Ok, I'm a retired Civil Engineer having spent much of my career dealing with water/wastewater.

1) Though water could be used much more efficiently in CA (I'm an advocate of the book "Cadillac Desert"), some of the previous responders are correct in saying:

a) Ag is about 80% of CA water use, and CA feeds the rest of the nation

b) History is full of water diversion for ag for better or worse (think Nile River, Tigris/Euphrates).

2) Desal is very energy intensive and expensive on a large scale (reclaiming wastewater or stormwater is much cheaper)

3) Some places get tons of water (ie Pacific Northwest) and if CA would agree to use water sensibly (ie leave enough for habitat, residents conserve, limit lawns, pools, allow reclaimed water), not grow thirsty crops (like alfalfa, rice, cotton) there would certainly be an economic incentive for Pacific Northwest to sell water to CA.

4) There have been pipeline proposals to bring water from Pacific Northwest (Columbia River Basin) and lower Mississippi to Colorado River basin, not sure of environmental impacts but certainly technically feasible.

5) I realize there is a popular "screw California" thread out there, but indeed this is where much of the Nation's fruits, veggies, etc come from, and in the present/future much of the solar energy (incidentally, Mexico also produces much ag/solar, should we parch them too?). I could also say that much of the defense/electronics industry resides here, part of it regionally/strategically important.

6) If cheaper/cleaner energy can be produced, I believe vertical fruit & veggie hothouses with driplines are the future. CA has much fewer "degree days" (net energy demand for hothouses due to warmer year-round climate) than other parts of the country. In short, lettuce, olives, avocados etc. need a Mediterranean climate & don’t grow well in the Midwest, East, most of the South.

7) I would like to see rainwater cisterns used en masse. A 5000 gal tank is only about $5k, and some simple treatment could make this water available for all non potable applications (all but kitchen & possibly bath water). A "swimming pool" doesn't necessarily need to be demonized...cisterns and pools are also what we use for fire fighting capacity (and sometimes solar heating/cooling storage, possibly heat pump use).

8) Humanity in our country may need to move northward, who knows. But with ag land in short supply (and the Southwest having such a short frost season), could the Nation really afford abandonment?