Iowa Code § 468.370
All bonds issued under the provisions of this part shall be issued pursuant to and in conformity with a resolution adopted by the board of supervisors, which shall specify the amount authorized to be issued, the purpose for which issued, the rate of interest they shall bear and whether payable annually or semiannually, the place where the principal and interest shall be payable and when it becomes due, and such other provisions not inconsistent with law in reference thereto as the board of supervisors shall think proper, which resolution shall be entered of record upon the minutes of the proceedings of the said board and a complete copy thereof printed on the back of each bond, which resolution shall constitute a contract between the drainage district and the purchasers or holders of said bonds.
Iowa Code § 468.370