The world's fresh water supply is dwindling every year, according to research in the United States.

Within 25 years, half the world's population could have trouble finding enough fresh water for drinking and irrigation.

The study was carried out at Colorado University, which surveyed river basins all over the planet to identify those under most pressure.

It found a third of the world's people already live in regions considered to be "water-stressed" - where there is not enough, or barely enough water to go around.

Areas at risk

Waterways under most pressure included China's Yellow River basin, the Zambezi River in Africa, and the rivers that lead into the Aral Sea in Central Asia.

Most of the water from those sources is used for irrigation, not drinking, according to the study's leading author, Kenneth Strzepek.

He says that with rising populations, half the world's people will find it hard to get enough water for crops and livestock within 25 years, and still have enough left to drink themselves.

With many of the world's freshwater rivers crossing more than one country, Professor Strzepek says political solutions, as well as scientific ones, need to be found.