California Code, Public Resources Code - PRC § 6927

The terms so fixed shall be operative for 20 years from the date of the grant.  Upon the expiration of such 20-year period and each succeeding period of 20 years the commission or other duly constituted authority, may prescribe for each ensuing period of 20 years a different royalty rate or rates to be paid as a condition of the continued exercise of the grant.