Water is plentiful on Earth; humans use less than 1/100 of one percent. This global abundance obscures local challenges. In many places, the tap is running dry. That's because there's a difference between usable water -- the kind that's clean and salt-free and located nearby, and unusable water -- everything else.
Many of the reservoirs of water humans draw on for farming, manufacturing and drinking are ground-water aquifers that took millennia to fill. These irreplaceable storage tanks are being depleted over just a few decades.
The era of cheap water has passed its peak, according to Peter Gleick, a California scientist who helped define the term "peak water" in 2010. The good news is there's plenty of water out there if the humans just learn to better manage it.
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