California Code, Streets and Highways Code - SHC § 27203

Within a reasonable time prior to the time that the boards of supervisors of the counties having any portion of their territory within the boundaries of the district are required by law to fix their tax rates, the board shall certify to the board of supervisors of each of the said counties the rate of taxation fixed for district purposes and at the time and in the manner required by law for the levying of taxes for county purposes each shall levy and collect a tax for district purposes in addition to any other tax levied by such board of supervisors at the rate certified by the board.