You don't even have to go further than electricity and water supply to see how much they rely on other states. But I'm sure there's a lot of other areas where they're reliant on other states as well.
Other than the Colorado River, does California depend on other states for water? If California seceded, wouldn't they retain their part of the Colorado River Compact?
Power, I'll grant you. They've been "exporting pollution" by having power plants built in other states for a while now...