Cal. Pub. Res. Code § 4531

The board shall divide the state into not less than three districts. In establishing these districts, the board shall take into account differing physical characteristics, including, but not limited to, climate, soil type, and principal forest crops. Insofar as possible, the board shall group together lands that have substantially similar characteristics and that will best be served by substantially similar regulations. Boundaries of such districts may be altered from time to time as the board determines is necessary.