Cal. Water Code § 75474

The board shall annually fix and collect a charge, as provided in Section 75471, until the board, on its own motion or on petition of landowners representing at least 25 percent of the land within the district or at least 25 percent of the voters of the district, directs that commissioners be named to make a determination of the benefits received and to establish one or more zones of benefit which shall reflect the proportion of benefits to be derived by the lands within the respective zone or zones from the operation of the district and from any contract the district may have entered into. The determination shall be effective as of the first day of July in the next succeeding year.