Iowa Code § 468.376: Funds available to pay bonds.

1. When refunding bonds shall be issued to pay for drainage improvements under the provisions of this part, all special assessments, taxes, and sinking funds applicable to the payment of such bonds previously issued shall be applicable in the same manner and the same extent to the payment of the refunding bonds issued under this part, and all the powers and duties to levy and collect special assessments and taxes or create liens upon property shall continue until all refunding bonds shall be paid.

2. The drainage district shall collect the special assessments out of which the said bonds are payable and hold the special assessments separate and apart in trust for the payment of the refunding bonds but the provisions of this part shall not apply to assessments or bonds adjudicated to be void.