GMS

Three Methods for Assigning CHD

By aquaveo on November 12, 2024

Do you have a Ground-water Modeling System (GMS) project where you are needing to edit values in your constant head cells? If it is a simple grid approach, academic-style project, editing each cell is likely fine. But if you have a large, more complex, conceptual model, editing the constant head cells may prove problematic. Here are some ways that you can accomplish this with a minimum of effort.

A constant head (CHD) cell is an area where the water levels will remain fairly steady no matter what the inflow or outflow may be. Lakes, coastal areas, etc. are good candidates of constant head boundaries. Assigning cells to be constant heads provides a steady boundary where the water levels will stay the same for the model duration.

The constant head cells are typically assigned or edited in one of three ways. One method is to directly edit the IBOUND array. When setting up a MODFLOW simulation or when editing packages, the MODFLOW | Global Options menu will let you choose the IBOUND package to edit the options there including the array. Copying and pasting data from an external spreadsheet into the array could save you time here.

Another method is to select a set of cells and use the 3D Grid Cell Properties dialog. You can right-click on a cell to bring up the dialog. Make sure you are on the MODFLOW tab and edit the “IBOUND” and “Starting head” rows. If multiple cells are selected, this will edit all selected cells.

The simplest method is to define the constant head zones using feature objects as part of a conceptual model in the Map module. When you set up a coverage using the feature objects, the Coverage Setup dialog will allow you to assign CHD to cells in the grid. You can change what the default assignment will be if you also select “Layer Range” and make changes using the Attribute Table.

Whichever method you choose to assign CHD, when you are ready to run your ground-water model, the Model Checker can be used to see if there are errors in the assigned constant head cells. Then you can correct those cells mentioned specifically by using the IBOUND array or the Cell Properties dialog as mentioned above.

Head on over to GMS and see which method of defining your constant head cells works best for you!

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Reliable Grid Conversion with the Voronoi UGrid from UGrid Tool

By aquaveo on October 22, 2024

Did you realize while planning your Groundwater Modeling System (GMS) project that a different unstructured grid (UGrid) would be a better fit? UGrids are very flexible and can be refined around streams, wells, basins, and other objects. But sometimes, one type of UGrid may fit better than another for a particular project. The Voronoi UGrid is one of the more flexible types of UGrids available in GMS. The Voronoi UGrid from UGrid Tool in the Toolbox is there to help convert a UGrid into a Voronoi UGrid.

For most UGrids, the attributes and boundary conditions assigned are associated with the edges, points, and nodes, all on the outer edges of the cells. For a Voronoi UGrid, those things are associated with the point at the center of the cell.

If your model is fairly straight-forward, has structured boundaries or rectangular domains, UGrids with rectangular cells might still be the better choice. Or, if you require computational efficiency, the Voronoi UGrid may not be your best choice. However, if those are not considerations, then using the Voronoi UGrid from UGrid Tool may be a good option for you.

With a Voronoi UGrid, you get even greater flexibility to refine around irregular boundaries thus improving your groundwater model’s accuracy. The cells of the Voronoi UGrid are created with an algorithm that depends solely on the geometry of the points, arcs, and polygons in the active coverage. Voronoi UGrid cells can have more sides (up to eight) than just the three or four sides allowed in most other UGrids. Therefore, it can fit into the boundaries of a model and be refined along arcs and at points much more accurately.

You will find the Voronoi UGrid from UGrid Tool in the Unstructured Grid folder of the Toolbox. The Voronoi UGrid from UGrid Tool only needs the input UGrid and a name for the new Voronoi UGrid it will generate. There are some limitations to the tool. Conversions may not occur if any created final cells would have more than 8 edges. The UGrid being converted must contain only 2D cells. And there can also be no disjoint regions where cells aren’t connected to the rest of the UGrid.

Head on over to GMS to see if the Voronoi UGrid from UGrid tTool will help your model!

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How to Prioritize Interpolating Multiple Rasters

By aquaveo on October 1, 2024

Do you have a project requiring multiple elevation rasters that are difficult to reconcile with each other? To manage the rasters, and get the best data for your project, our Groundwater Modeling System (GMS) has many tools in the Toolbox to help. One of these, the Interpolate Priority Rasters tool, gives you the ability to prioritize the elevation data from each raster.

