Oregon Revised Statutes Water Resources: Irrigation, Drainage, Flood Control, Reclamation § 553.520

(1) After determining the sum of money to be raised by any assessment, except a preliminary assessment, the board shall apportion the same among the lands liable therefor. Benefits used as a basis for apportioning maintenance and operation assessments and improvement assessments, except assessments levied for the operation, maintenance and improvement of irrigation works, may be adjusted from year to year in such manner that the adjusted benefits are in the same proportion to the new assessed valuation of the tract of land as the original appraised benefits are in proportion to the original appraised benefits plus the original assessed valuation of such tract of land.

(2) Adjusted benefits shall be made the basis for apportioning the maintenance and operation assessments and the improvement assessment, and shall not be used as a basis for apportioning construction assessments or preliminary assessments. Benefits determined to be accruing to lands upon the construction of irrigation works shall not be adjusted in any manner.