GMS

Solution for Overlapping Points in MODFLOW

By aquaveo on October 24, 2023

When using the Groundwater Modeling System (GMS), it is important to understand how cells work in MODFLOW models, especially with conceptual models. Conceptual MODFLOW models are defined using feature objects such as points, arcs, and polygons on a grid. GMS processes each feature object separately, and occasionally there may be more than one feature object in a cell. MODFLOW is able to handle more than one boundary condition in a cell simultaneously, however, there are some things you should note.

The use of coordinates is essential to GMS as a whole, but not to MODFLOW. GMS uses coordinates to keep track of the exact location of feature objects relative to each other, as well as relative to the grid and other model data. Because the cell is the smallest unit of measurement in MODFLOW models, it only cares about the contents of the cell and not the specific location within it. All feature objects within the cell are mapped to the cell center and used for the cell calculations simultaneously.

When importing MODFLOW data that wasn't created in GMS, there are no coordinates tied to that data, so GMS uses the cell center as a reference and places all points there. This poses a problem as GMS requires that all points are assigned to unique coordinates, so GMS will generate an error message if any two or more points share an x, y, and z location. The way to fix this is pretty straightforward, although it can become tedious depending on the number of points on your grid. To solve this problem, follow these steps:

Open the Attribute Table dialog by double-clicking on a point in the Graphics Window.
Make sure the Feature type is set to "Points", and the Show dropdown is set to "All".
Check the box next to Show coordinates.

Now you can identify which points share the same coordinates and make the necessary changes. GMS only cares if more than one point has exactly the same coordinates in the x, y, and z directions. Offsetting a point even slightly in one of the three directions is enough for GMS to no longer have a problem, and the calculations will come out the exact same as long as the point remains within the original cell.

Head over to GMS and use these tips to make sure your MODFLOW simulation runs smoothly!

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How to Refine an Unstructured Grid

By aquaveo on October 3, 2023

When working with a multi-layer unstructured grid (UGrid) in the Ground-water Modeling System (GMS), your project may require different levels of refinement in each layer to introduce complexity to the stratigraphy. GMS currently offers a limited number of ways to achieve this in a 3D UGrid module. Mapping coverage directly to a 3D grid simplifies vertical refinement considerably, but it results in all layers being of uniform size. This blog post will cover the most straightforward way to create a complex stratigraphy, as well as some things that may be useful to know about how refining grids works and what the limitations are.

The type of grid that will often get you the closest to where you want to be is a Quadtree grid. These are the steps you’ll want to follow:

Using the Refine command to refine a quadtree

Use the TINs/Horizons to UGrid approach to generate a grid that incorporates the desired pinchouts and ensures an appropriate level of refinement for the least refined layer.
Right-click on the Quadtree UGrid in the Project Explorer and scroll to the bottom of the UGrid Properties dialog window.
Make sure the Constraint dropdown is set to "None". If the constraint is set to "3D", you will not be able to refine the UGrid cells.
In the toolbar above the Graphics Window, check the box for Single Layer to isolate a layer of the grid for refinement. Highlight the cells you wish to refine using the Select Cells tool.
Right-click on the selected cells and choose Refine.

The newly generated UGrid maintains the original level of refinement on all layers except for the one where you just refined some of the cells. You can repeat this process as many times as needed to achieve the desired level of refinement.

Currently, there is no automated process for this kind of complex refinement, so while it is possible to use this process on a larger project, it may not be practical. If you have a limited number of areas to work on, this workflow lets you produce a grid that features your desired TIN pinchouts, with varying levels of refinement for different layers. It is also not possible to un-refine a grid, so you’ll always need to start with the least refined layer and work your way to the most refined.

Head over to GMS and try this method to refine your 3D grid today!

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Utilizing PEST Observations with MODFLOW 6

By aquaveo on September 12, 2023

MODFLOW 6 comes with an observation utility (OBS) in the Ground-water Modeling System (GMS). This allows you to calculate values like water levels, drawdown, and flow for specific locations throughout the simulation. This utility employs programs from PEST, which makes it similar to the observation feature available in GMS for MODFLOW-USG.

PEST was developed to be used in conjunction with complex environmental models. PEST is an inverse model that uses set parameters to launch the model multiple times until the output matches the observed values, eliminating the need for manual calibration of a MODFLOW simulation.

The way PEST Observations are added to MODFLOW 6 is different from how they are added to other versions of MODFLOW in GMS. With standard MODFLOW, observations are added to the simulation through the MODFLOW | Observations menu option. PEST observations are added to a MODFLOW 6 simulation by right-clicking on the simulation in the Project Explorer and selecting New Package | PEST Observations. This is where the PEST input data is generated for the simulation. The Generate PEST Observation Data button is what allows you to assign coverages as the head and flow observation coverages.

Example of PEST Observations for MODFLOW 6 in GMS

After running the simulation with PEST Observations, you can view the data using statistics tools, whisker plots, and observation plots. The statistics can be viewed in the form of a text file, which are found under the solution files folder in the Project Explorer. Running the MODFLOW 6 simulation with the PEST Observations package automatically generates new coverages with the observation arcs or points. The whisker and observation plots are accessed by making one of the new PEST observation coverages active, then selecting an observation point or arc in the Graphics Window.

If you right-click on one of the PEST Observation coverages you can select Observations, which will bring up a dialog that contains a table with all the data from the observation arcs or points, as well as a plot that displays all of the points on a graph.

Example of the Observation Plot dialog for MODFLOW 6 in GMS

Adding PEST Observations to your MODFLOW 6 model can be incredibly useful, so head over to GMS and see how it can enhance your project today!

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Exporting Transient Observation Data

By aquaveo on August 22, 2023

When working with transient data in the Groundwater Modeling System (GMS), having a way to view the generated Plot Wizard. The Plot Wizard is found under the Display menu at the top of the GMS window. The Plot Wizard opens to a dialog containing a list of all the available plot types. If you're not sure which plot type will contain the data you're looking for, a short description of the selected plot will appear on the right under the plot preview.

Once you finish the set up in the Plot Wizard and the plot is generated, you can right-click on the plot window and select "View Values" to bring up the View Values dialog. This dialog contains a simple data table which includes a list of all points and observed points, as well as a list of all time steps and their related value.

Exporting Transient Observation Data from the Plot Wizard

The second method to view observation data is by going to the MODFLOW menu at the top of the window and selecting Observations. This method opens a window which will allow you to export a CSV file that contains all the observation data. Make sure that "Use" is turned on for the Observation Wells coverage, and then click "Export Trans. Obs…" to bring up the Transient Observation Filename dialog. Although the "Save as type" dropdown may indicate that the data can only be exported as a TXT file, it will actually save as a CSV file. The CSV file contains a more comprehensive table of all the observation data than the Plot Wizard does.

CSV Format of Exported Transient Observation Data

The given data for the exported observation data is divided into several groups. Each group contains a list of observation times, the corresponding recorded value, and the computed value at those specific times. This is sometimes different from the output times from the calculated list, so it offers precise values for comparison, removing the need for estimation based on the output list.

Head over to GMS and check out how this can improve how you view transient data today!

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