Tex. Water Code § 60.110: REVENUE SET ASIDE FOR SINKING FUND.

(a) A resolution or an order authorizing the issuance of obligations under this subchapter shall provide that the revenue from which the obligations are to be paid shall, from month to month as it accrues and is received, be placed in a sinking fund and disbursed in the manner provided in Section 60.109 of this code.

(b) In determining the amount of revenue to be set aside, the commission shall provide that the amount to be set aside and paid into the fund in any year shall not be less than a fixed sum which shall be at least sufficient to provide for the payment of the principal of and interest on all obligations which mature and become payable each year and shall include a surplus or margin of 10 percent in excess of that amount.

Acts 1971, 62nd Leg., p. 110, ch. 58, Sec. 1, eff. Aug. 30, 1971.