'nother hydro-worker checking in... The concern is valid and worth thinking about. But, water has a cycle, it rains, it gets stored, it gets used, it evaporates, it rains. There's plenty unless you get greedy.

"Greedy" means insisting on building your largest cities in deserts, putting golf communities in deserts, irrigating cotton in deserts, polluting your water sources, etc. It also means piping water from where there's plenty to supply unthinking greedy people in the desert who still wash their cars twice a week with drinking water because it's so cheap.

What we may see someday: a few unpleasant events that in the end will benefit us because they will cause us to wise up and stop being reckless with something so important. Like: prices rising to reflect actual cost of providing water in unsustainable places.

As with all doomsday scenarios, the worriers are discounting the flexibility of humankind. If push comes to it, there is a lot that can be done; artificial recharge (directing rainwater to storage), dual piping (using funkier water for washing and sewage and conserving the crystal clean stuff for drinking) etc., and the one we don't like but mankind has used many times to survive--migration away from unsustainable places. 'course many people would rather fight to take somebody else's stuff, so wars are possible, but not too likely wherever there are more practical answers.