GMS

Using MODFLOW Native Files

If you have been using MODFLOW with GMS for any amount of time, you have likely noticed that GMS uses a modified version of the MODFLOW files. This is so that the MODFLOW files can be used more efficiently by the GMS interface.

However, there are times when a project may require using the native MODFLOW files. This is often necessary when opening a MODFLOW project that was not originally created in GMS, or when sharing a project with someone who does not have access to GMS.

When importing native MODFLOW files into GMS, there are some important concepts to keep in mind:

  • You will need to start by importing either the NAM file or MFN file. These files contain a directory for the other files in the MODFLOW project and how they should be opened.
  • It is important to keep all of the MODFLOW files together in the same directory. Having only the NAME or MFN file will not be enough to open the MODLOW project.
  • Files for the packages used with the project will typically have a file extension that matches the package. For example, the Wells package will have the extension "*.wel".
  • All native MODFLOW files can be opened and reviewed using a text editor if needed. See the MODFLOW user guide for information on the file format.
The Save Native Text Copy option

Native MODFLOW files can be exported from GMS by turning on the Save Native Text Copy option in the MODFLOW Global/Basic Package dialog. When exporting native MODFLOW files, keep the following in mind:

  • GMS will create a separate directory with the native MODFLOW files. This directory will typically be the project name with "_text" appended to it. For example, if the project is named "Aquifer", the directory will be named "Aquifer_text".
  • All files in this directory should be kept together.
  • As mentioned before, the files can be reviewed using a text editor.

With GMS 10.4, MODFLOW 6 native files can be imported and exported.

Being able to use native MODFLOW files can greatly enhance collaboration with projects. Try out importing and exporting native MODFLOW files in GMS today!

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Exploring MODFLOW Head Boundary Packages

GMS allows using a number of different MODFLOW head boundary packages (GHB, RIV, STR, SFR, etc.) to indicate flow in or out of your model. These packages often appear to be similar. To the differences between them, here we discuss a few of these packages.

Example of MODFLOW head boundary packages available in GMS
General Head

The General Head Boundary (GHB) is, conceptually, a fixed head far from the model where it is assumed to be a fixed head with time (i.e., the river or head will not be affected by the model stresses over time). The purpose of using this boundary condition is to avoid unnecessarily extending the model domain outward to meet the element influencing the head in the model. As a result, the general head condition is usually assigned along the outside edge of the model domain.

General head cells are often used to simulate lakes. General head conditions are specified by assigning a head and a conductance to a selected set of cells. If the water table elevation rises above the specified head, water flows out of the aquifer. If the water table elevation falls below the specified head, water flows into the aquifer. In both cases, the flow rate is proportional to the head difference, and the constant of proportionality is the conductance.

River

The MODFLOW River (RIV) package only tracks flow between the aquifer and the river. With the River Package, once water has entered the river, it is lost to the model.

Stream

Unlike the River package, the Stream (STR) package routes flow through the stream. The water can travel downstream and possibly re-enter the aquifer at another point. The Stream package also allow periodic drying. However, there are more restrictions than in the River package.

Also unlike the River package, the Stream package calculates the water depth based on the flow rather than it being manually entered. There are several different options available for calculating water depth, including using Manning’s equation or a depth versus flow table.

The Stream package divides streams into reaches and segments. It models effects of rivers on aquifers while tracking flow in river. Interaction between surficial streams and the groundwater for the Stream package uses Manning’s equation and simple channel hydraulics to compute the stage in the stream.

Streamflow Routing

This Streamflow Routing (SFR2) package is similar to the Stream package. Though it has more restrictions than STR, it has more sophisticated hydraulics and routing options.

These are just the most commonly-used of over a dozen different MODFLOW head boundary packages that can be used with GMS. Go ahead and explore the different MODFLOW packages available in GMS today!

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Troubleshooting Observations in GMS

After creating your observation points in GMS, have you ever reviewed the results from the observation wells and thought they didn't look right? It can sometimes happen that observation wells give back the wrong values or are missing altogether. This can happen for a number of reasons.

Here are a few tips to help ensure that your observation points give back accurate information:

Point Outside Grid
  1. Review the location of the observation points/wells. Make certain the point is located on the grid. A common issue is that the elevation of the point causes it to be above or below the grid, so be certain to review this.
  2. Make certain the observation points have correctly mapped over to the grid. If the point is outside of the grid area, then it will not be included in the model run.
  3. After being mapped over, check that the observation wells are in the correct layer of the grid. If the observation point is meant to be on more than one layer, make certain it shows up on each layer.
  4. Well Screen Equals Zero
  5. When using a well screen, check the length of the point. If the length is 0, then the observation point will not be able to record any results. Check that the top screen elevation is higher (greater) than the bottom screen elevation by a positive, non-zero amount.
  6. Check that the observation point is in cells that are active. If there are no head values, or the cell is dry, then it is unlikely that the observation point will provide any useful information.

These are just some of the items to look for when using observation points in GMS. For most of these issues, when you save the file, a warning message should appear.

If you have a current license of GMS with paid maintenance, you may contact technical support for additional help in using observations. For project specific troubleshooting, contact Aquaveo's consulting team.

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Using Advanced Data Services Options

In the current versions of GMS, SMS, and WMS, the data service options for importing online maps has changed. It was noted by some of our users that the Advanced option for the Data Services Options dialog was removed.

We are happy to say that the advanced options for the Data Services Options dialog has been restored. The new advanced options are only available when using the Import from Web command in the release of GMS, SMS, and WMS that went out at the beginning of May 2019.

To access the Advanced options from the Data Services Options dialog, click the Advanced button, just as before, to bring up the Select Online Source dialog. This dialog allows users to bring in new data sources for downloading data.

Select Online Source Dialog

Adding new data sources to the Select Online Source dialog can be done in any of three ways:

    Add New Source Dialog
  1. The Select Online Source dialog contains a list of all of the data sources currently available. You can select one of these sources and click the Duplicate Source button to create a copy of the data source. Then, with the copy selected, click the Edit Source button to reach a dialog where you can make modifications to the source such as limiting the layers downloaded from the source or changing the image format downloaded from the source.
  2. You can click the Add New Source button to reach a dialog where you can specify the url of a new data source along with any modifications.
  3. Finally, if you have an Online Source File with the information needed to reach a source, you can click the Add Sources from File to add the source to the available list.

Sources can be deleted from the available list by selecting a source in the list and clicking the Remove Source button. Only sources that have been manually added can be removed or edited.

To get access to the new advanced options for the Data Services Options dialog, visit our downloads page today.

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