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Example Problem 2. Representation of
Aquitards
Next we need to set up the scale of the drawing. From the "Special" menu select "Scale and Units". Change the choices so that it reads "Every 1 cm on the screen represents 100 units in the real world in both the x and y direction". Choose m as the units where it says "Label units as:". Click on the OK button.
Now view the entire page domain by pushing the zoom-out button on the lower left hand corner of the page. It will be convenient to have the origin of the drawing inside the drawing instead of at the edge of the drawing. To change that select "Special|Drawing Size..." and fill in the dialog box that appears. I suggest you use the values shown in the illustration below. Click on "OK" when you are done.
Go to the layers floater and make "MODFLOW Domain Outline" the active layer (click to the left of the eye icon). Select the Closed Contour Tool. The button should look like the one illustrated below. If it doesn't, select it with the mouse button and hold the mouse button down until a menu appears. Your choices will be symbols for "Closed Contour" at the top, "Open Contour" in the middle, and "Point" at the bottom. Once you have selected the closed contour, go to the drawing area and outline an area with dimensions of approximately 2000 m x 2000 m. Just click on each of the corners to outline the area. Use the scale on the sides of the drawing as a guide. If the mouse is over the drawing are, you can also see the coordinates of the mouse position beneath the toolbar.
When you are done, double click on the last corner and move the mouse slightly. A dialog box will appear. Enter 200 for the Value. You may also enter a contour name. The Value is the default grid spacing in the model - in this case 200 m. You can adjust any of the corners of the contour. To do so, select the arrow button and then select the corner you want to move and drag the corner to the new position. You can also move the whole contour by clicking inside it and dragging it to the new position. If you want to enter precise coordinates for a contour, the EditContours PIE that allows you to do so. If you have it installed, you can use it by selecting "File|Import <layer name>|Edit Contours"
Now change to the arrow tool. (Click on the button with an arrow.) Click on the contour that defines the domain outline and copy it to the clipboard. ("Edit|Copy") Next, switch to "Elevation Top Unit1" and paste the contour ("Edit|Paste"). You could change layers in the same way as before using the "Layers Floater". If the Layers floater is not visible, select "View|Show Layers Window". Another way to select the active layer is to select it in the combobox in the main Argus ONE window.
In the layers floater, the active layer has a check mark next to it and visible layers have an open eye icon. Invisible layers have a closed eye icon. (In the figure above, "MODFLOW Domain Outline" is the active layer and also the only visible layer.) Click on an eye icon to change a layer from visible to hidden and vice versa. Click to the left of the icon in an inactive layer to make it the active layer. You can show or hide all layers at once by clicking on the "All" or "None" buttons on the Layers floater.
Assign the contour you just pasted to the "Elevation Top Unit1" layer a value of -1000. To assign the value, double click on the contour and type the elevation in the dialog box that appears. Switch to "Elevation Bottom Unit1" and paste the domain outline contour to this layer as well. It will still be on the clipboard (unless you have copied something else in the meantime). Assign an elevation of -1100 to the bottom in the same way. (We'll look at another method to assign aquifer properties in the next example under "Using the Expression Editor to set Default Aquifer Properties and to Link Layer Properties".) Change to "Initial Head Unit1" and assign an initial head of 0 to the entire model by pasting the contour from the clipboard and assigning it a value of 0. Switch to "Hydraulic Cond. Unit1" and assign a hydraulic conductivity of 2.3e-5 to the entire model. Because this is a one layer model, you don't need to assign a value to the vertical hydraulic conductivity. Switch to Specific Storage, Unit1. Assign a specific storage of 7.5e-6 to the entire model. The layer is confined so you do not need to assign a specific yield.
An alternative method of assigning properties to the layers in this case would be to change the default values for each layer. To do this open the Layers Dialog box by selecting "View | Layers" from the menu or the "Layers" button. Select the "Hydraulic Cond. Unit1" layer from the list in the top half of the dialog box and then select the Kx parameter from the list in the bottom half of the dialog box. Click Fx button on the item for this parameter under the "Value" heading. The Expression Editor will appear. Enter the default value in the upper part of the Expression Editor. For example, for hydraulic conductivity, enter 2.3e-5.
