Wat. Code, § 25651

(a) If a refunding bond plan or modification of it provides for the raising of a fixed amount each year to be applied to the payment of interest on or redemption of refunding bonds in the manner provided in the refunding plan or modification of it, the annual assessment shall include a levy in the amount required to be raised by assessment in that year pursuant to the plan or modification of it.

(b) If the proceedings in connection with the issuance of refunding bonds or modification of them provides for the raising of an amount to be paid annually into a sinking fund to pay the principal or interest of the refunding bonds, the annual assessment shall include a levy in an amount sufficient to provide the sinking fund payments for the current year. The amount required to be raised for the sinking fund shall be reduced by the amount of surplus funds from other sources in the sinking fund in excess of the amount required to be therein at the time of levying the current annual assessment.