Cal. Water Code § 51400

When the board has authorized the sale of refunding bonds at less than par value and has determined that it is necessary or advisable that an amount be made available to pay interest on the refunding bonds from the date of sale to the date of maturity of the bonds to be refunded, the board shall declare whether it is necessary or advisable that a supplemental assessment be levied to raise the amounts required under the provisions of subdivisions (a) and (b) of Section 52560, or either thereof.