How much of the earth’s water is usable as a freshwater resource?

Earth contains about 91.9% of water but only 71% is usable as a freshwater resource. The 91.9% includes the water in the atmosphere that you can't see, like water in clouds, water as complete gas form, and water as ice.
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Geothermic gradient is the rate of increasing temperature with respect to increasing the depth in the earths interior. away from tech tonic plates boundaries, it is about 25-40 c/km (72-87 degrees f/mi) of depth near the surface in most of the world.

Humans have survived and thrived in mexico city by using technology to create greater access to land. when the spanish conquered the aztec capital, tenochtitlan, in the 16th century, they built mexico city by draining the lakes and filling in the lake water to create more arable land for agriculture. this technique of land reclamation has continued to be fundamental to human civilization in mexico city.