In the USA the unit gpd/sqft (gallon per day per square foot) is often used to measure the hydraulic conductivity k, and the specific discharge q. In Europe the standard unit is m/s (meter per second), following the unification initiated by Napoleon around 1800. European scientists consider this to be more convenient, but the unit gpd/sqft has the advantage that the magnitude of a value is easier to imagine. Furthermore, European engineers may be tempted to think that the specific discharge is a velocity, because it is expressed in m/s. However, it is not, as the average velocity is v = q/n, where n is the porosity. American engineers will not have that idea, because they are used to express a velocity in mph, and that seems to be quite different from gpd/sqft. What is the relation between the two units?