GFLOW is a highly-efficient stepwise groundwater flow modeling system. It is a Windows 95/98/2000/NT program based on the analytic element method. It models steady-state flow in a single heterogeneous aquifer using the Dupuit-Forchheimer assumption. While GFLOW supports some local transient and three-dimensional flow modeling, it is particularly suitable for modeling regional horizontal flow. To facilitate detailed local flow modeling, GFLOW supports a MODFLOW-extract option to automatically generate MODFLOW files in a user-defined area with aquifer properties and boundary conditions provided by the GFLOW analytic element model. GFLOW also supports conjunctive surface water and groundwater modeling using stream networks with calculated baseflow.

The thickness of the lines of the blue stream network is proportional to the streamflow rate. Finite difference grid dialog box is for setting up a MODFLOW model that includes all features of the analytic element model GFLOW.

Uses of GFLOW GFLOW is the most efficient groundwater flow modeling system on the market. It facilitates a stepwise modeling approach, allowing you to quickly set up an initial model and painlessly build up complexity as your understanding of the groundwater regime grows. For this purpose, GFLOW has very powerful elements: e.g., line-sinks with bottom resistance, drains, fully or partially penetrating wells, domains with differing hydraulic conductivity, bottom elevation, porosity and recharge. It also supports fully or partially penetrating slurry walls that may be open or closed, three-dimensional pathline tracing, flux inspection lines, stream networks with baseflow, overlandflow and streamflow, etc. What truly sets it apart are conjunctive surface water and groundwater solutions (stream networks) and instant extraction of a MODFLOW model. The stream network feature is similar to the MODFLOW Streamflow Package but much easier to implement. Streamflows offer valuable calibration targets in addition to the head targets commonly used in groundwater flow models. GFLOW lets you design a MODFLOW model inside the GFLOW graphical user interface of all or part of the model domain. The MODFLOW model will inherit all of the model properties and will be preconditioned with the GFLOW solution. This feature is often used to create a local detailed MODFLOW model with boundary conditions on the grid perimeter derived from the regional analytic element model. GFLOW is based on the DOS program GFLOW, which has been used in academia, government agencies and consulting firms for more than five years. The native windows program feels just like your other windows applications making it easy to use. GFLOW is an analytic element model similar in design to the US EPA program WhAEM, but has all the power of other commercial analytic element programs. And more! GFLOW Stepwise Modeling Perhaps the most practical advantage of the analytic element method is its operational efficiency. In the absence of a mesh or element network, the hydrologist is concerned only with entering hydrologic features in the model. Representing streams by strings of straight line elements and lakes by polygons is a rather intuitive task. Also, for initial modeling runs, a limited set of surface water features may be introduced. Later, when insight into the groundwater flow regime increases, more data may be added to locally refine the modeling. This stepwise modeling is not new. For example, Ward applied what he calls a "telescopic mesh refinement modeling approach" to the Chem-Dyne hazardous waste site in southwestern Ohio (Ward et al., 1987). However, Ward had to use three different computer models for the three different scales at which he was modeling. Conditions on the grid boundary of the "local scale" were obtained from the "regional scale" modeling results, while similarly the conditions on the grid boundary of the "site scale" were obtained from the "local scale" modeling results. In contrast, the analytic element method allows these different scales to be treated within the same model by locally refining the input data, thus avoiding transfer of conditions along artificial boundaries from one model into the other. When necessary, even three-dimensional flow features can be included. While uniquely suitable for groundwater flow modeling at different scales, current generation analytic element models have some limitations. For instance, both transient flow and three-dimensional flow are only partially implemented in analytic element models. Gradually varying aquifer properties cannot be represented in analytic element models. GFLOW also does not support multi-aquifer flow. Depending on circumstances and on the purpose of the modeling, however, these phenomena may be important. The GFLOW graphical user interface allows the user to carry the modeling beyond the limitations of the analytic element method. This is done by extracting a MODFLOW model out of a GFLOW model, transferring the internal boundaries (streams, wells, lakes) and the domains with differing aquifer properties as defined in GFLOW directly to the finite difference grid. The GFLOW steady-state groundwater flow solution is used to define either head or discharge specified conditions on the grid perimeter, and to precondition the MODFLOW solution procedure with heads at each cell center from the GFLOW solution. This procedure of extracting a MODFLOW model is quick and easy using the "grid" menu option in GFLOW. In this manner, the stepwise modeling procedure within the analytic element model is extended to modeling groundwater flow with MODFLOW and, when needed, contaminant transport with e.g., MT3D. Analytic Element Method The analytic element method was developed at the end of the seventies by Otto Strack at the University of Minnesota (Strack and Haitjema, 1981a). There are two books about the analytic element method. "Groundwater Mechanics" by O. D. L. Strack, 1989, contains detailed mathematical descriptions of the analytic elements and their numerical implementation. "Analytic Element Modeling of Groundwater Flow" by H. M. Haitjema, 1995, provides the basic theoretical framework for the analytic element method and focusses on its use. This new method avoids the discretization of a groundwater flow domain by grids or element networks. Instead, only the surface-water features in the domain are discretized, broken up in sections, and entered into the model as input data. Each of these stream sections or lake sections are represented by closed form analytic solutions: the analytic elements. The comprehensive solution to a complex, regional groundwater flow problem is obtained by superposition of all, a few hundred, analytic elements in the model. Traditionally, superposition of analytic functions was considered to be limited to homogeneous aquifers of constant transmissivity. However, by formulating the groundwater flow problem in terms of appropriately chosen discharge potentials, rather than piezometric heads, the analytic element method becomes applicable to both confined and unconfined flow conditions as well as to heterogeneous aquifers (Strack and Haitjema, 1981b). The analytic elements are chosen to best represent certain hydrologic features. For instance, stream sections and lake boundaries are represented by line sinks, small lakes or wetlands may be represented by areal sink distributions. Areal recharge is modeled by areal source distributions (areal sinks with a negative strength). Streams and lakes that are not fully connected to the aquifer are modeled by line sinks or area sinks with a bottom resistance. Discontinuities in aquifer thickness or hydraulic conductivity are modeled by use of line doublets (double layers). Specialized analytic elements may be used for special features such as drains, cracks, slurry walls, etc. Locally three-dimensional solutions may be added such as a partially penetrating well (Haitjema, 1985). GFLOW Output Graphical output can be sent directly to the printer. GFLOW also writes DXF files (AutoCAD) and BLN files (Surfer) with selected graphical data (analytic elements, contours, pathlines, and wellhead protection areas). GFLOW uses background maps ("base maps") in a special binary format (.bbm) for fast redraws. These maps can be created from DXF files, USGS DLG or STDS files. The US EPA is developing a national coverage of the .bbm files for its public domain program WhAEM. These .bbm maps can be selected using a map browser that is part of the WhAEM and GFLOW program. GFLOW is UCODE and PEST ready in that it can write ASCII input files for the GFLOW Solver which can be used by UCODE and PEST to run the Solver for parameter estimation purposes. GFLOW Hardware Requirements 32-bit Windows operating system (95/98/2000/NT) 15 MB of hard disk space 16 MB RAM