In our last post, we looked at how the Leakage Factor can help you determine the ideal spacing for your model’s internal nodes, the Spatially-Variable Area Sinks (SVAS). While the Leakage Factor is a valuable guide, it’s just the first step. The ultimate test of whether your SVAS nodes are spaced correctly is a direct check of the model’s mathematical accuracy. This is where the Extraction Points analysis comes in.
What Is the Extraction Points Analysis?
The extraction points analysis is a method for checking the accuracy of your SVAS node spacing by directly comparing two different calculations of vertical flow:
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Modeled Extraction: This is the extraction rate that AnAqSim is modeling at a specific point using the spatially-variable area sink functions.
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Extraction From Heads: This is an independent calculation of flow based on the head differences and vertical hydraulic conductivities between layers.
At the precise location of an SVAS point, these two values are equal, meaning the flow equation is solved perfectly. However, as you move away from a node, the Modeled Extraction becomes an approximation. The goal is to make these two values as close as possible across the entire model.
How Do I Use This Analysis?
AnAqSim provides two primary ways to perform this check:
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Spot Check with the Cursor: You can use the cursor inspector tool to hover over any point in a multi-layer domain. The data pane on the left will display the values for both Modeled Extraction and Extraction From Heads. If the two values differ significantly, it means your SVAS node spacing in that area is too large and should be reduced.
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Plotting a Transect: For a more detailed analysis, you can plot the extraction values along a line. This creates a graph that shows how well the two values match along a specific cross-section of your model. A good match indicates that your SVAS node density is sufficient in that area, while a poor match signals that you need to increase the density to improve accuracy. (To plot a transect use the “Conditions along a line” feature under the Analysis tool)
By using this analysis, you can strategically adjust your SVAS node spacing, ensuring your model is both mathematically accurate and computationally efficient.
Pro-Tip: When to Refine Your SVAS Node Spacing
While it might be tempting to get your SVAS node spacing perfect from the very start, it’s usually best to save this step for the end of your model development.
As you begin designing your model, you’re likely focused on the big-picture layout, such as defining different boundary conditions and domains. At this stage, your model will change frequently as you add, remove, and modify elements.
To ensure your model solves quickly during this iterative process, start with a wide SVAS spacing. This approach significantly reduces the number of equations the model needs to solve, which helps the model converge faster. This way, you can focus on the foundational design and layout without long wait times.
Once you have finalized the model’s overall design and are in the polishing phase, you can go back and strategically refine your SVAS spacing. By doing this last, you can use the
Leakage Factor and Extraction Points analysis to optimize the density of your SVAS nodes for maximum accuracy and efficiency. This workflow allows you to build a robust model without getting bogged down in minute details too early in the process.