Multi-Layered Finite-Element Aquifer Flow Model
AQUIFEM-N is a quasi-three-dimensional groundwater flow model with additional capabilities for simulating contaminant transport in two dimensions.
The most common application of AQUIFEM-N is for simulating groundwater flow in regional aquifers, i.e., two-dimensional flow in plan. Such aquifers can be confined or unconfined, or changing status from one to the other. AQUIFEM-N allows transmissivity to vary as the water-table elevation varies, or alternatively allows the user to choose a linearized solution in which transmissivity is based on an approximate saturated thickness.
Groundwater flow in systems of aquifers is characterized by the fact that flow is almost horizontal within aquifers and almost vertical through separating layers known as aquitards. This is well-suited to AQUIFEM-N's quasi-three-dimensional modeling approach which also allows for layers bifurcating, pinching out to zero thickness and drying out.
Three-dimensional flow in a single aquifer can be handled to a limited extent by using leakage coefficients between layers to represent vertical hydraulic conductivities divided by the separation of the layers. The number of layers is limited to about 10 in typical applications.
Contaminant transport in a regional aquifer can be simulated using AQUIFEM-N but this requires an assumption or the knowledge that the contaminant of interest is well mixed over the thickness of the aquifer. AQUIFEM-N does not allow the simulation of quasi-three-dimensional transport, because of the ease with which artificial (non-physical) spreading would occur between aquifers.
AQUIFEM-N can predict steady flow in a vertical section, either in a plane vertical slice or in an axi-symmetric wedge. AQUIFEM-N includes a capability for iteratively moving the grid to match the phreatic boundary condition at the water table, e.g., inside an earth dam. Contaminant transport can also be simulated in a vertical section.
AQUIFEM-N has internal memory management capabilities such that very large groundwater flow problems can be solved using disk storage instead of memory. Transport problems must be solved in available memory, however.
AQUIFEM-N is supplied together with a number of ancillary programs for grid generation and graphical output. Program GENOPT has a large number of options for generating triangular finite-element grids, while Program PLOT has options for displaying grids, contour plots, velocity vectors and x-y plots such as times series or cross-sections. Graphical output can be displayed to the screen, or saved as files containing Hewlett Packard Graphics Language (HPGL) or PostScript. Such files can be sent to a variety of plotters and laser printers.
AQUIFEM-N - THE MODELING PROCESS
AQUIFEM-N is a particular implementation of a particular numerical method to solve particular balance equations for flow and solute transport in aquifers. The most important part of the modeling process, however, is that of conceptual modeling, of deciding how to represent a physical system with the model, such that all the important features of a problem are adequately included.
AQUIFEM-N and its documentation are logically structured to encourage the user to systematically select the model geometry, assign aquifer properties, choose physically reasonable boundary conditions, choose a solution method and run the model.
AQUIFEM-N INPUT REQUIREMENTS
AQUIFEM-N requires the user to create one or more ASCII data files, either by hand (using a text editor) or using other software to facilitate this process.
The first step is to develop a finite-element grid. Program GENOPT provides great flexibility in grid design, and three methods for optimally renumbering the nodes of the finite-element grid to reduce storage and execution time.
Aquifer properties include the aquifer bottom elevation, aquifer thickness, hydraulic conductivities or transmissivities (including arbitrary orientation to any anisotropy), specific yield, aquifer storage coefficient and leakage coefficient through an adjacent aquitard. The AQUIFEM-N user's manual explains which properties are needed for which kinds of situations. Properties can be spatially uniform throughout an aquifer or model layer, uniform within zones of nodes or elements, or unique for any or all nodes or elements.
Boundary conditions can be of three kinds: prescribed head, prescribed flux or mixed. In all cases, the boundary conditions can vary in space and time. The AQUIFEM-N user's manual provides advice on how to use each type of boundary condition, but also on how to represent different types of physical features. AQUIFEM-N can represent rivers, streams, lakes, ponds, pumping wells, flowing artesian wells, distributed recharge and evapotranspiration, leakage to or from adjacent aquifers, and many other features. Special features include rising water table nodes which can be used to simulate streams, where the water table rises to the land surface, and excavations, where the land surface is lowered to meet the water table.
AQUIFEM-N allows a number of possible solution strategies. The time integration method can vary between fully implicit and the Crank-Nicolson method. Time steps can be constant or varying. Iterative solutions can be controlled in several ways.
