You know that only 10 total people live in those two big counties on the lower right?
I don't think California's going to secede, but if it did, a lot of people would simply leave the sparsely populated inland counties for the US, rather than fight it out.
They'd probably be replaced by settlers from the cities who'd support the secession and hold on to the water for SF and LA.
Orange, San Bernardino and Riverside county are shown in red here. Old map. Those counties went heavily for Hillary. Just like former Republican stongholds San Diego, Ventura and San Luis Obispo counties.