>>8 Desalination is a energy costly and expensive process, which is why it's not redily being used, at the moment alternative decontamination processes are being reaserched. The Mojave desert sounds interesting, but with the land that it'd take up, I'd suggest that you'd be better off building solar farms and using that electricity to power the Desalination Plants while also powering the local community.
My personal thought experiment has lead me in a few directions, mostly focused on producing most of our food using aeroponic/hydroponic methods to drastically cut down the major water consumer on the planet (as you've pointed out, Agriculture). But my personal fav so far would be to use Plasma Gasification as our major desalinization and recycling method. Theoretically, it would devide the introduced substance (sea water) into a cloud of gas (rich in chemical elements, such as, and the major part would be H2) then the salt would be crystallized on the bottom of the reactor chamber, with petro-chemical waste turned into an oil rich goo. All of which could be then collected, then sold/used in any imaginable way. Best off, the hydrogen would then be able to be collected, shipped to where water and energy is needed, then burned as fuel, water being it's "exhaust" (it is rocket fuel after all). The heat produced within the reactor chamber is enough to power the reactor, along with a small town. It literally pays for it self in energy and chemical production alone, water being just a side product... which would be sold for an additional profit.