Idaho Code § 43-1321
If it appears to the board of county commissioners that the holders of evidence of title to a majority of the acreage with water rights delivered by the irrigation district within the boundaries of an existing irrigation district have properly signed a petition and approved the partition of the irrigation district, that the holders of evidence of title to such land signing the petition are also holders of title to the water rights appurtenant to the land and it appears that the majority of the acreage with water rights delivered by the irrigation district described in the petition is of an agricultural character, the board shall issue an order partitioning the irrigation district as set forth in the petition. The board shall not modify the boundaries set forth in the original petition. Provided however, the board may permit any holder of evidence of title to land lying within the boundaries of the original district of forty (40) acres or more in size used primarily for agricultural purposes to include such land in the new irrigation district, if such landowner has filed a separate petition with the board establishing that such land meets all the criteria required to have been joined in the original petition, including ownership of water rights appurtenant to the land. Supplemental petitions may be accepted at any time prior to the date of hearing.
Idaho Code § 43-1321