"The idea of peak water, despite its flaws…signals we are at the end of the age of cheap, easy water," Meena Palaniappan of the Pacific Institute told environmentalresearchweb. "In the same way that peak oil has meant the end of cheap, easy-to-access sources of petroleum, peak water means we are going to have to go further, spend more, and expect less in the realm of freshwater. Peak water reminds us that water, which we used to think was widely available and inexpensive, can no longer be taken for granted."

Although the planet contains around 1.4 billion cubic km of water, nearly 97% of this is saltwater in the oceans. Most of the 35 million cubic km of freshwater, meanwhile, is locked up in glaciers in Antarctica and Greenland, in permanent snow cover, or in deep groundwater inaccessible to humans, write Palaniappan and colleague Peter Gleick in PNAS. But people are able to use freshwater from rivers, surface lakes, accessible groundwater, soil moisture and rainfall.

"Obviously there are tremendous differences between oil and water, particularly that water is primarily a renewable resource, while oil is non-renewable," said Palaniappan. "There is a certain fixed amount of water on the planet that has been here since the origins of the planet, while the amount of oil on the planet is constantly declining. But there are ways in which we can see a peak oil type effect in the water sector."

Palaniappan and Gleick have defined three types of peak water: peak renewable water, the total annual supply of water from sources such as rainfall, rivers, streams and groundwater basins that are recharged relatively quickly; peak non-renewable water, for aquifers containing fossil groundwater accumulated over many thousands of years and with a very slow recharge rate, or for groundwater systems that cannot be recharged when overpumped because the basin becomes compacted; and peak ecological water.

"We define peak 'ecological' water as a certain theoretical point where the value of ecological services provided by water is equivalent to the value of human services provided by water," explained Palaniappan. "After this point, taking more water for human use causes ecological disruptions far worse than the value it adds for humans. At the point of peak ecological water, society will maximize the ecological and human benefits provided by water."

According to Palaniappan there are rivers and watersheds throughout the world where we've already surpassed peak ecological water. "Worldwide, humans have over-appropriated flows of water to the point where many rivers no longer reach the sea or are too polluted to sustain life – the Colorado River, the Nile, and the Yellow River (Huang He) in China are a few examples," she said.

Similarly, unsustainable use of groundwater resources is ongoing in the Ogallala Aquifer in the great plains of the US, the North China plains, California's Central Valley, and many basins in India.

Although there is always the "backstop", more expensive option of desalinating ocean water, regional water shortage is already causing ecological and political problems. While the high value of oil makes it economic to transport the liquid huge distances around the planet, the same is not true for freshwater. What's more, various substitutes are available for at least some of oil's applications but water has no viable alternative.

With that in mind, Palaniappan and Gleick say they will continue to work on a more sustainable system of water management that provides for basic human needs for water and ensures environmental flows.

"We've done a lot of work on a 'soft path' for water supply which focuses on the need for efficiency, conservation, and human capital," said Palaniappan. "The soft path strives to improve the overall productivity of water use rather than seek endless sources of new supply. It delivers diverse water services matched to the user's needs, and works with water users at local and community scales."

The researchers reported their work in PNAS.