Cal. Gov. Code § 26203
In years other than those in which a census is taken by the United States, the board of supervisors by ordinance may cause a census of the county or any district therein to be taken which shall be validated by the Demographic Research Unit of the Department of Finance. The board of supervisors shall cause a census of a judicial district to be taken upon presentation to it of a petition signed by not less than 40 percent of the qualified electors residing in the district whose names appear upon the great register of the county at the last general election. The persons taking the census shall enumerate all the inhabitants of the county or district. The census shall be validated by the Demographic Research Unit. The expenses of taking that census is a county charge.
Ca. Gov. Code § 26203