Each district by its board each year within 15 days after the close of its session as a board of equalization shall levy an annual assessment upon the land within the district in an amount sufficient to raise all of the following:
(a) Interest due or that will become due on all outstanding bonds of the district and interest which the board believes will become due on district bonds authorized but not sold, all respectively before the close of the next ensuing calendar year.
(b) Principal of all bonds of the district that have matured or that will mature before the close of the next ensuing calendar year.
To the extent that provision is otherwise made as permitted by law for the payment of bond principal and interest, levies for principal and interest pursuant to this section need not be made.