For many years there has been a need for redefinition or more precise definition of certain ground-water terms used in publications by members of the U.S . Geological Survey. Another problem has been the expression of the coefficient of permeability (herein redefined as hydraulic conductivity) and the coefficient of transmissibility (herein redefined as transmissivity) in inconsistent units that included the U.S . gallon, the foot, and in some expressions, the mile. Such inconsistent units and the attendant confusing numerical conversion factors used in flow equations, such as 527.7, 264, and 114.6, makes it unnecessarily difficult for hydrologists, especially in foreign countries, to follow and use our published results. Because of this it is advisable that basic ground-water flow equations in publications by members of the Geological Survey contain only the pure dimensionless numbers that result from the derivation of the equations, such as 2, 2.30, e, π , and 4, and that numerical results having dimensions should be expressed in consistent units of measurement.

If in the solution of problems it is necessary or desirable to use inconsistent units, suitable conversion factors should be included so that the result is expressed in consistent units of length and time. For example, if a discharge rate is given in U.S. gallons per minute, conversion factors such as 7.48 gal ft^-3 and 1,440 min day^-1 should be included. Many hydrologists in English-speaking countries including the United States are already using consistent units in the fps, cgs, or mks systems of measurement.

To meet the growing need for consistency, J. T. Callahan, then acting chief, Ground Water Branch, in a memoradum of October 21, 1965, appointed the Committee on Redefinition of Ground-Water Terms.