replenished.

Major rivers that now run dry for large parts of the year include the Ganges, the Indus, the Yellow river, and the Colorado.

Of the world's total irrigated land, 20% has been damaged by a build up of salinity that slowly impairs the fertility of the soil.

Worldwatch says the number of people living in countries described as "water-stressed", where supplies are already barely enough, is 470 million.

But by 2025, that number is projected to have climbed to 3 billion. That will mean more intense competition for grain imports.

"Whether the US, Europe and other exporters will produce sufficient surpluses to meet those import demands is only half the issue.

"The other half is whether the exports will be offered at a price that poor, food-importing nations - especially those in south Asia and sub-Saharan Africa - can afford."

Worldwatch says water scarcity is now "the single biggest threat to global food production". And there is a further twist.

"Hanging over these worsening water problems is the prospect of climate change.