Desalination is a plausible solution for water for consumer use--that is, urban and suburban locations.
It is not a very plausible solution for agricultural use-- too expensive. Do you realize that those people take the water and just dump it on the ground?
*(well, some of the suburban people just spray it on the ground, too. But they spray millions and millions of gallons on lawns. Sounds like a lot... but agriculture uses trillions of gallons.)
Water rights are complicated. Since the rule is, whoever grabbed it first owns the rights to the water, the people who own it aren't necessarily the ones who use it most responsibly. http://www.washingtonpost.com/...
Agriculture is 80% of California's water use (although only 1.5% of California's economy) The big problem is almonds. Who would have thought that such a niche foodstuff would drive agricultural water? https://www.bostonglobe.com/bu...
Trillions? Yep: http://science.nasa.gov/scienc...