All revenues derived from the operation of the water department, from the sale of water works bonds, or from the sale by the water board of any of its property, shall be applied by the water board solely for the payment of expenses incurred in the exercise of authority or in the discharge of duties imposed on the board by this charter or by general law, for the payment of the cost of operating, improving, and managing the water system, and for the payment of principal, interest and sinking fund requirements necessary for the discharge of any water works bonds heretofore or hereafter issued.