GMS

Exploring MODFLOW Head Boundary Packages

By aquaveo on June 19, 2019

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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GMS

Troubleshooting Observations in GMS

By aquaveo on May 29, 2019

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:

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.
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.
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.

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.
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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GMS

Using Advanced Data Services Options

By aquaveo on May 22, 2019

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.

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

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.
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.
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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GMS

SMS

WMS

Splitting UGrid Layers

By aquaveo on May 8, 2019

Have you been working with a project in GMS only to discover that the project’s 3D unstructured grid (UGrid) needs to include another layer? Fortunately, it’s rare that a UGrid needs to have another layer, but every once in a while a layer needs to be added to an existing UGrid. GMS provides a way to divide UGrid layers quickly.

One thing to note: whenever a UGrid is changed—such as with adding a new layer—the existing MODFLOW simulation attached to the UGrid will be removed. It is therefore best to make certain the UGrid is correct—including having the necessary number of layers—before building the MODFLOW simulation.

In order to add a new layer to an existing Ugrid, do the following:

Using the Select Cell tool, select a cell on the layer you want to split.
Right-click on the selected cell and select the Split Layer command to start the process of dividing the UGrid layer.

UGrid layer before and after being split

When GMS finishes processing, it creates a new UGrid with the additional layer, leaving the original UGrid intact. The layer to which the selected cell belonged is divided into two layers on the new UGrid. GMS averages the distance between the top and bottom of the layer, then divides the layer equally to create the two new layers. It is recommended to carefully review the new UGrid to check for any unintended anomalies.

As mentioned above, any MODFLOW simulations contained in the original UGrid are not copied to the new UGrid. A new MODFLOW simulation must be created for the new UGrid.

Another option is to create a new UGrid with the additional layer and leave the existing UGrid as is. This option is best if you need to finely control the layer elevations.

For adding layers to complicated UGrids, you may want to consider using Aquaveo’s consulting services.

Now that you’ve seen the basics of splitting a UGrid layer, try it out in GMS today!

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GMS