Fortunately, all the problems from climate change and depletion point to similar and overlapping solutions. Unfortunately, the solutions will require radical change. It will also considerably reduce California food output, decrease meat in our diet and increase prices.
There are a few realities that I think will form the boundaries of what happens. Within those, we can make choices to direct how agriculture ends up.
First, oil is a substitute for labor. As oil becomes too expensive to be used for current mechanization, human and animal labor will fill in behind it or the enterprise will stop. Increasing energy use efficiency can give us some elasticity in substitution, but re-fitting machines also costs money
California is a great growing region, with very large areas of world class soils and a long growing season close to millions of comparatively rich eaters. Unless we get into paleodrought hydrology, we have enough water to sustain less agriculture (about 2/3rds) as well as desert-friendly cities.
Food prices will go up substantially, either because of reduced supply as land goes out of production, or oil or labor costs, or both. If meat and dairy production are forced to internalize their environmental costs, large concentrated operations will end.
The next many decades will see a constantly moving climate with increasing major perturbations. Under those conditions, rather than lock in expensive responses, the guiding principles should be preserving future capacity and being able to return to productivity after a flood, drought or fire.
These are the drivers that we will be reacting to. We should make decisions about how we want to respond to these constraints. We are a wealthy enough society that we have a lot of resources to put into shaping the agricultural sector.
On the public record 
I keep thinking about drovers’ roads, which were common all over England and Europe, and existed for a while in New England. Western cattle drives were drovers’ roads, really, just not through inhabited land. As the last mile gets more expensive, maybe the cattle drives will come back, with the cattle following pasture.
This could fail horribly without the extensive traditions and use-laws that it used to have. England didn’t face many droughts, after all; they did resent having too much grass go to passing herds, but OTOH the herds left droppings, so it tended to even out.
Funny factoid; to get geese all the way to London, one would drive the flock through warm tar and then powdered shell, to give them tough feet.