Groundwater modeling
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. The various data arrays in a MODFLOW model are like the pages in a groundwater system book. As you view them one by one, you see the spatial extent and physical properties of the aquifer, such as its thickness, porosity, and hydraulic conductivity (figure 1.8). You see the pumping stresses, flow patterns, and recharge and discharge volumes, and you see the piezometric head map that depicts the levels of water within the different layers of the groundwater system. The Arc Hydro Groundwater tools include a special extension called MODFLOW Analyst that enables MODFLOW descriptions of groundwater systems to be viewed and mapped in ArcGIS, thus enabling a much larger audience to understand the behavior of groundwater systems simulated with MODFLOW (see chapter 8).