Section 171.505 - How does BIA calculate my annual operation and maintenance assessment?(a) We calculate your annual operation and maintenance assessment by multiplying the total assessable acres of your land within the service area of our irrigation facility by the annual operation and maintenance assessment rate we establish for that facility.(b) We will not assess lands that have been re-classified as either permanently non-assessable (PNA) or temporarily non-assessable (TNA) or lands that have been granted an Annual Assessment Waiver.(c) If your lands are under an approved Incentive Agreement, we may waive your assessment as described in the Incentive Agreement (See § 171.610 ).(d) Some irrigation facilities may charge a minimum operation and maintenance assessment. If the irrigation facility serving your farm unit charges a minimum operation and maintenance assessment that is more than your assessment calculated by the method described in subpart (a) of this section, you will be charged the minimum operation and maintenance assessment. We provide public notice of any minimum operation and maintenance assessments annually in the FEDERAL REGISTER (See § 171.565 ).