Introduction

The AEM family of computer programs, presently SLAEM, MLAEM/2, and MLAEM, are based on the Analytic Element Method developed by Dr. O.D.L. Strack. For a description of the Analytic Element Method, see "Groundwater Mechanics" by O.D.L. Strack (Prentice-Hall, 1989). The computer programs are intended for modeling regional groundwater flow in systems of confined, unconfined, and leaky aquifers. SLAEM (Single Layer Analytic Element Model) is the single-layer version of the program. MLAEM/2 (Multi Layer Analytic Element M odel) can access two layers while the number of layers supported by MLAEM is limited only by hardware. All programs run under Microsoft Windows 95, 98 and NT. The programs are native windows applications and are accessed via a modern and flexible Graphical User Interface (GUI) as well as via a command-line interface. The latter capability makes it easy to drive the program from other programs such as Arc-View, ARC/INFO, and PEST. The programs create files from data entered graphically via the GUI. These files can be read in later. The programs read DXF-files and produce BNA files that may be read by other programs such as SURFER.

HISTORY OF SLAEM/MLAEM

SLAEM was the first analytic element computer program commercially available. It was used in the early 1980's by several consulting firms in the Twin Cities Metropolitan Area. SLAEM was originally written on a minicomputer and was accessed via a modem. Since then, the program has been improved and extended, ported to the IBM-PC platform, and marketed on a limited scale. Both SLAEM and MLAEM have been used in the US, Europe and Africa on numerous projects. Successful applications of MLAEM include the construction of a model of the Toppenish Basin prepared for the Yakima Indian Nation, the groundwater models of Hennepin County by the Hennepin County (Minnesota) Conservation District, the model of Dakota County (Minnesota), the National Groundwater Model (NAGROM) of The Netherlands prepared by RIZA, the Twin Cities Metropolitan Model prepared jointly by the Minnesota Pollution Control Agency and the University of Minnesota, and the regional model of the groundwater system around Yucca Mountain, Nevada, prepared by the University of Minnesota for the USGS. Most of these models are in active use, and the AEM programs have proven to be flexible, accurate, and powerful tools for regional modeling. Prices have been dramatically lowered (in the case of SLAEM by a factor of 10), and significantly updated both in terms of computational power and ease of use. All programs now have the latest advances in the analytic element method built in and are driven by a state-of-the-art graphical user interface.

AQUIFER FEATURES SUPPORTED

All programs support the same analytic elements so that a single description will cover all three products. The elements that are designed specifically for multi aquifer systems, however, are not available in SLAEM. The programs contain all elements that were supported by their DOS versions in addition to the newly developed ones. The older elements are not nearly as powerful as the new ones that replace them and are supported only for the sake of continuity; their entry is not built into the GUI, and they will not be described herein. The aquifer features currently supported by all programs are the following:

• Inhomogeneities in the aquifer parameters, namely, the base elevation, thickness, and hydraulic conductivity. These inhomogeneities are bounded by polygons of straight line segments and may be nested and have common boundaries.
   
• Curvilinear features that may be combined to form open and closed strings of the following types: 
   
  • Given head and resistivity (stream bottom). 
  • Given head and multi-resistivity (lake and river bottoms that change their behavior according to the head in the aquifer). This covers the case of portions of lakes and rivers that are dry.
  • Given extraction or infiltration rate per unit length.
  • Constant, but unknown, head and given total discharge (drain with given flow rate).
  • Leaky wall (normal flux is related to the difference in head across the wall and the resistivity of the wall).
  • Impermeable wall.
  • Given flux out of or into one side of the element.
     
• Areas inside of which extraction or leakage occurs. These areas are bounded by polygons and may be of the following types:
   
  • Given extraction or infiltration rate per unit area.
  • Given head and resistivity (lake and river bottoms).
  • Given head and multi-resistivity (lake and river bottoms that change their behavior according to the head in the aquifer) This covers the case of lakes and rivers that become dry.
  • Given resistivity for the computation of leakage between aquifers.

The properties of these aquifer features are entered in the program via the GUI independently of the analytic elements that are used to model them. The modules AQUIFER and ATARD (for aquitard) are used to enter the properties that apply inside a polygon. The aquifer properties are constant inside the polygon, but the aquitard properties (resistivity and/or head) are interpolated by a radial basis interpolator between the values entered by the user.

THE GRAPHICAL USER INTERFACE (GUI)

The graphical user interface makes it convenient to enter and modify data, usually on a background map read in DXF format. The look of the display depends on whether the user is entering or modifying the geometry and properties of elements or examining the results of a run. The two windows are the Input Window (see figure 2) and the Results Window. Both windows consist of a series of individual windows that the user may arrange as desired and a menu bar, tool bars, and command window. The latter window supports access to the command-line interface. The Input Window with a plot of an aquitard polygon is shown in figure 2. The individual windows are labeled. The vertical bar on the left of the screen is the tool bar for the entry of aquifer features, data, and analytic elements. The vertical sliders on the right are tools for zooming in and out of an area. The pull-down menus are accessed from the two headings file and window at the extreme left hand side of the menu bar.

Figure 2. The Graphical User Interface

The analytic elements currently supported by the AEM programs are the following:

1. Curvilinear analytic elements of an order up to 12.
    
2. Straight line doublets used for simulating the effect on the flow of the boundaries of inhomogeneities. These doublets may be of an order up to 30.
    
3. Multi-quadric area-sinks. These analytic elements simulate an extraction rate that varies over an area according to a multi-quadric interpolator based on the distance between a control point and the point in the aquifer where the extraction rate is evaluated. This interpolator is identical to the one used in the aquitard module for interpolating head and resistivity.

Figure 3. Several Analytic Elements created in MLAEM
 

All elements support over-specification as introduced recently by Jankovic and Barnes (1997), "High Order Line Elements for Two Dimensional Groundwater Flow." The multi-quadric area-sinks were developed by Strack (1997), "An Area-Sink With Multi-Quadric Density Distribution." The solutions produced with these elements are extremely accurate. Examples of results obtained by the use of the various elements are shown in Figures 1, 3, and 4, and 5.

UPGRADE POLICY

Upgrade policy for programming errors

Fixes for programming errors will be available free of charge for one year after purchase of the program except for MLAEM which is five years.

Upgrade policy for improvements to the programs

Upgrades may be obtained as they become available at discounts that will vary with the nature of the upgrade. MLAEM will be upgraded free of charge for any improvements or additions to the program for one year after purchase. Purchasers of licenses for MLAEM will be given a free upgrade to the version of the program with superblocks when this version becomes available. Holders of licenses of MLAEM may obtain customized versions upon request.

TECHNICAL SUPPORT

Support is available via e-mail. There is no charge for support if the question is related to programming errors or errors in the documentation. Other support (consulting) is available at a rate of $70/hour and will be provided within one business day from the time of the request. MLAEM licensees are entitled to 20 hours of free support.