Iowa Code § 468.512
Except where the provisions of section 468.511, providing for vote in proportion to assessment are invoked, any person or corporation owning land or right-of-way within the district and assessed for benefits may have the person's or the corporation's vote cast by the person's or the corporation's agent or proxy authorized to cast such vote by a power of attorney signed and acknowledged by such person or corporation, and filed before such vote is cast in the auditor's office of the county in which such election is held. Every such power of attorney shall specify the particular election for which it is to be used, indicating the day, month, and year of such election, and shall be void for all elections subsequently held. The vote of the owner of any land in a drainage or levee district in any election, where the vote is not determined by assessment, may be cast by absent voters ballot in the same manner and form and subject to the same rights and restrictions as is provided in section 468.511 relating to vote by absentee ballot when votes are determined by assessment.
Iowa Code § 468.512