California Code, Government Code - GOV § 68207

(a) The population of each county is as shown and determined by the last preceding decennial census taken under the authority of the Congress of the United States, except that whenever a new decennial census is taken under such authority after the effective date of this section the population shown and determined thereby shall be deemed the population of the county commencing on the first day of July in the year next succeeding the year as of which such census is taken and not before.

(b) The Department of Finance, before the first day of December of each year, shall certify to the Controller the population of each county as determined by the department pursuant to Section 13073.5. The Controller shall forthwith ascertain from that certification whether any county has a population, determined as provided in this subdivision, which would raise it to a higher classification under this chapter than as determined by subdivision (a). If any county is ascertained to be within such a higher classification, the Controller shall promptly certify that fact to the board of supervisors of that county. For the purpose of this chapter, the population of each county on the first day of July of each year shall be the population determined by the Department of Finance.