If you live on a 20' by 50' plot that gets six inches of rain per year, that's 500 cubic feet, or enough water for ten people to drink all year. And that's low rainfall even by Arizona standards (Phoenix gets 7 inches while Tucson and the plateaus are wetter), a small plot of land (1/43 acre), and ignoring any reuse or importation. The Southwest has plenty of water for people to live their lives, as long as it's utilized efficiently. It's agriculture that isn't suitable.
It's also worth recognizing that the western United States was settled first, as people came down the Pacific Coast after crossing the Beringia land bridge during the previous ice age. California in particular has always been quite densely populated, and the only (IIRC) indigenous irrigation systems were built by the Hohokam near the Colorado Plateau.
>The population [of California] cannot be tabulated by tribes, but there can be no question that it was several times larger than any other area north of Mexico and that the destruction has been correspondingly greater.
It's also worth recognizing that the western United States was settled first, as people came down the Pacific Coast after crossing the Beringia land bridge during the previous ice age. California in particular has always been quite densely populated, and the only (IIRC) indigenous irrigation systems were built by the Hohokam near the Colorado Plateau.
EDIT: https://repository.si.edu/bitstream/handle/10088/23978/SMC_8...
>The population [of California] cannot be tabulated by tribes, but there can be no question that it was several times larger than any other area north of Mexico and that the destruction has been correspondingly greater.