Great conversation, if you don't mind I'll try to put together a group of thoughts here rather than individually.
I was imprecise when I said "ration drinking water" rather the more accurate term would be "ration the water allocation to citizens" (that is what the city meters) and rationing isn't like you get your 150 gallons and then poof, it is really more like if you use more than your 'ration' your water bill will be much much higher. Which is, as others have noted, an economic allocation mechanism.
The rice fields are being drained. Over time as contracts run out and federal allocations are cut, those fields too will go dry.
It is hard to 'see' the regulation if irrigation water, and it is true there is no regulation on water pumped from underground. Having spent the summer working on a farm, the farms in the central valley get allocation from the water system that goes through it, and those big pipes with the wheels on them get closed and locked. When you are getting your allocation they are opened for a specific amount of time, as the system is calibrated to pass a specified amount of acre feet per minute/hour/day which is dumped into the irrigation canals around your farm. Once the allocation is up the pipe is closed for that period.
I agree that long before anyone decides to fund a giant river rerouting however some of the hydroponics experiments going on are going to yield much more water efficient farms.