43 CFR 426.21 - Interest on underpayments.
(a) Definition of underpayment. For the purposes of this section underpayment means the difference between what a landholder owed for the delivery of irrigation water under Federal reclamation law and what that landholder paid.
(b) Collection of interest on underpayments. If a landholder has incurred an underpayment, Reclamation will collect from the appropriate district such underpayment with interest. Interest accrues from the original payment due date until the district pays the amount due. The original payment due date is the date the district should have paid the United States for water delivered to the landholder.
(c) Underpayment interest rate. The Secretary of the Treasury determines the interest rate charged the district based on the weighted average yield of all interest-bearing marketable issues sold by the Department of the Treasury during the period of underpayment.