Whenever a public agency shall have executed a contract under this act and the payments thereunder are to be made either wholly or partly from the revenues of the public agency's systems, or any part thereof, or a combination of such systems, the duty is hereby imposed on the public agency to establish and maintain and from time to time to adjust the rate or fees charged by the public agency for the services of such systems, so that the revenues therefrom, together with any taxes and special assessments levied in support thereof, will be sufficient at all times to pay:
Miss. Code § 49-17-749