Finally Switch to "Wells Unit1". You may wish to hide most of the layers at this point to have less clutter on the screen. We want to add a single well to the center of the model. We need to switch to the Point tool to do so. Click on the "Closed Contour" button and hold the mouse button down until a pop-up menu appears. Select the bottom selection which is the point tool. Click in the center of the model. A dialog box will appear. Enter -1000 for the top elevation, -1100 for the bottom elevation and -4e-3 for the stress. The -4e-3 represents a pumping rate of 4x10-3 m3/s out of the aquifer. A positive value would represent injection of water into the aquifer. By default A "Source" icon will be assigned to the well. You can change it to any of the other possible icons or have no icon at all using the same dialog box used to assign the pumping rate. In the illustration below, I've changed the icon to "None". You can re-edit the information for this well by double-clicking on the well with the Arrow Tool.
Now change to "MODFLOW Grid Density".
If you don't want to bother with setting up the density contours yourself, you can import the contours I used in this example. "Select File|Import MODFLOW Grid Density...|Text File..." and select the file named theis1_Grid_Density.exp. It's in the "example1" folder. (You can export the contents of any layer in much the same way. In this case, I selected "File|Export|Export MODFLOW Grid Density..." and then chose a file name.)
If you do wish to create the contours yourself, here is how we can do it. We will need to set up a grid with large cells near the edge and smaller ones in the center. This will make the flow simulation more accurate in the center where we will have a well. Near the edges, accuracy is less important to us so we want to have larger cells to speed up model execution. To maintain sufficient accuracy, adjacent rows and columns should not vary in thickness by more than a factor of 1.5. By setting up contours on the "MODFLOW Grid Density" layer, we can control how the grid is formed. Each contour will have a grid density that will control the spacing between rows and columns. There are several ways we can set up the contours that will work.
1. Draw a series of squares with the Closed Contour tool and give them densities ranging from 40 m in the center of the model to 150 m near the edge.
2. Choose "Special | Allow Intersection". This will allow contours to intersect one another. Hold the Shift key down while making a series of open contours at a 45o from one corner to the other. (Holding down the Shift key will ensure that the contour can only be drawn at an angle that is a multiple of 15o.) Assign these contours ranging from 150 near the edge to 40 near the center. It is important that the ends of these contours exactly meet one another. If they don't there will be a narrow row and/or column formed between the ends of the contours. If you have the cursor near the vertex of another contour, you will see that the cursor changes to a hollow cross. If you click the button while the cursor is a hollow cross, the next vertex will be exactly on top of the nearby vertex.
3. Choose "Special | Allow Intersection". This will allow contours to intersect one another. Hold the Shift key down while making a two open contours at a 45o from one corner to the other. The two contours will make an "X". It doesn't matter what density you assign to these two contours; you'll delete them before you create the grid. They will only serve as guide lines. Now draw a series of contour as illustrated below. Assign these contours ranging from 150 near the edge to 40 near the center. Finally delete the two guide lines you created initially.
When you are done, switch to "MODFLOW FD Grid" layer and select the "Magic Wand" Tool. Click it anywhere inside the area to be gridded and it will make a grid for you. When it's done, check to make sure that none of the rows or columns is more than 50% larger than any of it's neighbors. If you find any that are, either go back to "MODFLOW Grid Density", edit it and regrid or adjust some of the grid lines manually. I find it easiest to manually move the grid lines. To change grid lines manually, first put the cursor over a grid line. The cursor will change shape. You can then drag the grid line to a new position. Remember, the grid sizes you specify in "MODFLOW Grid Density" is more of a suggestion than a command. Argus ONE will adjust the cell size to completely fill each contour on the "MODFLOW Grid Density" layer. It is usually better to edit the grid density than to edit grid lines manually because if for some reason you must regrid the model, you may have to manually adjust the grid lines again.
