The World Resources Institue has released a ranking this week of the 18 river basins around the world facing the “extremely high” levels of water stress, meaning that “80 percent of the water naturally available to agricultural, domestic, and industrial users is withdrawn annually”: http://bit.ly/1ijQ8tU

According to the list, Iran's Qom river faces the highest level of stress in the world.

Water supply has become a major environmental and political issue in Iran. The government has had to make contingency plans for rationing in Tehran, and riots have broken out over plans to divert a river from Esfahan province.

Five of the 18 basins on the list are in China. Indonesia, Afghanistan, India and the United States each have two.

The Colorado River and the Bravo Basin of the Rio Grande are America's most stressed basins. As Andrew Maddocks and Paul Reig of RWI point out, more than 30 million people depend on the Colorado for water and the seven states it supplies comprise 19 percent of America's GDP.

Last year authorities had to make major cuts to the amount of water flowing downstream for the first time in the heavily managed river due to record-low water levels in the reservoirs of Lakes Mead and Powell.

Full list on WRI site: http://bit.ly/1ijQ8tU

Joshua Keating is a staff writer at Slate focusing on international news, social science and related topics. He was previously an editor at Foreign Policy magazine.