GMS

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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Calculate the Water Level Below the Top of an Aquifer

By aquaveo on August 1, 2023

The Data Calculator is a useful feature that is available in the Groundwater Modeling System (GMS). The Data Calculator performs many functions and can enhance your project and simplify the visualization of data. This blog post will provide an overview of how to use the Data Calculator in GMS to pinpoint locations where the water level in a selected aquifer falls below the top elevation in a MODFLOW simulation.

Calculating the Water Level Below the Aquifer

As you might expect, a MODFLOW simulation containing aquifers needs to be completed before you are able to use the Data Calculator to find the time step where the water level falls lower than the top elevation in the simulation. The datasets required for this calculation are the top elevation dataset and the head dataset.

Before opening the Data Calculator, you will first need to duplicate the top elevation dataset. This is because the Data Calculator isn’t able to recognize the datasets directly under the MODFLOW simulation in the Project Explorer. Right-clicking on the top elevation dataset and selecting duplicate will create a copy of the dataset under the grid folder in the Project Explorer, and this duplicate can now be read into the calculator. If desired, you can right-click on the duplicate dataset and rename it to something that makes more sense to you.

The next step is to go to the Edit menu at the top of the screen and select Dataset Calculator to open the Data Calculator dialog. There is also a Data Calculator macro on the top row, which will bring up the same dialog. Follow these general steps to calculate the data once the Data Calculator is open:

  1. On the left side of the dialog, find and select the Head dataset.
  2. On the right side of the dialog, either select a specific time step from the list, or check the box that says “Use all time steps”.
  3. Click the “Add to Expression” button to add the Head data at the selected time steps to the calculation.
  4. Click the subtraction button (-), or use your keyboard to type the subtraction symbol.
  5. Select the copy you made of the top elevation dataset and click “Add to Expression”.
  6. In the Result field, enter a name for the new dataset.
  7. Press Compute, then click done to close the dialog.

After generating the new dataset, you can right-click on it in the Project Explorer and select “View Values”, which will let you view the data values for the selected time step. Any values denoted by a negative number indicate a water level lower than the highest point of the aquifer.

Head over to GMS and see the many ways the Data Calculator feature can be useful to you and your project!

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Understanding the MODFLOW Translator

By aquaveo on July 11, 2023

When importing a MODFLOW file into Groundwater Modeling System (GMS), you may need to translate the file to ensure compatibility with GMS's features and tools. This blog post includes details about the versions of MODFLOW supported by GMS, how GMS uses the translator dialog to transform the MODFLOW file into a version that GMS is able to read and alter, and methods you can use to determine the file version.

Example of the MODFLOW Translator

GMS supports MODFLOW versions 88, 96, 2000, 2005, MODFLOW-NWT, MODFLOW LGR, and MODFLOW USG. However, MODFLOW 88 and 96 are only supported as imports and require conversion to MODFLOW 2000. When importing a MODFLOW file into GMS, if the file was not created in GMS 6.5 or later, translation is necessary for full compatibility with GMS's features and tools, so the MODFLOW Translator dialog will appear. GMS will create a copy of the file before performing the translation which will ensure that the original file is preserved, however you should still always double check that all the data was converted successfully and hasn’t been changed and that none of the data has been lost. During the translation process, you can select the appropriate MODFLOW version from the list provided by the translator for accurate interpretation and conversion.

You can also alter the MODFLOW version inside GMS between supported versions by going to Global Options under the MODFLOW menu. It should be noted, however, that while most versions can be changed back and forth, it can’t be changed back from MODFLOW USG Transport.

There are specific indicators for each version of MODFLOW that can help you to determine what kind of MODFLOW file you are working with if you are unsure which file type it is, which you can view by opening the file as a text file. In a MODFLOW 88 file, the third line of the basic package file contains an IUNIT array with 12 or 24 slots, 24 being the more common option. A MODFLOW 96 file, on the other hand, lacks the IUNIT array and instead features the keyword "FREE" on the third line, indicating that data is in free format, or the third line is entirely blank. Files with a *.dis extension are likely to be MODFLOW 2000, 2005, or NWT models, all of which are supported by GMS.

Certain features and versions of MODFLOW are not supported in GMS. If you have more questions about what MODFLOW features are not supported by GMS, you can follow this link to Aquaveo’s wiki for more information. Head over to GMS and see how this can work for your own MODFLOW models today!

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