Together with energy water gives us and the rest of the planet life. Without it plants cannot through the incredible process of photosynthesis make energy for us to eat. Without it we die. Quite important thus.
Only around 0.007 of all existing water on the planet is freshwater that we can use. We have about 4000 000 000 000 m3/year of freshwater to use. Distribute to a population of 7 700 000 000 we get about 519 m3 per person/year or 519 000 litres per person/year.
(This estimation comes as any other estimation with a lot of uncertainty and how we deal with that uncertainty is a question of moral. This thought experiment of calculation has for example not included the water need of plants and animals. It just highlights unfairness in water access where some humans have plenty clean from a tap while some have to walk miles to get some contaminated.)
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