When building a MODFLOW model, all of the input parameters must be entered using a consistent set of units. For example, if the grid is created using length units of meters, and the stress periods are defined as days, all hydraulic conductivity values must be entered in units of meters/day.
Unit management can be simplified using the Units dialog accessed from the Units button the Global Options\Basic Package dialog. The Units dialog is used to define a standard unit for length, time, mass, force, and concentration. These units are then used to display a units label next to all of the edit fields in the MODFLOW, MODPATH, and MT3DMS dialogs. These labels do not alter values that are entered in the edit fields. They are simply placed as a reference to remind you what units have been chosen in the Basic Package Dialog.
Related Links:
Building a MODFLOW Model
MODFLOW Interface