When fifty or more landowners within a proposed watershed district, or twenty percent of the landowners within the proposed district, whichever is less, desire to form a watershed district, they shall file a petition with the board of supervisors. The petition shall define the boundaries of the proposed district, the number of acres of land involved, reasons for requesting creation of the district, the proposed name for the watershed district and other information pertinent to the proposal. In the event of a challenge to the validity of signatures on a petition, the burden of proof shall be on the sponsors of the petition.