What is Inch Of Water?

Legal Definition
The unit for the measurement of a volume of water or of hydraulic power, being the quantity of water which, under a given constant head or pressure, will escape through an orifice one inch square (or a circular orifice having a diameter of one inch) in a vertical plane. Jackson Milling Co. v. Cbandos, 82 Wis. 437, 52 N. W. 759
-- Black's Law Dictionary