Where the office of a commissioner becomes vacant during his term the vacancy shall be filled for the unexpired portion of the term by appointment by the majority of remaining commissioners, and if not so filled within 30 days after the vacancy, it shall be filled by appointment by the board of supervisors pursuant to Section 1778 of the Government Code.
Source:
Section 6054.3, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=HNC§ionNum=6054.3..