Whenever the board of directors for any intercommunity water or natural gas supply association determines, by resolution, that interest or necessity demands the acquisition, construction, repair, extension, improvement or betterment of any water system or natural gas transmission and distribution system, the association is hereby authorized to make and issue revenue bonds, payable solely out of the net income to be derived from the operation of such system, and to pledge irrevocably such income to the payment of the bonds, the proceeds of the bonds to be used solely for the purchasing, acquiring, constructing and making of necessary improvements, extension, repairs and betterments of the systems or for the purchase and acquiring of wells, cisterns, reservoirs or other sources of supply, rights-of-way, pipelines and pumping plants or other machinery necessary for the operation thereof and the land and real estate upon which the same are situated or to be situated.