The board may from time to time make appropriations from individual public utility funds for the following purposes:
(a) The payment of operating expenses of the public utility and all or such portion of the general salary and expense fund of the district as the board determines is justly apportionable thereto.
(b) Repairs and reconstruction.
(c) Payment of interest and sinking fund on the bonds or indebtedness incurred or assumed for acquisition, construction, or completion of the public utility.
(d) Extensions and improvements.
(e) A reserve fund.