ORS § 545.395
An irrigation district may provide for the reclamation, improvement or irrigation of the lands within the district in units. When a district does so, the assessments against the lands in the district may be apportioned by the board of directors to the lands owned or held by each person so that the lands in each unit shall pay the cost of reclaiming, improving, maintaining and operating the lands in the unit. Within the units the assessments shall be apportioned in accordance with ORS 545.381, 545.385, 545.387, 545.389, 545.391 and 545.413. Land noncontiguous to an irrigation district may be included in the district as a unit at the time of the organization of the district or at any subsequent time. Prior to the completion of the works for the reclamation of any units the lands in the units may be assessed as appears equitable to the board, subject to the rights of land owners in the district to have the assessments adjusted by the board of equalization and to appeal therefrom. However, all reclaimed or improved lands, whether irrigated or not, shall be subject to assessment for the payment of any obligation of the district.
ORS 545.395