What are four main sources of water on earth?

Answer 1

At least in Saudi Arabia, the four main sources are:
1. Surface water, e.g., rivers and lakes.
2. Ground water, held in aquifers underground.
3. Desalinated seawater.
4. Reclaimed wastewater.
http://saudinf.com/main/a54.htm

These folks know how to manage water when it becomes a scarce resource.

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Cheers,
Bruce