CA's central valley is productive enough to supply the entire U.S. with enough food to eat, and they have water coming out their ears (if they had not torn out all their dams and flush most of it down the rivers in order to save a certain small fish.)
Climate change will destroy the agricultural sector in the Central Valley. Already, the state is facing a severe drought, and the situation is likely to escalate in the future. Also, the water-rich Northern California is heavily Republican, and they are unlikely to support the secession.