How water raises the political temperature between countries
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 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 hydro-power on the river Indus. China, Nepal, India and Bangladesh all spar over the rivers rising in the Himalayas and which 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, as well as the severely depleted Aral Sea. Argentina and Uruguay have taken their dispute over the river Plate to the International Court of Justice in The Hague, while Mexico and the US argue over rights on the Rio Grande and Colorado.
Last month, Baghdad demanded that Syria cease pumping water from the Iraqi portion of the Tigris. Elsewhere in the Middle East, Palestine and Israel, and Iraq and Iran, row 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 between Botswana, Mozambique, Zambia and Zimbabwe, while 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 UN, there are more than 250 internationally shared rivers covering nearly half the total land surface of the earth, as well as innumerable shared aquifers. Around 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 last century, only seven minor skirmishes over water were documented.
However, nearly all the world's major rivers are expected to come under increased pressure to provide farming, industry and drinking water for the three billion extra people expected to be born before the world's population starts to drop. By 2025, says the UN development programme, 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.
The disputes are not just between countries but between states and rural and urban users. The Yellow river in China, the Ganges, the Mekong and other Asian rivers do not always reach the sea in dry seasons, leaving farmers short and blaming factory users higher upstream.
The river Kaveri is the bone of serious contention between Tamil Nadu and neighbouring Karnatgaka states, and water from the Vansadhara river is disputed between Andrah Pradesh and Orissa states.