ORS § 545.387
After an affirmative vote at any regular or special election called or held under the Irrigation District Law, a district issuing bonds may proceed to levy and collect assessments for any purposes of the irrigation district on a benefit basis instead of on the basis of the number of irrigable acres. The valuation of lands for determination of benefits shall be made by three competent, disinterested viewers appointed by the governing body of the county. The viewers shall classify the lands included in each ownership or smallest legal subdivision and fix the assessments according to the productive value of water and land prepared to receive water. The assessments shall be determined without regard to permanent improvements, such as buildings or orchards. When fixing the amount of assessments, the viewers shall provide proper deductions for partial water rights appurtenant to any tract of land within the district not furnished by the district. However, no change in method of assessment shall be made except with the consent of the holders of outstanding bonds.
ORS 545.387