The total amount of water on earth is fixed then why studies show that 40 percent of the world population live in water stressed areas

 Water on earth is not distributed evenly; and the amount of SAFE DRINKING WATER is not accessible evenly.

Further, a large portion of the earth's water is ocean water (about 70% of the Earth itself is ocean water, which is not readily available for human consumption (or for consumption of other animals —or plants, for that matter; and not all people live near the ocean (from which seawater can be "converted" into potable water) or near places where potable water can be obtained.

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