All water improvement districts in any and all of the cities of the first and second class of the State of Arkansas that may be designated as sites for United States Veterans' Hospitals and their respective boards of managers and boards of improvements, or any of them, are authorized to extend, develop, and improve water plants and systems, and to acquire real estate as to them may seem necessary and advisable for this purpose. They may issue their certain negotiable promissory obligations in payment thereof, payable out of the surplus, from year to year, of the revenue arising from the operation of any such plant of the district, after all operating expenses have been paid or provided for, and to sell and negotiate such obligations for the purpose of securing cash funds with which to so extend, develop, and improve the plant and to acquire real estate.