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Chunmiao Zheng, Software Editor Ground water modeling requires a wide range of models for different types of problems and applications. FEFLOW is an advanced Finite-Element subsurface FLOW and transport modeling system with an extensive list of functionalities, including variably saturated flow, variable fluid density mass and heat transport, and multispecies reactive transport. It is a proprietary code and not freely available; it supports an impressive array of features of interest in subsurface flow and transport and is well documented, in terms of both peer-reviewed papers in the scientific literature and a comprehensive set of manuals and white papers. The program has been under development since 1979 by the Institute for Water Resources Planning and Systems Research Inc. (WASY GmbH) of Berlin, Germany, which has recently become a part of DHI Group. For more information, see http://www.wasy.de/english/products/feflow/index.html. FEFLOW v 5.3 (patch 1) was reviewed and tested by two reviewers. One reviewer used two Windows XP computers (a desktop P4 3.2 GHz and a laptop Dual Core 2.33 GHz) each with 2 GB RAM; the other reviewer used a laptop P4 3.0 GHz with 1.5 GB RAM. One of the reviewers has also had experience executing FEFLOW on Linux platforms. FEFLOW was downloaded and installed directly from the WASY Web site in Germany—a process that took approximately 10 to 30 min depending on Internet connection speed. A handsomely packaged box is also available as the delivery method. After installing the full package, the contents of the WASY folder containing FEFLOW amounted to 450 MB of disk space, including demo and help folders. FEFLOW requires a license key and is capable of running in licensed stand-alone mode or by connection to a remote license server. FEFLOW’s graphics are X-Windows based, so the installation provided the user with an X server (Hummingbird Exceed). Other important tools installed were FEFLOW Explorer 2.0, an OpenGL-based data explorer, and WGEO 5.0, an image georeferencing tool. FEFLOW can be modified and removed using a standard Windows installation manager interface. Stability of the Windows computers was not affected by the presence of FEFLOW. Execution can lead to significant resource demands on the computer host and concomitant slowdowns of other applications, but terminations are uncommon even during execution times of 8 to 12 h or more. The software performance was good. It was able to solve a range of classical benchmark problems readily. Both triangular and rectangular finite elements are supported, with a range of direct and indirect solver options, including algebraic multigrid techniques (Stüben 2001). FEFLOW autodetects the number of available processors and invokes a multithreaded parallel mode accordingly, but the user can specify the number of threads to use if desired. Numerical stability is usually good, but complex problems involving strong density coupling and unsaturated flow can present convergence problems. The graphical user interface (GUI) contains many features of use to the modeler, including mouse-driven mesh construction, boundary condition specification, and property editing. It is literally possible to build a georeferenced flow and transport simulation from existing spatial data sets without having to use a text editor. One reviewer, who is an experienced ground water modeler but new to FEFLOW, tested a portion of FEFLOW’s functionality related to ground water flow and contaminant transport. Throughout the testing, the program performed as advertised. While the reviewer found the GUI not particularly intuitive, the software appeared stable (the program did not crash once)—with warning messages appearing anytime the reviewer did something that might otherwise crash the program. While running FEFLOW to set up and review simulations and results, there were no apparent hiccups in any other programs running concurrently. It took the reviewer about 2 d to complete the demonstration exercise, after which, the reviewer felt confident that he could dive in and set up his own ground water flow and contaminant transport models. FEFLOW supports an array of data import and export filters. It has its own internal formats for mesh and “Finite Element Problem” data, but it can also read and reconstruct simulation files from SWS (Surface Water Modeling System) and GMS (Groundwater Modeling System) produced by Environmental Modeling Systems Inc. For spatial meshing and gridding, FEFLOW can import a range of formats including AutoCAD DXF, ESRI shapefiles plus a variety of simple ASCII formats, and many bitmap formats for gridding and georeferencing operations. This makes it easy to construct complex grids aligned and shaped with geographic data sets (Figure 1); the combination of FEFLOW and a GIS package (e.g., ArcMap) forms a powerful tool for spatial analysis of ground water problems. Extracting data can be tricky sometimes, especially when trying to export nodal values of material properties or fluid velocities in formats that can be read into other packages. However, one reviewer succeeded in extracting a vertical slice through a conductivity distribution from a three-dimensional (3D) FEFLOW model and inserting this into a MODFLOW model without too much effort. The native FEFLOW file format for problem definitions supports both ASCII and binary forms; the ASCII format is larger but allows FEFLOW files to be generated and/or modified programmatically by external applications. A screen capture of a program window for FEFLOW showing grid refinement around four wells on a base map. FEFLOW has three graphical output tools: an internal viewer, FFPLOT, and FEFLOW Explorer. The internal viewer is functional and performs a range of two-dimensional (2D) contouring and particle tracking functions, plus data export as points, ESRI shapefiles and time series, and Golden Software’s GRD files. The internal viewer also supports a 3D mode, but the quality and flexibility of the graphics in this mode are not high. FEPLOT is a routine tool for constructing annotated map-style graphics from FEFLOW runs—but an advanced user may prefer to use GIS and drafting tools for this task instead. The graphics tool that really impressed the reviewers is FEFLOW Explorer—it is here that the user can really see into the simulation results. Explorer allows the user to construct complex 3D animations, fly-throughs, and renderings of the FEFLOW grid and solution data together with superimposed GIS data and georeferenced bitmaps—the output is presentation quality (Figure 2). Advanced visualization with FEFLOW Explorer 2.0 showing contours of a simulated 3D head distribution. FEFLOW comes with a variety of documents available on-line and as part of the installation package. As well, there is a Web forum to discuss FEFLOW modeling topics with other users. A demonstration exercise and associated tutorial are provided to familiarize new users with the most commonly used features of FEFLOW. The demonstration exercise guides the user through a typical simulation problem: from importing base maps, mesh generation, and boundary condition assignment to solvers and processing of