Tex. Local Government Code § 324.096

(a) The expense of operation and maintenance of a facility of which the revenues are pledged to the payment of bonds is a first lien on and charge against the income of the facility. As long as any of the bonds or interest remain outstanding, the board shall charge or require the payment of fees for the use of the facilities, except drinking water or sanitary facilities. Fees must be equal and uniform within classes defined by the board and must be in amounts that will yield revenues at least sufficient to pay the expenses of operation and maintenance and to make the payment prescribed in the bond resolution for debt service. "Debt service," as defined in the bond resolution, may include: the payment of principal and interest as each matures, the establishment and maintenance of funds for extensions and improvements, an operating reserve, and an interest and sinking fund reserve.

(b) Except as provided by a contract entered into by the board, the board may determine the rate of fees charged for the use, operation, or lease of facilities, services, or equipment of the district. The board shall fix the fees in amounts sufficient to comply with the covenants in the bond resolution.