Output from AQUIFEM-N is to ASCII output files and to unformatted output files suitable for display with Program PLOT or other software. There are numerous options for controlling the timing and content of model output. Water balance calculations are performed and can be output as required.
AQUIFEM-N is supplied with Programs GENOPT and PLOT. Although not as easy to use as some graphical interfaces for other models, these programs are very powerful, and do provide some advantages. For systematic investigations, these programs can be repeatedly called from within batch files or scripts. GENOPT allows more control over grid design than some interactive packages. PLOT allows numerous plots to be displayed on one screen, and even allows a primitive form of animation.
PreAQ is an interactive graphical preprocessor for preparation of input files for AQUIFEM-N. Finite-element grids can be created on an empty screen or superimposed on an imported base map. Aquifer properties, boundary conditions and all other parameters for simulating both flow and transport can be assigned interactively. Existing AQUIFEM-N grids and data sets can be imported directly for further editing.
AQPost is an interactive postprocessor designed specifically for graphical presentation of results obtained with AQUIFEM-N. Results are presented as color maps such as contours of heads or concentrations, perhaps overlaid on a base map or finite-element grid, with flow paths or arrows showing the direction and magnitude of flow. Maps can be saved in many formats or printed or plotted directly, with similar options to those for UniGraph.
UniGraph is an object-oriented graphic editor for creating and editing vector-based pictures. It provides support for PreAQ and AQPost, allowing the digitization of base maps prior to developing a grid with PreAQ, and enhancement of maps prepared with AQPost, i.e., by adding borders, title blocks and annotation. It can also be used as a stand-alone CAD or drawing package. UniGraph can import DXF files produced by the majority of commercial CAD and GIS packages, existing AQUIFEM-N grid files, blanking and grid files from SURFER, and strings of ASCII coordinates defining point, line or polygon objects, created with any text editor or word processor. Pictures can be saved in GKS, DXF, Microsoft Windows bitmap (BMP) or Microsoft Windows Metafile (WMF) formats. They can be printed directly to HP LaserJet, Canon, IBM and Epson printers, plotted directly to HPGL-compatible plotters, or saved as HPGL or PostScript files which can later be printed or imported into other applications.
All three programs are written in Borland Pascal using the Turbo Vision library and version 6.0 of the MiniGKS graphics system. All programs run under both DOS and Windows.
The recommended minimum computer configuration for these programs is a 486 with 2 MB of RAM. An XMS driver must be installed and 2 MB of disk space is needed for the programs and auxiliary files.
Another powerful graphical interface for AQUIFEM-N on Unix machines and PCs is provided as part of MAPTEK's VULCAN Software for the mining and environmental industries.
AQUIFEM-N versus MODFLOW
AQUIFEM-N has many similarities with MODFLOW, a popular finite-difference model developed by McDonald and Harbaugh (1988). Anderson and Woessner (1992) document the similarities and differences between MODFLOW, another finite difference model known as PLASM, and AQUIFEM-1, the precursor to AQUIFEM-N. There are more similarities than differences, but some are as follows:
- MODFLOW is based on finite differences with square or rectangular cells, whereas AQUIFEM-N is based on the finite-element method with linear triangular elements.
- MODFLOW claims to be three-dimensional whereas AQUIFEM-N claims only to be quasi-three-dimensional. The reason for this is that fully three-dimensional finite-element models connect a node in one layer to more than the nodes directly above and below that node in other layers. MODFLOW and AQUIFEM-N both connect nodes to the nodes directly above and below.
- Some users argue that it is easier to represent the boundary of complex aquifers using triangular elements rather than squares and rectangles. The same is true for internal features, such as rivers or streams. It is certainly easier to refine a finite-element grid near areas of interest, such as near wells, while maintaining larger elements away from those areas.
AQUIFEM-N is available for 386 PCs with 80387 coprocessors, 486 PCs, Pentiums and Macintosh computers. AQUIFEM-N is available for single users only. At present, AQUIFEM-N is supplied with its own associated grid generation and plotting software, but several graphical interfaces will be available shortly, both for PCs and for UNIX-based workstations.
486/Pentium or Macintosh with 4 MB RAM. Please specify PC or Mac and amount of RAM when ordering.
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Τελευταία Ενημέρωση 27 Ιουλίου 2004 - Last Revised on July 27th 2004
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