results. The documentation is easy to follow with plenty of screenshots to keep users on the right track. Help is readily accessible throughout a simulation exercise with convenient “Help” buttons available on most GUI forms (F1 can be pressed at anytime as well). The support staff are friendly and expert. WASY maintains a public bug list and issues documented patches frequently. These are available for download and installation directly. One reviewer corresponded with an experienced FEFLOW user who commented that “support from WASY is very good… The manual and the on-line help are great, but they do not answer all questions or replace the knowledge of their staff.” The reviewers were most impressed by FEFLOW’s capability to handle saturated/unsaturated flow, and transport and reaction simultaneously in the one mouse-driven GUI package. The fully-3D finite-element nature of FEFLOW is a significant advantage for complex ground water modeling applications. This is all backed up by credible peer-reviewed journal papers on the various methods and solvers, so that users can have confidence in density-coupled simulations, dispersion modeling, unsaturated flows, and in reaction and sorption kinetics. FEFLOW handles multiple free surfaces, discrete fractures and has convenient tools for mapping material properties and boundary condition constraints based on spatial domains. FEFLOW contains the excellent triangulation algorithm by Shewchuk (2002) for fast and optimal gridding, plus has PEST (Doherty 2002) support built-in, and includes fluid age and thermal conduction calculations. There is also a full developer application programming interface that allows users to add custom code modules directly into the FEFLOW simulator. Being able to work on other tasks while FEFLOW was solving a CPU-intensive transport simulation was helpful as the users could run FEFLOW on one PC without having to switch to another one to do other tasks. Well-documented file formats are important because in the event that the users cannot do something they would like in FEFLOW or FEFLOW Explorer or with the interface manager, they can still write their own utilities for pre- and postprocessing. One reviewer particularly liked that he could tackle his own flow and transport simulation using FEFLOW after only a few days of use. The program is stable and gives users the ability to “play” with the program and push buttons to learn about it without fear of crashing the program. The error handling in the program is excellent and is so often lacking in other programs. There are several things that could be better in FEFLOW. First, the GUI, while perfectly functional, is starting to show its age. The reviewer who is new to FEFLOW does not like the “look” of the GUI because it has an outdated blocky feel and does not follow the standard Windows “look.” This does not affect the ability of FEFLOW to carry out the user’s wishes, but it can make some things more obscure or laborious than need be. The definition of local grid coordinates, coordinate origin, and problem measure is confusing. Interruption of simulation time-stepping can be difficult, especially for large complex problems where the mouse status is not polled for long periods during matrix inversions. Color representation in the solver window sometimes collapses to a dithered mode, unnecessarily. By default, FEFLOW renders contour maps over the whole domain for each time step. On some platforms, this can slow down time-stepping performance. It is possible to run FEFLOW in “batch mode,” thereby avoiding repetitive renderings, but it is not clear how to start batch mode on some platforms. Importing/exporting data for 3D models can be laborious since these operations need to be done for each layer at a time. Second, while the FEFLOW multispecies reaction model is more than useful, it would be nice to have a greater range of example applications to choose from so that users could see how to relate the example problem settings to the complicated reaction formalism discussed in the FEFLOW white papers. Without this, the reaction model will likely be underused. Third, FEFLOW incorporates an adaptive layering technique that allows slices between layers in a 3D model to move up and down to minimize solution error. While the user is able to fix slices at will (e.g., to stratigraphic interfaces), it is often useful to include extra “moveable” slices for improved vertical resolution for head gradients, tracer fronts, and so on. The difficulty is that knowledge of the vertical location of a moveable slice as a function of time and space throughout the simulation is not easily accessible. This can complicate the process of model calibration and solution interpretation. Finally, a comprehensive search option is not available in the help documentation but would be useful as the help buttons did not always connect the user with the information sought. The help documentation is structured or layered in the same manner as the GUI, which may cause some difficulty for novice users who are still getting used to the GUI. For example, while working on the demonstration exercise, one reviewer added too many constant-head boundary condition cells along one edge. The documentation did not instruct him on what to do in this eventuality, nor did the help associated with the form tell him how to delete a boundary node. Deleting such a node is trivial in the end, and it took only a little time to figure out through trial and error, but the lack of help was a little frustrating. Our impression of FEFLOW is that it is a stable and credible ground water simulation code well suited for sophisticated users. The support is excellent and conscientious. The demonstration exercise and tutorials are clear and easy to follow with plenty of screenshots, making it easy to learn the basics of the interface in a short time. FEFLOW could provide better 2D charting and plotting support, but the 3D Explorer is first class. While a search feature would be helpful for the help documentation, the program is stable enough that users can click around and work with the GUI until they find what they need. All in all, FEFLOW is a well-documented and powerful GUI-based tool for professional subsurface hydrology simulations. The reviewers ranked the software’s capability, reliability, ease of use, and technical support on a scale of 1 (worst) to 5 (best). The following rankings are the average of three sets of scores from both reviewers and the editor: Capability: 4.7 Reliability: 4 Ease of use: 4 Technical support: 5 For software download and pricing information, visit the Web site of WASY GmbH in Berlin, Germany: http://www.wasy.de/english/produkte/FEFLOW/download.html. The goal of Software Spotlight is to help readers identify well-written, intuitive, and useful software. Independent reviewers from government, industry, and academia try out full working versions of software packages and provide readers with a concise summary of their experiences and opinions regarding the capability, stability, and ease of use of these packages. Chunmiao Zheng can be reached at the University of Alabama, Department of Geological Sciences, Box 870338, Tuscaloosa, AL 35487; [email protected].