CA Gov't Code Section 71043

The determination of whether a judicial district or former judicial district has a population above or below 40,000 shall be made on the latest occurring of the following bases:

(a)

As shown by the last preceding federal census of the district or of the aggregate cities and other political subdivisions situated within the district, whichever is greater.

(b)

As shown by a subsequent census taken pursuant to Section 26203.

(c)

As may have been found to be the fact in any proceeding for declaratory relief brought in a court having jurisdiction.