Cal. Water Code § 60110

If the department determines that the persons or property within a proposed district will benefit directly or indirectly from the use of or right to use the ground water supplies therein or the replenishment waters thereof, the board of supervisors of the principal county shall forthwith divide the proposed district into five (5) divisions, according to and based upon the population, as estimated by the board of supervisors of the principal county from the register of voters used at the last general election, in such manner as to equalize, as nearly as practicable, the population in the respective divisions. Such divisions shall be numbered first, second, third, fourth, fifth, and one director, who shall be a resident thereof, shall be elected for each division by the voters thereof, and shall take office as hereinafter provided.