GMS

Assigning Elevations in GMS

By aquaveo on January 16, 2019

GMS offers several options for importing, exporting and manipulating elevation data. With so many options, sometimes it can be confusing when choosing which method to use. If you sometimes struggle with elevation data, you’re in luck, because in this blog post we will be exploring different ways that you can utilize your elevation data to accomplish your goals. Listed below are several ways in which you might be interested in using your elevation data.

  • Assigning elevations in the conceptual model (e.g. to drain nodes)

    Using the Select Points/Nodes Tool , double-click on a point/node such as a drain node. This brings up the Attribute Table dialog. Make sure the Feature Type is set to point/node. Here you can set the type of point/node (such as to a drain or a river) and set the bottom elevation. Things such as river arcs and drain nodes require elevations to run the model in MODFLOW.

  • Interpolating scatter sets/rasters to MODFLOW elevations

    Right-click on the scatter set or raster and select Interpolate To and select MODFLOW layers. This will bring up the Interpolate to MODFLOW Layers dialog. Select the dataset you want to interpolate on the left side of the dialog and the layer you would like to interpolate to on the right-side. With both the dataset and the desired layer selected, click Map. This will add the selection to the Dataset=>MODFLOW data queue. Select Interpolation Options if you want to change the interpolation method. Click OK to exit the dialog and interpolate the scatter set to the layer.

  • Making sure nothing in your conceptual model assigns a polygon elevation that would overwrite the interpolated values

    It is important to note that if you have top and/or bottom elevations assigned as areal properties to a polygon, and you map this coverage to MODFLOW, any scatter points or raster elevations previously interpolated to MODFLOW as the top/bottom elevations that lie within the polygon will be overwritten.

  • Pulling datasets out of MODFLOW (e.g. Layer → 2D Dataset) for manipulation and/or use elsewhere

    Another great feature available in GMS is the ability to pull elevations from a MODFLOW layer to create a 2D dataset. This 2D dataset can then be manipulated and/or used elsewhere for various purposes. This can be done in one of two ways; by using the Layer → 2D Dataset option, or by using the MODFLOW Layers to Scatter option.

  • Layer → 2D Dataset vs. MODFLOW Layers to Scatter (preferred)

    It is possible to create 2D datasets from layer arrays in MODFLOW by going to the Global Options in MODFLOW, and selecting the array you want to create a dataset of (starting heads, top elevations, bottom elevations). In the array dialog box select Layer → 2D Dataset or Grid → 3D Dataset. The dataset that this creates will have ids, i, j, and f values.

    The preferred method for creating datasets from MODFLOW layers is by selecting the layer and then selecting Grid | MODFLOW Layers to 2D Scatter Points. In the MODFLOW Layers → Scatter Points dialog, you can select to create scatter points within a selected coverage and chose the desired coverage. If you use this option, there must be a polygon in the coverage for the points to map to. This dialog also gives many other options that are extremely useful and convenient. When using this method, the 2D scatter set will have x and y coordinates and the f value.

As you can see there are several ways for you to take advantage of the many options available in GMS when working with elevation data. Whether building from a conceptual model, or maybe even building a conceptual model from a MODFLOW simulation, there are many ways to use your elevation data in GMS. Practice using your elevation data in GMS 10.4 today!

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GMS

Incorporating Geology into a MODFLOW Model

By aquaveo on December 26, 2018

Have you created a MODFLOW model and would like to incorporate geological features in between the MODFLOW layers?

The first step is to create the solid to be used. In GMS, solids are representations of stratigraphy used for site characterization and visualization. Solids can be created in any one of three ways:

  1. Convert horizons to a solid.
  2. Convert one or more TINs to a solid.
  3. Manually create a solid by right-clicking in the Project Explorer and selecting the desired type of solid from the New | Solid menu.

Using the first two methods will allow the solid material composition (what the solid is made of) to be automatically interpolated from the horizon or TIN information. Using the third method will allow you to select the material for each solid reated. The third method also requires knowledge of the XYZ coordinates and other attributes, depending on the type of solid being created (cube, cylinder, sphere, or prism).

Once you’ve created your solids, these steps will integrate the solids into the MODFLOW model:

  1. Right-click on the grid and select Classify Material Zones… to bring up the Classify Material Zones dialog.
  2. Select the solids folder you just created.
  3. Select the Classify algorithm you want to use.
  4. Enter the name of the material set being created.

The Centroid algorithm assigns the solid to the cell if it passes over the centroid of the cell. The predominant material algorithm assigns the solid to the cell if it is the predominant material in the cell (the material making up the highest percentage of the cell). This method maintains your grid, while interpolating the materials to the grid, so that you can have multiple materials within a layer.

Try adding solids to your MODFLOW models in GMS today!

