For the purposes of this article there shall be three geographical zones which shall be designated (a) the “southern zone,” which shall consist of the Counties of Imperial, Orange, Riverside, and San Diego; (b) the “central zone,” which shall consist of the Counties of Kern, Los Angeles, San Bernardino, San Luis Obispo, Santa Barbara, and Ventura; and (c) the “northern zone,” which shall consist of the remaining counties in the state.