Whenever the legislative body determines that it is desirable for the general welfare and benefit of the people, and for the interests of the local agency, it may incur an indebtedness evidenced by negotiable bonds for the purpose of acquiring land to be conveyed to the United States for federal purposes in consideration of the benefits to be derived by the local agency from the use of the lands by the United States for such purposes. The bonds may be issued in an amount not exceeding, together with all existing bonded indebtedness of the local agency, 5 percent of its taxable property as shown by the last equalized assessment of the local agency.