How little water is needed to sustain human habitation?

Supposing a roughly mars sized planet, how little surface water would need to exist to sustain a population of a size similar to pre-industrial earth (say 750 million?) The situation is somewhat unique in the sense that I'm hoping for a very significant portion of the planet to arid, Tatooine style. Is that actually feasible? Note, not all water needs to be on the surface. The population has the means to collect water from underground and atmospheric sources. But I imagine some significant body of water would need to exist.

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How little water is needed to sustain human habitation?

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