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Working in Model Mode You enter Model mode two ways. If you are creating a new model during this session at the computer or modifying an existing grid, you progress from Grid mode to Model mode. If you open a model in Grid mode that already has a grid, you will go directly into Model mode. Model mode is used to add layers to the two-dimensional grid created in Grid mode, to specify the hydrologic properties for the model, and to set the stresses and stress periods for the model. After you complete these steps, the model is ready to run. The steps to follow in Model mode are discussed in this section. You will notice that certain commands must precede others within a Graphic Groundwater mode. For example, you cannot enter Model mode until you create a grid in Grid mode, and you cannot run a model in Model mode until you specify hydrologic properties for each grid cell. Most menu commands in Model mode, however, can be accessed in any logical order. For example, you can first establish all the layers for the model, and then set the values for the different properties in any order you choose. Alternatively, you can create layers one at a time, first setting all the hydrologic properties for each layer before creating a new layer. Table 2.1 Aquifer properties and discretization data for problem 14 (after Andersen, 1993).
Select the Preferences command under the Edit menu, or alternately, click on the Preferences button (the one with the large ‘P’ as an icon) in the toolbar. The Preferences window will appear. The Preferences window is actually seven separate windows accessed by clicking on one of the tabs aligned along the top edge. Click on the Basic Information window. Now click on the Transient option button, and activate (click on) the River and Well check boxes (make sure that they are checked). Click the Save Results check box. Click on the Solver Packages window. Click on the SIP option button and enter 5 into the Number of Parameters text box. Click on the Calculated Seed option button. Click on the Output Control window. Click on the Hydraulic Head check box (make sure it displays a check mark) and the Rivers check box (within the Budgets frame). Click on the All Time Steps option button. Finally, click on the Finished button. The process outlined above has created a MODFLOW model that is transient, includes the well and river packages, will be solved with the strongly implicit procedure with a calculated seed and five iteration parameters, and outputs hydraulic head and river flow from river cells at the end of every time step. Click on the Overlays menu and the Well command. The Overlay window (with ‘Well’ as a title) will appear. You can move this window out of your way by dragging it by its upper right corner anywhere on the screen. Alternately you may minimize the window by clicking the small command button in the upper right corner of the Overlay window. Clicking this button again will restore the Overlay window. Before you set the discharge for a well, you must first add a well stress to the model. Click on the Add Stress button. Note that the Stress drop down list becomes active, and the number of the stress you created is displayed. All wells in a stress must share the same layer (actually they must share the same slice; more on slices later), period, and discharge. This problem has one well. Click on the cell in the upper left corner of the grid. The cell is highlighted, which means it appears in inverted colors. Enter a value of –2500 in the Discharge text box. Click on the Set Stress button. Immediately, the number 1 will appear in the center of the highlighted cell (or a dot if the cell is too small to display the number). The inner border of the cell will change. Click the mouse anywhere in the Model window outside of the grid. The cell in the upper left corner will no longer be highlighted. Its inner border is now green, indicating that it includes an external stress, in this case a pumping well. The value in the cell represents the stress number for this well (Chapter 3 will explain the concept of Cell Stresses in greater detail). Click on the Overlays menu and the River command. The Overlay window will change, displaying ‘River’ as the title and adding additional text boxes and command buttons. Click on the Add Stress button. Enter values 0, 200, and –2, respectively, into the Stage, Bed Conductance, and Bed Bottom Elevation text boxes. Move the mouse pointer to the cell in column 4, row 1. Drag the pointer vertically down in column 4. Note the box, or focus rectangle, drawn on the screen. Release the mouse button. Any cell that touches the box becomes highlighted. Click on any cell in the grid. Press and hold the Shift key. Point and click on any cell not already highlighted. Note that this will highlight a cell and that any cell already selected remains highlighted. Drawing focus rectangles or selecting cells while you hold the Shift key down are the two means for selecting groups of cells for data entry. Make sure that every cell in column 4 is highlighted. Click on the Set Stress button. Again the cells will contain a value (1). Click on the Model screen anywhere outside of the grid. The inner border of the cell changes to green, indicating that these cells contain a river stress, all belonging to river stress 1, all with river characteristics as set in the Overlay window text boxes. Graphic Groundwater is a consistent unit model. In other words, once one property is set in terms of feet, all other properties that include length must measure length in feet. We set the length unit for this model when we set the scale. Since we want our hydraulic conductivity in units of feet per day, every other property that includes time must measure time in days. Notice the Stress Periods menu at the top of the screen. Select the Stress Period Qualities command under the Stress Periods menu. A window that includes Duration, Time Steps, and Multiplier text boxes will appear. The duration of the stress period must be in days in order to maintain consistent temporal units (Table 2.1 specified hydraulic conductivity in feet per day). Enter the values from Table 2.1 into the appropriate text boxes, then click on the Finished button. Point and click on the Model menu, then the Flow command. The Run Model window will appear. The default values in the Tolerance, Maximum Iterations, and Accelerator text boxes are fine for this model. Press the Start button. If everything is right with the world, and your model, Graphic Groundwater will pass your data to MODFLOW. Hydraulic heads will be computed at each of 20 time steps, up to a total model duration of 365 days. This will take only a few seconds. Note that you can cancel the model at any time just by pressing any key. The progress of the model (maximum error at the end of each iteration) is indicated in the Error list box. If the errors become unreasonable, cancel the model and check your input data. When MODFLOW converges (the Error list box stops scrolling) click on the Finished button. Click on the Results menu, then on Head. Graphic Groundwater displays the Results window and the hydraulic head in the center of each cell for the first time step. The values for other time steps are displayed by clicking on the arrow adjacent to the Step drop down list in the Results window. Select time step 20. Note how the model time, listed in the Results window, changes. Point and click on the Display menu, then the Contour command. The range in hydraulic head is displayed in a window. Select a starting contour line and a reasonable contour interval. We suggest a starting contour of -8 ft, with a 0.5 ft interval. Point and click on the Start button. Graphic Groundwater will calculate, then display a contour map of hydraulic head. Unlike property overlays, a result overlay cannot be changed from within Graphic Groundwater. You can select a cell however. Select the cell in Column 4 and Row 1 (4,1). Graphic Groundwater displays the head at the bottom of the screen in the center panel of the status bar. You can move to other cells and observe their heads with the arrow keys supplied on most keyboards or point and click on individual cells. Compare the head in Cell (4,1) for each time step with those listed in Table 2.2. You can change the information displayed on the screen. Point and click on Preferences under the Edit menu (or click on the Preferences button on the toolbar). Click on the View Preferences window. Turn on/off the features that you want to include or exclude by clicking on the appropriate check box. Point and click on the Results menu, then Flow into Stress Cells, then the Rivers command. Graphic Groundwater displays the flow in cubic feet per day from each cell to or from the river. Compare the flow rate in Cell (4,1) at the end of every time step with those listed in Table 2.2. If you set up the model correctly, the numbers should match to the second or third decimal place.
Table 2.2. Hydraulic head and river budget at Column 4, Row 1. Head in feet, river budget in cubic feet per day (data after Andersen, 1993).
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