Wat. Code, § 35474

The charges to pay revenue bonds and interest thereon shall be fixed by the board upon a flat rate per acre or connection or on a metered basis or on a combination of a flat rate and metered basis and shall be sufficient to raise the amount specified on the face of the bonds when issued and one year’s interest thereon unless the district has accumulated a surplus fund in the treasury of the district which is available for the payment of all bonds and interest that will accrue for payment during the current year, in which event the charges may be made so that the aggregate amount to be raised by the sale of water when added to the surplus fund equals the face value of the bonds that will mature within one year and the interest thereon.