At the time of levying the annual assessment, there shall be added by the board to the annual assessment on land within any improvement district or distribution district within any district:
(a) The installment, if any, for which the land is liable in that year by reason of an improvement district or distribution district assessment levied pursuant to Chapter 1 or 2 of Part 7 or Part 6.5 of this division.
(b) An amount which the board may consider necessary for the operation of the works in or for the improvement district or distribution district for the ensuing year.