When, by reason of disturbed or disrupted economic conditions due to the prosecution of war or due to the operation of laws, rules or regulations of governmental authorities, whether enacted, passed, promulgated or issued under or due to the emergency or necessities of war or national defense, the contracting or purchasing by the district, for any one or more of the purposes mentioned in sections 70-637 to 70-640, is so restricted, prohibited, limited, allocated, regulated, rationed, or otherwise controlled, that the letting of contracts therefor, pursuant to the requirements of said sections, is legally or physically impossible or impractical, the provisions of said sections shall not apply to such contracts or purchases.