Peak water

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From Circle of Blue: Water News (Peter Gleick):

Water Number: Three (3) definitions of “peak water.”

Peak Renewable Water: This is the limit reached when humans take the entire renewable flow of a river or stream for our use. Water is renewable, but there is a limit to how much can be used. Humans have already reached “peak renewable water” limits on the Colorado River. We use it all and can’t take any more. In fact, of course, we probably shouldn’t even take as much as we do, for ecological reasons (see “Peak Ecological Water” below). Increasingly, we are reaching peak renewable limits on many of our rivers and streams. The Yellow River in China no longer reaches the sea much of the year. The Aral Sea has been devastated because the entire flows of the Amu and Syr Darya rivers have been consumed. The Nile Delta is typically dry much of the year.

Peak Non-Renewable Water: While much of our water supply is renewable, there are “non-renewable” water sources as well, where our use of water depletes or degrades the source. This most typically takes the form of groundwater aquifers that we pump out faster than nature recharges them — exactly like the concept of “peak oil.” Over time, groundwater becomes depleted, more expensive to tap, or effectively exhausted. Central Valley aquifers are overpumped, unsustainably, to the tune of 1-to-2 million acre-feet a year. So are groundwater aquifers in India, China, the Great Plains, and other places. This cannot continue indefinitely — it runs into peak non-renewable water limits.

Peak Ecological Water: The third definition, and perhaps the most important (and difficult) one, is peak “ecological” water — the point where any additional human uses cause more harm (economic, ecological, or social) than benefit. We’re good at measuring the “benefits” of more human use of water (semiconductors manufactured, or food produced, or economic value generated), but we’re bad at measuring on an equal footing, the ecological “costs” or harm caused by that same use of water. As a result, species are driven to extinction, habitat is destroyed, water purification capabilities of marshes and wetlands are lost. For many watersheds around the world, we are reaching, or exceeding, the point of “peak ecological water.”

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