10 million people live in, say, around Los Angeles. But to supply those 10 million people with water a fair percent of the watershed of California is tapped.
Yes, and that is especially wrong because it is unnecessary. Believe it or not, Los Angeles receives enough rainfall to account for more than 90% of its water use. But about 99% of that water runs straight into the ocean (where it causes brackishness during rains, because so much water is shed so quickly!) because Los Angeles has been paved all to hell, and has no ability to retain water. It's like a runner that's skipping salt.
In, say, parts of New York of the south, water is more abundant. But to feed 10 million people anywhere takes land to grow food, to find a place to dispose of their sewage and trash, etc. etc.
Sewage is a big issue. At best you need enough room to compost the poop, and since nobody here wants to be a night soil man we have a whole expensive infrastructure for piping the shit around... using water. A lot of water. And then, the water is maybe used for irrigation. But we could at least be using AIWPS and getting clean water out of the other end of the system, albeit at some cost in space. Which brings us back to what you were saying, of course.
Food is actually a much smaller issue. Vertical gardening on aeroponics can produce a whole lot of produce in a very small space with very little resources. There are dozens if not hundreds of such operations across the country so far, and they are reproducing rapidly.
Trash is a huge issue, but it should be a lot smaller. Notably, all packaging should be recyclable, and what isn't recyclable should be compostable. It should be outright illegal to sell anything that comes in a non-recyclable package. That would go a long way towards solving this problem.