GMS:MODAEM

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MODAEM
Model Info
Model type Analytic element, Steady state analysis
Developer Vic Kelson of Wittman Hydro Planning Associates
Tutorials MODAEM Tutorials

MODAEM is an analytic element model developed by Vic Kelson of Wittman Hydro Planning Associates (WHPA) of Bloomington, Indiana. The key feature of analytic element models is that they do not require the discretization of the interior of the model domain into cells and elements as is the case with finite difference and finite element models. Rather, the model is defined by “analytic elements” representing line sources and sinks such as rivers and drains or specified head and specified flow boundaries. Wells are also represented as points, and recharge and aquifer properties can be defined on polygons. MODAEM then formulates a set of equations based on these entities and the equations can be solved for any location in the horizontal plane.

MODAEM is perfectly suited for the conceptual model approach used by GMS since the input data to MODAEM is consistent with the GIS feature objects (points, arcs, and polygons) used to define a GMS conceptual model. In other words, once a MODAEM conceptual model is defined, the model can be immediately executed. There is no need to build a grid or mesh. A background grid is utilized for displaying the MODAEM results using contour lines. However, the background grid is purely for visualization purposes and has nothing to do with the accuracy of the model. MODAEM also supports particle tracking/streamlines.

One of the caveats of the analytical element approach is that a simplified representation of the aquifer must be used. The aquifer properties (K, porosity, bottom elevation, aquifer thickness) can be assigned to polygons, but they are assumed to be constant within each polygon. Currently MODAEM supports steady, confined and unconfined two-dimensional groundwater flow, although streamlines are computed in three dimensions.

Linking MODAEM and MODFLOW

GMS also includes a feature to link MODAEM models with MODFLOW models. The main application of this linkage is for defining boundary conditions for a local scale model. A regional model can be built with MODAEM and then linked to a local scale MODFLOW model. This is accomplished as follows:

  1. Build and calibrate the regional scale model using MODFLOW.
  2. Build a conceptual model for the local scale MODFLOW model. When setting up the coverage properties for the source/sink coverages, be sure to toggle on the MODAEM Head and Observed Flow options in the Source/Sink/BC's list. If there are more than one MODAEM model in the project, it will be necessary to select which model should be linked to the MODFLOW model using the MODAEM models combo box at the bottom of the Coverage Setup dialog.
  3. Mark the boundaries (or at least the upstream and downstream boundaries) of the MODFLOW conceptual model as MODAEM Head boundaries.

At this point, whenever the Map → MODFLOW command is executed, GMS will automatically launch the MODAEM model and calculate a head value for each grid cell coinciding with the MODAEM head boundary. Furthermore, a flow value is computed for the boundary using MODAEM and assigned to the MODFLOW boundary as an observed head. This makes it possible to monitor the flow budget across the boundary to ensure that the changes made to the local scale model (insertion of remedial wells, etc.) do not violate the flow conditions inherited from the regional scale model.

MODAEM Output File

When MODAEM is executed, it generates an HTML formatted text output file. If the execution is successfully completed, an icon External Text File Icon.svg will appear under the corresponding MODAEM conceptual model in the Project Explorer representing the output file. Double-clicking on this icon loads the output file into the default web browser.

MODAEM Links