You could probably get another ~30% reduction in residential use by eliminating lawns. Depending upon how you go about it, trees are probably more important in that environment though as a cooling element. Our backyard patio increases heat by a good 5-10 degrees on sunny days.
But it's still not much water saved. The only reason to do it would be to sell it politically - "everyone has to sacrifice". Which I'm fine with. But if your goal is to make residents do this but tell farmers they can do whatever they want, it's kinda pointless.
Edit: I also think there's somewhat responsible ways to have lawn in California. Subsurface irrigation can reduce lawn water usage by 30-40%. And certain drought tolerant grasses require 60-80% less water. These setups seem very rare though.
It might be easier to convince people to not grow lawns if we'd do away with some of the stupid laws and ordinances that require people to have "well groomed" lawns in places where such grass is an unnatural imported occurrence.
Perhaps even offer some tax breaks or other incentives to "zeroscape" or landscape their yards using native plant life suited to the climate. A yard without a lush verdant lawn need not be an ugly yard. It can still have plants and look nice without grass, if we'd "normalize" more nature/planet friendly practices instead of punishing people for not being destructive to their environment.
I bring this point up because here in Utah many neighborhoods will actually fine you if you don't have a lush green lawn in your yard. (Unless of course you're rich, then somehow you magically get approval to go the zeroscape route if you so desire.)
I would definitely like to see it taken farther as you have suggested -- give tax breaks and subsidies for people xeriscaping or replacing lawns with synthetic turf.
And it definitely works. I knew a lot of people in HOAs that mandated residents eliminate a certain amount of lawn due to these fines.
The only reason it started growing so quickly is because A/C became available. Prior to that they couldn't use swamp coolers mid summer due to the monsoon season making it too humid to work. Water, power, etc, all of that was already there.
A better example is someplace like Las Vegas.
https://kinder.rice.edu/sites/default/files/files/2017/08/Sc...
They could start by banning lawns and golf courses in the desert. No reason someone living in Phoenix needs a verdant green lawn. Maybe place some limits on unsustainable agriculture in the desert. I imagine such bans would be a tough sell in most parts but it's hard to imagine how else the land can recover.