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GMS

Troubleshooting MODFLOW

By aquaveo on December 5, 2018

Are you experiencing issues with your MODFLOW simulation? Unable to get your model to converge? Even after properly constructing a model in GMS, you might still find that your model won’t converge or it terminated with an error. Below are some hopefully helpful suggestions on why the model might not be converging and what you can do about it.

To begin, look at what might be causing the convergence issues to occur. The model might have improper aquifer properties which should be reviewed and adjusted if needed.

Another possibility is that there is an unbalanced flow budget. An unbalanced flow budget can manifest itself in two ways. One way is when the inflow is greater than the outflow, then the model can experience sometimes extreme flooding, and the model in turn will not converge. The other way in which there can be an unbalanced flow budget is if the outflow is greater than the inflow. If all the cells in a model are caused to go dry, then the model will not converge. A high outflow may be caused by things such as high conductivity and high pumping rates.

Another possible issue that might cause MODFLOW to have some issues is if you have a specified head for all grid cells in the model. This is because when all cells are Specified Head boundaries, then there is nothing for MODFLOW to compute and the model will terminate with an error.

Some other common issues include: improper initial conditions, improper boundary conditions, wetting and drying issues (as mentioned above) and a highly sensitive model. If the area is known to be highly sensitive, this might cause MODFLOW to not converge due to the speed at which flow can be affected.

Now that we know of some issues which might cause MODFLOW to struggle, we can take a look at three basic troubleshooting steps.

Basic Troubleshooting Steps:

  1. Review the command line output from MODFLOW, and check to see where the issue began to arise. This will enable you to better pinpoint the cause of the error.
  2. Before running MODFLOW, make sure you always run the model checker to see if there are any errors that will prevent convergence. Immediately repair the errors found in the Model Checker. The Model Checker can be found by clicking MODFLOW | Check Simulation…
  3. Look in the MODFLOW output file (*.out) to search for missing values.

Further detail for some specific issues is also given below.

  • If the head is going to drop below the cell, use the MODFLOW-NWT solver.
  • Cells usually go dry due to a low recharge or a high conductivity. Adjust these parameters to better calibrate the model.
  • A transient water table will cause a fluctuation in the heads and the cells may go dry. In this case it is best to use the rewetting option which is available in all flow packages. However when possible MODFLOW-NWT is still the best solution to this problem.
  • Relax the maximum residual or head change criteria. It is best to not increase these values beyond about 1% of the value.

Hopefully with these troubleshooting tips, you can get your MODFLOW simulation up and running in no time. Additional information can be found in the Frequently Asked Questions section of the MODFLOW user manual under the question "My model hasn’t converged. What can I do?" If you are still struggling to get your model to work, consider using Aquaveo's consulting services for expert assistance.

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GMS

Aquaveo Visits Namibia

By aquaveo on November 21, 2018

From November 6-9, Aquaveo had the privilege of traveling to Windhoek, Namibia to conduct a training and provide consulting sponsored by BGR for several employees of MWAF. The Ministry graciously provided space in its building for the training classes. During the training classes, Alan Lemon helped them refine a MODFLOW model of the Ohangwena II aquifer.

Northern Namibia has a long history of droughts, and Namibia is the driest country in sub-Saharan Africa. Surface water is not a reliable source for potable water in the area due to the infrequency of rain storms to replenish the watershed. Because of this, the Namibian government—through its Ministry of Agriculture, Water, and Forestry (MWAF)—partnered in 2012 with the German Federal Institute for Geosciences and Natural Resources (Bundesanstalt für Geowissenschaften und Rohstoffe, or BGR) to research and develop an aquifer discovered near the northern border with Angola.

The Ohangwena II aquifer is found below an area about 75 km by 40 km, and has nearly 20 billion cubic meters of fresh water. This is enough to last for well over 400 years at current usage levels, and some think there may be even more water there. This kind of resource would be invaluable to the Namibian people.

Creating a valid working model of the aquifer will help the government of Namibia and BGR to come up with a solid groundwater management plan. Such a plan will help them better manage this amazing resource so that the people living in northern Namibia will have the water they need while also preventing needless wasting of hundreds of years of water located in the precious aquifer.

During the trip, Alan had the chance to visit the Okaukuejo Waterhole in Etosha National Park, where he saw a number of different animals, including rhinoceroses, crocodiles, and springbok. The watering hole is visited by tens of thousands of animals each year, from small birds and mammals to some of the largest and most dangerous animals in Africa. The national park is near the area covered by the Ohangwena II aquifer.

We want to thank BGR and MAWF for inviting us to their facility, and for the opportunity to work on such an important project. If you are interested in having Aquaveo assist with your projects, check out our consulting services.

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GMS

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