GMS

Utilizing the Transient Step Function Method

By aquaveo on November 14, 2023

The Groundwater Modeling System (GMS) provides tools for working with transient MODFLOW simulations. When working with transient dataM in a MODFLOW model, it is important to understand stress periods and how GMS calculates their values. There are two different methods you can use to define the values for a stress period: the continuous time series method, and the step function method. This blog post will cover how to input the data for a stress period in the XY Series dialog so that GMS will calculate their values correctly.

Transient MODFLOW simulations use time intervals called stress periods to define the values of transient stresses such as pumping rates, and river stages. The values for stress periods are entered in the XY Series dialog, which is opened from the Attribute Table dialog. For a continuous time series, you need to enter only one data point per time step. When you enter only one value per time step, GMS assumes that the value continues to increase through the stress period, creating a straight line connecting the two points for a smooth transition. But because GMS needs a constant value for each stress period, it will take an average of the starting values of that particular stress period and the starting value of the following stress period.

The step function method tells GMS that there is only one value for each stress period, rather than a continuously increasing value. To input data in the XY Series dialog so that GMS knows that there is only one constant value through the entire stress period, you’ll need to enter both a start value and an end value. The figure below shows an example of the format to use.

As you can see in the above figure, you need the end value of a stress period to match the start value of the following stress period. This creates "steps" in the data, telling GMS to read a constant value through the entire stress period, rather than having a gradual increase from beginning to end.

Head over to GMS and try creating a step function for your transient MODFLOW model today!

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Solution for Overlapping Points in MODFLOW

By aquaveo on October 24, 2023

When using the Groundwater Modeling System (GMS), it is important to understand how cells work in MODFLOW models, especially with conceptual models. Conceptual MODFLOW models are defined using feature objects such as points, arcs, and polygons on a grid. GMS processes each feature object separately, and occasionally there may be more than one feature object in a cell. MODFLOW is able to handle more than one boundary condition in a cell simultaneously, however, there are some things you should note.

The use of coordinates is essential to GMS as a whole, but not to MODFLOW. GMS uses coordinates to keep track of the exact location of feature objects relative to each other, as well as relative to the grid and other model data. Because the cell is the smallest unit of measurement in MODFLOW models, it only cares about the contents of the cell and not the specific location within it. All feature objects within the cell are mapped to the cell center and used for the cell calculations simultaneously.

When importing MODFLOW data that wasn't created in GMS, there are no coordinates tied to that data, so GMS uses the cell center as a reference and places all points there. This poses a problem as GMS requires that all points are assigned to unique coordinates, so GMS will generate an error message if any two or more points share an x, y, and z location. The way to fix this is pretty straightforward, although it can become tedious depending on the number of points on your grid. To solve this problem, follow these steps:

Open the Attribute Table dialog by double-clicking on a point in the Graphics Window.
Make sure the Feature type is set to "Points", and the Show dropdown is set to "All".
Check the box next to Show coordinates.

Now you can identify which points share the same coordinates and make the necessary changes. GMS only cares if more than one point has exactly the same coordinates in the x, y, and z directions. Offsetting a point even slightly in one of the three directions is enough for GMS to no longer have a problem, and the calculations will come out the exact same as long as the point remains within the original cell.

Head over to GMS and use these tips to make sure your MODFLOW simulation runs smoothly!

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How to Refine an Unstructured Grid

By aquaveo on October 3, 2023

When working with a multi-layer unstructured grid (UGrid) in the Ground-water Modeling System (GMS), your project may require different levels of refinement in each layer to introduce complexity to the stratigraphy. GMS currently offers a limited number of ways to achieve this in a 3D UGrid module. Mapping coverage directly to a 3D grid simplifies vertical refinement considerably, but it results in all layers being of uniform size. This blog post will cover the most straightforward way to create a complex stratigraphy, as well as some things that may be useful to know about how refining grids works and what the limitations are.

The type of grid that will often get you the closest to where you want to be is a Quadtree grid. These are the steps you’ll want to follow:

Using the Refine command to refine a quadtree

Use the TINs/Horizons to UGrid approach to generate a grid that incorporates the desired pinchouts and ensures an appropriate level of refinement for the least refined layer.
Right-click on the Quadtree UGrid in the Project Explorer and scroll to the bottom of the UGrid Properties dialog window.
Make sure the Constraint dropdown is set to "None". If the constraint is set to "3D", you will not be able to refine the UGrid cells.
In the toolbar above the Graphics Window, check the box for Single Layer to isolate a layer of the grid for refinement. Highlight the cells you wish to refine using the Select Cells tool.
Right-click on the selected cells and choose Refine.

The newly generated UGrid maintains the original level of refinement on all layers except for the one where you just refined some of the cells. You can repeat this process as many times as needed to achieve the desired level of refinement.

Currently, there is no automated process for this kind of complex refinement, so while it is possible to use this process on a larger project, it may not be practical. If you have a limited number of areas to work on, this workflow lets you produce a grid that features your desired TIN pinchouts, with varying levels of refinement for different layers. It is also not possible to un-refine a grid, so you’ll always need to start with the least refined layer and work your way to the most refined.

Head over to GMS and try this method to refine your 3D grid today!

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Utilizing PEST Observations with MODFLOW 6

By aquaveo on September 12, 2023

MODFLOW 6 comes with an observation utility (OBS) in the Ground-water Modeling System (GMS). This allows you to calculate values like water levels, drawdown, and flow for specific locations throughout the simulation. This utility employs programs from PEST, which makes it similar to the observation feature available in GMS for MODFLOW-USG.

PEST was developed to be used in conjunction with complex environmental models. PEST is an inverse model that uses set parameters to launch the model multiple times until the output matches the observed values, eliminating the need for manual calibration of a MODFLOW simulation.

The way PEST Observations are added to MODFLOW 6 is different from how they are added to other versions of MODFLOW in GMS. With standard MODFLOW, observations are added to the simulation through the MODFLOW | Observations menu option. PEST observations are added to a MODFLOW 6 simulation by right-clicking on the simulation in the Project Explorer and selecting New Package | PEST Observations. This is where the PEST input data is generated for the simulation. The Generate PEST Observation Data button is what allows you to assign coverages as the head and flow observation coverages.

Example of PEST Observations for MODFLOW 6 in GMS

After running the simulation with PEST Observations, you can view the data using statistics tools, whisker plots, and observation plots. The statistics can be viewed in the form of a text file, which are found under the solution files folder in the Project Explorer. Running the MODFLOW 6 simulation with the PEST Observations package automatically generates new coverages with the observation arcs or points. The whisker and observation plots are accessed by making one of the new PEST observation coverages active, then selecting an observation point or arc in the Graphics Window.

If you right-click on one of the PEST Observation coverages you can select Observations, which will bring up a dialog that contains a table with all the data from the observation arcs or points, as well as a plot that displays all of the points on a graph.