In managing and controlling a water system, the board of supervisors has authority, upon such terms and conditions as the board shall prescribe, to establish the rate or rates for the use of water furnished or provided by the county. All revenues derived from such charge, are available solely for payment of the cost of maintaining the water system and the payment of principal and interest of bonds as they become due. The board may from time to time, out of any surplus of such revenues, transfer to the appropriate funds of the county such sums as in its judgment may be available for payment of principal and interest of county bonds issued pursuant to the provisions of this article.