Section 13(b)(12) of the Fair Labor Standards Act exempts from the overtime provisions of section 7 any employee employed in agriculture or in connection with the operation or maintenance of ditches, canals, reservoirs, or waterways, not owned or operated for profit, or operated on a sharecrop basis, and which are used exclusively for supply and storing of water, at least 90 percent of which was ultimately delivered for agricultural purposes during the preceding calendar year.