(a) The commission shall pay from revenue raised under § 60.103 of this code:
(1) all expenses necessary to the operation and maintenance of the improvements and facilities, including the cost of the acquisition of properties and materials necessary to maintain the improvements and facilities in good condition and operate them efficiently, the wages and salaries paid to the employees of the district, and other expenses necessary to the efficient operation of the improvements and facilities;
(2) the annual or semiannual interest on any obligations issued under this subchapter and payable out of the revenue of the improvements and facilities; and
(3) the amount required to be paid annually into the sinking fund for the payment of any obligations issued under this subchapter and payable out of the revenue of the improvements and facilities.
(b) No expenses other than those authorized by Subsection (a) of this section may be paid from the revenue of the improvements and facilities as long as the principal and interest on any obligations issued under this subchapter remain outstanding and unpaid. Any revenue received in excess of that required for the purposes stated in Subsection (a) of this section may be used by the commission to pay the cost of improvements and replacements which are not listed and may establish a depreciation fund.