Cal. Water Code § 4275

Except as provided in Article 5 (commencing with Section 4300), the department, on or before the 15th day of August of each year, shall certify, to the auditor and the board of supervisors of each county having a watermaster service area, or part of a watermaster service area, the amount required, in order to pay the apportionments for the ensuing fiscal year, to be levied on the land used in the storage or diversion, conveyance or distribution of the water stored or diverted under the right and the land on which the water is, or is entitled to be, used.