Because groundwater systems are largely hidden from sight, groundwater simulation models are used to infer the properties of groundwater systems and to simulate the flow and storage of water within the subsurface under variable conditions, including natural conditions and human-made influences such as pumping. The most widely
used groundwater simulation model is MODFLOW, produced by the USGS, which is as much of a standard in the groundwater field as ArcGIS is a standard for geographic information systems