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versions of the MODFLOW code. This makes use of custom model packages, such as the state <br />modification to the ET and stream packages, difficult to implement. These packages are <br />eliminated from additional consideration for the SPDSS for both of these reasons, and since <br />these packages provide no compelling advantage over the other packages under consideration. <br />Each of the interface packages selected for consideration in the evaluation are described in more <br />detail in following sections. A general description of each package is provided, along with a <br />summary of approaches to define a conceptual model that can be extended to a numerical model. <br />This description focuses on components that are significant to the SPDSS. These packages will <br />be screened against requirements defined in Task 50.2 in order to recommend a selection in Task <br />50.3. <br />3.2.1 GMS <br />The Department of Defense Groundwater Modeling System (GMS) has been developed at the <br />Environmental Modeling Research Laboratory at Brigham Young University in cooperation with <br />the US Army Corps of Engineers Waterways Experiment Station. The data centered modeling <br />approach used by the State for the RGDSS has been implemented using GMS (Version 3.1) as <br />one of the central components. The conceptual model approach to model definition was <br />pioneered by GMS, and the current version (5.1) includes tools that allow rapid creation of a <br />numerical model from a conceptual model. This interface package has a large and active user <br />base and is the preferred interface package for Corps of Engineer projects. Development of this <br />package is ongoing, with significant upgrades released periodically. <br />A conceptual model in GMS is defined in map coverages that are similar to those in a GIS. <br />These coverages can include points, lines with vertices and nodes, and polygons. Data can be <br />associated with these entities to represent parameters such as hydraulic conductivity <br />distributions. GMS is able to directly import and use ESRI shape files for many features, in <br />addition to providing robust tools for creating and editing features within the package. GMS also <br />publishes the specification for internal file types, facilitating use of external programs to move <br />information from external GIS form to internal GMS form. Version 5.1 of GMS has greatly <br />eased migration of GIS information into the conceptual model format. GMS can read ESRI files <br />and has full ArcObj ects capability incorporated into the interface. If an ArcGIS license resides <br />on the same machine, GMS can directly access the most of the capabilities of ArcGIS from <br />within GMS, including extensive map layout formatting. Import tools allow mapping of GIS <br />fields to a data type used by GMS. Version 5.1 will only import steady-state data using the GIS <br />interface, however, the beta version 6.0 appears to have extended this capability to include <br />importation of transient data. GMS also has extensive and sophisticated capabilities for <br />interpolating randomly distributed data values onto a model grid. Complex stratigraphy can be <br />represented as 3-dimensional solids that can be used for three-dimensional visualization and for <br />model building. <br />A complete conceptual model can be built within GMS that is independent of the final model <br />grid. Conceptual model information resides largely in GIS map layers internal to GMS. A <br />source/sink layer type is used to define model elements such as wells, drains, and streams, along <br />12 <br />