The standard of measurement of the flow of water shall be the cubic foot per second of time; the standard of measurement of the volume of water shall be the acre-foot, being the amount of water upon an acre covered one foot deep, equivalent to forty-three thousand five hundred and sixty cubic feet. The miner’s inch shall be regarded as one- fiftieth of a cubic foot per second in all cases, except when some other equivalent of the cubic foot per second has been specifically stated by contract, or has been established by actual measurement or use.