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

Answer:  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.