In any instance where the district is using, selling or leasing water for beneficial purposes or furnishing other service under the provisions of this chapter and there is reasonable certainty of a permanent fixed income from this source, the district board, upon previous written approval of the county legislative authority of the county within which the major portion of the district is situated, shall have authority to pledge the revenues derived from a fixed proportion of the gross income thus obtained and to issue bonds of the district payable from the utility bond fund and to sell the same to raise money for district purposes.