Fifteen years ago Ismail Serageldin, an Egyptian who was vice-president of the World Bank, shook politicians by predicting that the wars of the 21st century would be fought not over oil or land, but about water.
So far he has been proved wrong, but escalating demand for water to grow food and provide drinking water for burgeoning urban populations has raised political tensions between many countries.
In Asia, there are disagreements over the right to dam shared rivers.
India and Pakistan are in semi-permanent dispute over hydropower on the river Indus.
China, Nepal, India and Bangladesh all spar over the rivers rising in the Himalayas that flow through neighbouring countries, providing water for nearly 500-million people on the way.
Tensions run high between Uzbekistan, Kazakhstan, Kyrgyzstan and Tajikistan over the Amu Daria and Syr Daria rivers.
Argentina and Uruguay have taken their dispute over the river Plate to the International Court of Justice in The Hague, whereas Mexico and the United States argue over rights on the Rio Grande and Colorado.
Last month, Baghdad demanded that Syria cease pumping water from the Tigris. Elsewhere in the Middle East, Palestine and Israel, and Iraq and Iran, quarrel over water supplies from the Shatt al-Arab waterway and Turkey’s dams.
In Africa, the Chobe, a tributary of the Zambezi, has caused tension among Botswana, Mozambique, Zambia and Zimbabwe, and there have been incidents between Mauritania and Senegal over control of the Senegal.
Shares of the Niger, Volta and Zambezi are all disputed.
According to the United Nations, there are more than 250 internationally shared rivers covering nearly half the total land surface of the earth, as well as innumerable shared aquifers.
About 300 potential conflicts around the world have been identified, but history suggests very few, if any, are expected to develop into armed conflict. In the past century, only seven minor skirmishes over water were documented.
But in the future nearly all the world’s major rivers are expected to come under increased pressure to provide water for farming and industry as well as drinking water for the 3-billion extra people expected to be born before the world’s population starts to drop.
By 2025, says the UN, nearly one in three people will live in countries that are affected by water shortages.
These already affect 450-million people in 29 countries and, according to the World Water Forum, tensions over water rights and allocations are expected to mount.
Last year the Pentagon predicted that water disputes would rise up the agenda in global politics in the coming years.
It argued that water was central to border disputes. Conflicts in Chad, Yemen and Somalia, it said, have all been linked to water scarcity.