Money deposited in the water fund shall be used for the following purposes only; for the operation and maintenance of the water system of the city; for the making of additions and betterments to the extensions of the water system of the city; and for the payment of principal and interest of revenue bonds against the water department or transferring from the water fund of any moneys therein for the purpose of meeting sinking fund requirements for the retirement of revenue bonds issued against the water department. (Ord. 82, § 3, 1938.)