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

New and Improved Lidar Tools in GMS 10.4

By aquaveo on November 7, 2018

With the release of GMS 10.4 (beta), a set of new and improved tools for importing and handling lidar files is now available. The new tools provide much faster import times, and most processing happens on the fly as the lidar display options are adjusted.

Merging and Display Options

As with previous versions of GMS, either a single or multiple lidar files can be imported. However, the options available for handling the lidar data have changed significantly, offering more precise control and more options that will allow better use of the lidar file data.

One of the best new features is that GMS 10.4 allows merging multiple lidar files into a single file. This is accomplished by selecting the desired lidar files in the Project Explorer, then right-clicking and selecting Merge. Enter the desired filename and allow GMS to process the merge.

After the merging is complete, you can import the new lidar file into your project.The merged file has all of the lidar cloud points that were contained in the original lidar files, though not all of them may be visible. The visibility of lidar cloud points can be adjusted using the display options for the merged lidar file. Access them by right-clicking on the merged lidar file in the Project Explorer and selecting Display Options… to bring up the Display Options dialog.

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This dialog allows you to change point size, how many are displayed, which kinds are displayed, which points to exclude, and how to exclude them. This allows you to choose the level of complexity without overwhelming you with too many options. The following image shows the merged lidar file with 5 million points displayed.

Interpolation Options

  • UGrid: If your project has an existing unstructured grid (or one that has been imported or created), you can interpolate the lidar files to that UGrid. Simply select the desired lidar files in the Project Explorer, then right-click on them and select Interpolate to UGrid… to bring up the Interpolate Lidar to UGrid dialog. This allows you to select the target UGrid, set the new dataset name, and set the sampling distance.
  • Raster: This option uses the lidar display options set at the time of interpolation and creates a raster file. The Interpolate Lidar to Raster dialog allows the number of X and Y cells to be set, and changing those affects the cell size. The merged raster is then imported into the Project Explorer.

Try these new features today in GMS 10.4 (beta).

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GMS

What We Learned at the 2018 Aquaveo User Conference

By aquaveo on October 24, 2018

Aquaveo held its first annual User Conference from October 16-17, 2018! It was amazing to get to meet so many of our users at once. We hope everyone had a great time.

For us, it was exciting to meet users from all over the world. We met users from the United States, Canada, Portugal, Germany, the United Arab Emirates, Guyana, and Thailand. When asked why they came, the attendees said they came to learn:

  • How Aquaveo’s products can help with their projects
  • More of what Aquaveo has to offer
  • What Aquaveo is all about

In listening to what our users had to share with us, we took away a couple key items that you want to see improved in our software.

  1. You want modeling to go faster. We understand the demand to give results in less time is growing, so we will be looking at ways to make the process of modeling take less time and still be accurate. Some of the innovations added into Aquaveo’s software already significantly reduce the time it takes to complete a model. We hope to add more automation and tools to speed up modeling times.
  2. You need to be able to work with big data. More data is becoming available and you want to be able to use it. This means processing large files covering larger areas in more detail. At Aquaveo, we are working on doing this, including optimizing our code and adding new tools to work with large files such as lidar files.

We’d like to thank the following for participating in our user conference:

If you couldn’t make it to the Aquaveo User Conference this year, we will be having another one next year. Watch our website and Facebook page for future details. See you next year!

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