Cal. Pub. Util. Code § 22556

The board may charge and collect fees, tolls, and rentals for the use of all or a part of the aerial facilities of the district in such amounts and at such rates as, so far as possible, will produce revenues sufficient to pay the operating expenses of the district, provide for repairs and depreciation of the properties of the district, pay the interest on the bonded indebtedness of the district, and provide a sinking or other fund for the payment of the principal of the indebtedness as it becomes due.