You have now finished entering data for the model. To export the MODFLOW input files and run MODFLOW, switch to the MODFLOW FD Grid layer. From the PIEs menu, select "Run MODFLOW/MOC3D". You need to have MODFLOW-96 installed in C:\Modflw96.3_2\bin\. You will be given the option of just creating the MODFLOW input files or both creating them and running MODFLOW. Do the latter. You will be prompted to choose an export file name. You can use the default name and choose an appropriate directory. Argus ONE will show a progress bar while it is exporting the MODFLOW input files. When MODFLOW begins to run, a DOS window will appear on the screen. This will disappear when MODFLOW is done.
When the program is complete, select "PIEs|MODFLOW/MOC3D Post Processing". Select "MODFLOW (Head and Drawdown)" and click on the "Select Data Set" button.
Select the file Theis.fhd. (This is a text file containing the heads. Theis.fdn
contains drawdowns.) After a short wait, a dialog box will appear. From
it you can choose any of the data matrices produced by MODFLOW. Our model
only has one stress period so all of our choices are for stress period 1. We had 100 time steps but only choose to produce output
on every fifth step so all our time steps are multiples of 5; "TS 5",
"TS 10", "TS 15" ... "TS 100". Choose the last
time step, S:100. Next choose the type of chart that you would like to produce.
In this case choose "Contour Map". Click the OK button. Now change to the MODFLOW Post
Processing Charts layer and double-click on the Contour Map that is created
to customize it. For example, you may wish to change the contour interval
to an even number. You can also add text to the map using the "A"
button. Just push the "A" button with your mouse, click on the
map and type the text you want.
Now let's take a look at the Theis.lst file. This is the main MODFLOW output file. Save the model in Argus ONE and, if you wish, close Argus ONE. Open Theis.lst with any word processor. What we want to look at is the Water budget. To get to it fast, do a search for "DISCREPANCY". You should find some text that looks like the table below. Some of the numbers may be slightly different in your version.
VOLUMETRIC BUDGET FOR ENTIRE MODEL AT END OF TIME STEP 1 IN STRESS PERIOD 1 ----------------------------------------------------------------------------- CUMULATIVE VOLUMES L**3 RATES FOR THIS TIME STEP L**3/T ------------------ ------------------------ IN: IN: --- --- STORAGE = 2.5080E-03 STORAGE = 4.0000E-03 CONSTANT HEAD = 0.0000 CONSTANT HEAD = 0.0000 WELLS = 0.0000 WELLS = 0.0000 TOTAL IN = 2.5080E-03 TOTAL IN = 4.0000E-03 OUT: OUT: ---- ---- STORAGE = 0.0000 STORAGE = 0.0000 CONSTANT HEAD = 0.0000 CONSTANT HEAD = 0.0000 WELLS = 2.5080E-03 WELLS = 4.0000E-03 TOTAL OUT = 2.5080E-03 TOTAL OUT = 4.0000E-03 IN - OUT = 0.0000 IN - OUT = 0.0000 PERCENT DISCREPANCY = 0.00 PERCENT DISCREPANCY = 0.00
The most important number here for our purposes is the percent discrepancy. In this case it is 0.00. That is good. If the percent discrepancy is more than approximately 1%, the model is not sufficiently accurate. You should always check the percent discrepancy of every time step to make sure the error is sufficiently small. To see the results when the model isn't accurate enough, go back to the Stress/Solvers tab by selecting "Edit Project Info" from the "PIEs" menu. Change the solver to SOR. Then on the SOR page, change "converence criterion" to 0.1. Run the model and make a new contour map. It will probably look strange. In the example below, the contours are not symmetrical around the well. If you look at the percent discrepancy in the water budget, it will be well over 1%. In this case, it's easy to see that something is wrong just by looking at the contour map because we know that the drawdown should be uniform. In most models, you don't know what to expect. Looking at the water budget is one good way of recognizing problems with a model. (However, a low water budget error does not ensure that there aren't problems with a model.)
So far the only type of output we have looked at is a contour map. You might be interested in looking at some of the other types of output too. Argus ONE can produce Color Maps, 3D Surfaces and cross sections. The results you'll get are illustrated below. If you want to save both the old map as well as the new one, rename the layer with the map before selecting "PIEs|MODFLOW Post Proc..."
This simple problem introduces most of the techniques you will need to use MODFLOW-96 successfully with Argus ONE and the MODFLOW-GUI PIE.
Example Problem 2. Representation
of Aquitards
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