Gleick provides three categories of water usage to illustrate different issues: peak renewable water, peak non-renewable water, and peak ecological water. The first concerns surface or rapidly-renewed subsurface water supplies.
The nonrenewable category refers to sources that are accessible, but for geologic reasons are effectively non-rechargeable. Usage of these sources ultimately leads to depletion and increased costs for extraction.
Water use affects not only humans, but all of nature.
Gleick suggests that the US has already exceeded "peak" conditions in all three categories. He provides this supporting data:
Water use peaked over 30 years ago. One must conclude that if more water could have been economically extracted from the environment, it would have been.
Gleick is interested in business and investment issues. He sounds an alarm by indicating the potential risk to food supplies from over-extraction of water, but finishes on a high note by pointing out that even though water usage has leveled off, our economy has continued to grow. In other words, water productivity has continued to increase and it is not yet clear that water availability has been a hindrance to the economy.
Lester R. Brown, president of the Earth Policy Institute, is directly concerned with the security of the world’s food supply in his book Full Planet, Empty Plates: The New Geopolitics of Food Security. He is greatly concerned about the tendency to use water in an unsustainable manner because the ability to produce food is tied to the water supply. If water usage is unsustainable, then food production is unsustainable. He refers to such a condition as living in a "food bubble."
Brown provides this list of the "bubble" countries.
Note that most countries in the Middle East make his list. The causes and political ramifications of food shortages in that region have been discussed in Climate change, Food security, and Revolution.
Brown provides a short summary of the water/food supply issues in the major countries. A measure of a country’s capability to utilize water supplies to produce food is its ability to increase or maintain land under irrigation.
In China, Brown is concerned about the long-term health of agriculture in the North China Plain, an area that produces half of its wheat and a third of its corn. The region receives little rainfall and is dependent on groundwater.
"....concerns are mirrored in the unusually strong language of a World Bank report on China’s water situation that foresees ‘catastrophic consequences for future generations’ unless water use and supply can quickly be brought back into balance."
Of most concern to Brown is India, a country of over a billion in population that is barely able to feed its people today, yet is expected to add another half billion before its population tops out.
"In communities where underground water sources have dried up entirely, all agriculture is rainfed and drinking water is trucked in. Tushaar Shah, a senior fellow at the International Water Management Institute, says, ‘When the balloon bursts, untold anarchy will be the lot of rural India’."
Brown expresses this fear:
And he leaves us with this ominous note:
A declining water supply competes with soil erosion, desertification, population growth, global warming, and diversion of food stocks to fuel as threats to world food security. Brown’s book was intended to warn us of the danger of following such an unsustainable path.