The standard of measurement for flowing water, both for determining the flow of water in natural streams and for the purpose of distributing it therefrom when appropriations have been made for direct flow, shall be one cubic foot per second of time. The standard of measurement of the volume of water shall be one acre-foot, equivalent to forty-three thousand five hundred sixty cubic feet, and when water is stored in any natural or artificial reservoir, this standard shall be used for determining the capacity of storage reservoirs, the amount stored, and the amount used therefrom, except that for public water supplier appropriations, the standard of measurement may be in terms of gallons.