California Code, Government Code - GOV § 69508.5

(a) In courts with two judges a presiding judge shall be selected by the judges each calendar year and the selection should be on the basis of administrative qualifications and interest.

(b) If a selection cannot be agreed upon, then the office of presiding judge shall be rotated each calendar year between the two judges, commencing with the senior judge.  If the judges are of equal seniority, the first presiding judge shall be selected by lot.

(c) Notwithstanding subdivisions (a) and (b), the Judicial Council may provide by rule of court for the qualifications of the presiding judge.

(d) In a court with one judge, whether as the result of a vacancy in a judgeship or otherwise, a reference in a statute to the presiding judge means the sole judge of the court.