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).
Scope of the book
The book is divided into nine chapters. The first three chapters introduce the subject, describe the core framework data model, and present an exposition of how to represent 3D objects in ArcGIS. The next three chapters present components of the groundwater data model for geological mapping, wells and aquifers, and 3D hydrostratigraphy. The final three chapters describe time series, groundwater modeling, and implementation of the groundwater data model in ArcGIS. It is important to understand some of the limitations of this book—we are describing an information model for groundwater data. This is not a book about groundwater simulation modeling. Arc Hydro Groundwater can be used to help create groundwater simulation models and to store the results of groundwater simulations, but Arc Hydro Groundwater is not itself a groundwater simulation model. Likewise, although we devote considerable attention to hydrogeology, Arc Hydro Groundwater is not intended to capture all the detailed cartographic aspects of geologic and hydrogeologic mapping. Our focus is on the groundwater environment and on the properties of the water flowing through that environment.