Bonds payable from such special fund shall not be an obligation of the reclamation district and they shall state on their face that they are payable solely from a special fund derived from a certain fixed proportion (naming it) of the gross income derived by the district from the sale, rent or lease of water or power, as the case may be, and such fixed proportion of such gross income shall be irrevocably devoted to the payment of such bonds until the same are fully paid.