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.