A US conservation group says the world's impending water shortage could reduce global food supplies by more than 10%.

The group, the Worldwatch Institute, based in Washington DC, says the shortage could lead not only to hunger but also to civil unrest and war.

Sandra Postel is the author of a book published by the institute on the water crisis, entitled: "Pillar of Sand: Can the Irrigation Miracle Last?"

Heavy reliance on irrigation

She says: "Without increasing water productivity in irrigation, major food-producing regions will not have enough water to sustain crop production."

It will be a thirsty as well as a hungry world

"Some 40% of the world's food comes from irrigated cropland, and we're betting on that share to increase to feed a growing population."

Postel says in the book that one threat to the productivity of irrigation is excessive pumping of groundwater from subterranean aquifers.

Earlier research by Worldwatch illustrated the scale of the problem:

• Between 1991 and 1996, the water table beneath the north China plain fell by an average of 1.5 metres a year.
• Almost everywhere in India, the water table is falling at between one and three meteres annually.
• Mexico City is sinking because of the amount of water being pumped out from beneath its foundations.

Two further problems are the growing diversion of irrigation water for use in cities, and the build-up of salinity in the soil.

The book says there is an annual "water deficit" of about 160 billion cubic metres - enough to produce nearly 10% of the world's grain.

Countries which are short of water are buying more and more on the world grain market.

Jordan imports 91% of its supplies from abroad, Israel 87%, Saudi Arabia 50% and Egypt 40%.

Less water will be there to feed more people

The book says the number of people living in countries suffering from water shortages is expected to rise from 470m today to 3bn by 2025.

One region that relies heavily on irrigation is north-east Africa. There, Worldwatch says, competition for the waters of the Nile will get fiercer.

The total population of Ethiopia, Sudan and Egypt stands at 157m today.

By 2050 it is projected to reach 388m.

A mix of strategies

Postel says the world needs a "blue revolution" to double water productivity in the next 30 years.

She says drip irrigation systems, which deliver water straight to the plant's roots, can cut water use by from 30-70%.

They also raise crop yields significantly, and are in use in the US, Europe and Asia.

Farmers in Texas have improved their water efficiency to more than 90% by using efficient sprinklers.