When multiple rasters are interpolated to the geometry, multiple datasets will be created, one for each raster. A raster catalog can help consolidate this information but it doesn’t prioritize data. With the Interpolate Priority Rasters tool, you can specify exactly which raster takes precedence first, second, and so on. It will create one dataset that contains all the prioritized information. This ensures that the information most important to your model can be retained and used.

Rasters downloaded from separate sources, or from the same database but at differing times, can contain discrepancies in the data. When the rasters overlap, those differences can create problems with the model. If major discrepancies exist, some of the other tools in the Toolbox can help correct them before using the Interpolate Priority Rasters tool.

In GMS, the Interpolate Priority Raster tool can be used with both 2D and 3D UGrid geometries. All that is needed for this tool is a geometry selected, a dataset used to provide default values for interpolation, a method for interpolation, and the selected rasters placed in the order of priority. Once run, a new dataset is created with the integrated data. If the new dataset is meant to be the elevation of the target geometry, you will need to map it manually because it will not be done automatically, but you will have an interpolated raster that is consistent across the model.

Head on over to GMS and try the Interpolate Priority Rasters tool in your models.

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Problem Solving and the Fix Layer Errors Tool

By aquaveo on September 10, 2024

When creating multilayer models, defining layer data can be challenging. This is true for cases involving embedded seams, pinchouts, and truncations. Fortunately, Ground-water Modeling System (GMS) contains a suite of tools for interpolating and manipulating layer elevation data. With the tools in GMS), even complex geologic strata can be modeled quickly and easily. When interpolating layer data for the purpose of defining MODFLOW layer elevation arrays, there are often cases where the interpolated values overlap. In some cases, the best way to fix such a problem is to experiment with the interpolation options or to create some "pseudo-points" to fill in the gaps between sparse scatter points. In other cases, the overlap may correspond to a pinchout or truncation in the layer. In such cases, the elevations need to be adjusted so that there is a small but finite thickness for all cells in the overlapping region.

The first step in fixing layer errors is to use the Model Checker to determine if elevation overlaps occur. If they do occur, the Fix Layer Errors button at the top of the Model Checker dialog can be used to bring up the Fix Layer Errors dialog. Four options are available for fixing layer errors, including Average, Preserve Top, Preserve Bottom, and Truncate to Bedrock. The Average method is useful for modeling the transition zones adjacent to embedded seams. The Preserve Top method can be used to model truncated outcroppings. With the Preserve bottom method, at each cell where an overlap is found the bottom elevation is unchanged and the top elevation is adjusted to top = bot + min thickness. The Truncate to bedrock option differs from the other methods in that it can be used to alter several layers at once.

These are hypothetical scenarios that outline problems and solutions you may encounter when using the Fix Layer Errors Tool in GMS:

You are trying to construct a UGrid model that includes refinement around streams and pumping wells with refinement reserved for the top layers, and inactive cells with a thickness of less than 10 cm.

You want a UGrid that includes refinement around streams and pumping wells, yet reserved for the top layers. You get the impression that reserved refinement is achievable only by using the preserve top method.
It is correct that the preserve top method is a way to apply refinement. The average method does not have any way to refine certain areas, unless the 2D geometry being used as the base already has that refinement. In that case, the refinement would be added to all layers. In other words, refine the 2D grid first, but use the TIN method to create the grid based on that refinement. Use your judgment on if method one works better for your elevation data.
You would like all cells with a thickness of less than 10 cm to be inactivated. Yet you find that the only way to inactivate cells less than 10 cm is by using the average method.
Using the average method and setting the minimum thickness to 0.1m ≠ inactive thin cells. It means cells less than the minimum thickness are not created. Cells are not inactivated until after a MODFLOW simulation has been added. As such, they cannot be inactivated during the creation process. If this is not the result, and there does appear to be thin cells for a TIN grid, further action would be required.

Access the latest version of GMS 10.8 to make use of the Fix Layer Errors tool today!

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