In the case of a district that contains only unincorporated territory in more than one county, the board of directors may be elected or may be appointed by the boards of supervisors of the counties in which the district is located. If the board of directors is appointed by the county boards of supervisors, the boards of supervisors shall appoint directors according to the proportionate share of population of that portion of each county within the district, provided that each board of supervisors shall appoint at least one director.
Source:
Section 5783.3, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=PRC§ionNum=5783.3..