1. Petitions for including additional land within a duly created and existing watershed subdistrict may be filed with the supervisors of the soil and water conservation district and in such cases the provisions of sections 278.170 to 278.190 in respect to the creation of a watershed subdistrict shall be observed.
2. If sixty-five percent of all landowners of the area proposed to be included in the existing subdistrict voting in the referendum do vote in favor of the inclusion, the proposed area shall be included in the existing subdistrict and the soil and water district supervisors shall declare the area duly included and the action recorded in their official minutes.