"The abandonment of a city like Los Angeles, would be just as quick as say New Orleans was if the taps ran dry. i.e. about a week."

For Las Vegas and Phoenix, probably half that.

The crux of the problem is our high water usage. The U.S. national average of water usage is 180-200 gallons per person per day. Even at the lower estimate, for a family of four, that is over 250,000 gallons per year, or 20,000+ gallons per month.

Rainy Seattle gets about 36" of rain per year; if that arrived evenly at 3" per month (it doesn't), that's still only 1860 gallons for a 1000sf house, 3720 for a 2000sf house, and 4650 gallons for a 2000sf house with a 500sf garage, with a grand total annually of 55,800 gallons per year. That provides 4650 gallons per month, far short of the average use, and would provide a maximum of 38 gallons per person per day, far short of that 180 gallons.

During the recent election, one of the arguments against our state government was that "it doesn't have an income problem, it has a spending problem". With water, we have a usage problem.

Just for fun, I did a little basic math for a city in SoCal, picked at random, Canoga Park. It's average rainfall is 16.83"/year, mostly falling in 5 continuous months (Nov-Mar), the other 7 months are almost dry.

If you had four people living in a 1000sf house with a 500sf garage, you might be able to collect 15,000 gallons of water.
At the usual water usage of 180 gallons per person per day, that 15,000 gallons would last about a full 20 days.

If you rationed it to 50 gallons per day (12x4)*, it would last about 300 days. If you could ration it to about 10 gals per day per person, you might be able to squeak through a good year. Otherwise, you would have to create enough additional collection area to make up the 3,250-gallon shortfall. And that's a decent year, and that's paring it awfully close to disaster.

*****
According to Population Reports the four most basic water needs are for drinking, sanitation, bathing and cooking. Worldwide, the estimated average need is 12 gallons per person per day.

Sue