This tutorial gives a short introduction in creating a groundwater flow model from scratch. It yields a preliminary model that will be enhanced even more in section 11.6.
All steps in this tutorial were demonstrated during the first live iMOD webinar, recorded on May 2016. You can watch the recordings via the webinar page on the iMOD website (http://oss.deltares.nl/web/imod/webinars). (Be aware that some parts of the tutorial might be improved or edited in the mean time)
This is what you will do:
• Create the basic input files that are necessary to simulate a simple groundwater flow model;
• Enhance the model with an extraction well to compute the drawdown caused by the well;
• Simulate flowlines that describe the catchment area of the well;
• Experiment with extraction rates to compute the maximum sustainable yield without extracting water from the sea.
For this tutorial you need the following iMOD Data Folders:
• ISLAND.PNG/ISLAND.PNGW: this image sketches the outlines of the island
This file is located in/below the folder:{installfolder} \TUTORIALS \TUT_INITIAL_MODELING.
Note: {installfolder} refers to the full path of the directory you installed iMOD in (e.g. D:\iMOD).
Note: If you are a left-handed person and you converted your mouse button settings, ’left mouse button’ should be ’right mouse button’ and vice versa in these tutorials.
Beside this data you will need the iMODFLOW executable to make the final model computations.
1. If iMOD is installed correctly, the iMODFLOW executable is placed next to the iMOD-GUI executable. Also the keyword MODFLOW is added to the file IMOD_INIT.PRF with reference to the iMODFLOW executable.
Example of a content of an iMOD_INIT.PRF file.
USER ”D:\IMOD\IMOD_USER“
MODFLOW ”D:\IMOD\IMODFLOW.EXE“
See section 11.2 and chapter 9 for more information about the folder structure in iMOD and a description of IMOD_INIT.PRF (see section 9.1 for more information about this PRF file). Please restart iMOD after changing the IMOD_INIT.PRF file.
2. Launch iMOD by double clicking the iMOD executable in the Windows Explorer, and start by selecting the option Create a new iMOD Project. Click the Start button.
One of the first things one would like to display is an image of the outline of our island that we’re going to model. Let’s do that.
3. Select the option View from the main menu and then select Add Background Image ... from the dropdown menu. This will start the Add Background Image dialog.
4. Select the option Add from the dialog and select the file
{installfolder} \tutorials \TUT_INITIAL_MODELING \island.png from the Windows
Explorer, see section 5.3 for more information about this window.
5. Select the Apply button that closes the window.
6. Whenever the image does not appear click the Show Background Image (
) on the main menu.
Figure 11.49: Example of showing a topographical map using the main menu ’View’, ’Show Background Image(s)’ option.
This island represents a small tropical island (3 x 3 km) somewhere in the Pacific. It is surrounded by shallow waters with crystal clear water (light blue), white beaches all around (yellow) and meadows (light green) with some bush areas (dark green) on the hills. In the centre of the island there exists a small settlement with a few houses that use groundwater for watering their fields and cattle and use it as primary source for drinking water. They extract groundwater at the centre of the island (blue circle).
Many people have discovered the beauty of this island and plans arise to build a resort on the island. This will increase the pressure on the natural water resource and the question to be answered is: “How much water can be sustainably extracted from the subsoil, without attracting seawater in the near future to the pumps?”.
With this very simple example we will use iMOD to build a hypothetical model of this island. By means of this example we will illustrate the methodology in iMOD to create a groundwater flow model. At this stage we will ignore any effects of density-driven components caused by salt water. The following steps will be undertaken:
• build an IDF-file for the surface level of the island which will be our uppermost boundary of the system modeled;
• create an IDF-file that defines the boundary conditions of the model, for which part the groundwater head needs to be computed and for which part this is known beforehand;
• create an IPF file that describes the location and rate of the pumping well;
• create a runfile that describes the necessary files and values for the simulation;
• simulate the model using the runfile;
• create startpoints for the particle tracking simulation and carry out this simulation;
• modify the extraction rate in order to search for the maximum sustainable yield.
Okay, a lot of work needs to be done, so let’s go!
Our model will describe the groundwater flow between the surface level and the bedrock in the subsoil. Our first task is to get a digital representation of the surface elevation. Often this is available in the form of a Digital Elevation Model (DEM), unfortunately, we’re lacking this DEM for our island, so we have to sketch it ourselves.
7. Select the option Edit, Create Features from the main menu, then select the option Create an IDF from... and finally the option Polygons/Lines (GEN).. to start the Create IDF window.
With this functionality in iMOD we’re able to create simple features in the format that iMOD needs to perform a model simulation. In this case we would like to create the outline of the surface level and therefore we need to draw the contours of the surface level and assign appropriate levels to it. After that we can tell iMOD to interpolate from the contours. Okay, let’s start this by digitizing the shore of the island, the outer yellow contour, with an elevation of 0.0 m+MSL.
8. Make sure you’ve shown the topographical image of the island (repeat step 3. upto 6. whenever you don’t see the image);
9. Click the Draw button (
), this will start the Select window;
10. Select the option Polygon from the Shape types;
11. Click the Ok button.
Your cursor has been changed into the following cursor symbol
which means that you can start drawing a polygon. figure 11.50 gives an idea of how this first polygon should look like.
Figure 11.50: Example of how the first polygon should look like.
12. Click your left mouse button on the graphical canvas at the location of the first point of the polygon to be drawn. Repeat clicking your left mouse button to insert more point of the polygon. Whenever you are satisfied click your right mouse button to stop this process. Note: if you are a left-handed person and you converted your mouse button settings, ’left mouse button’ should be ’right mouse button’ and vice versa in these tutorials.
See section 4.4 for more details how to modify the polygon once you’ve created it.
Now we have to assign a surface level of 0.0 m+MSL to the drawn polygon.
13. Click the Information button (
) to display information on the content of the GEN file.
Figure 11.51: Example of the ’Attribute Values for ...:’ window.
With this Attribute values for ... window we can observe/change the attributes that can be added to the shapes (polygons, lines). We have to add a new attribute to the data to store the contour values of the surface level. Let’s do that.
14. Click the Add Attribute button (
) and enter the label [Level] in the field "Give an attribute name" in the Input window that arises.
Figure 11.52: Example of the ’Input’ window to add an attribute.
15. Click the OK button to return to the Attribute Values for... window. Observe that an extra column has been added to the table.
16. Enter the value [0.0] to the input field Level.
17. Select the option Use following column for gridding/interpolation and select the attribute [Level] from the dropdown menu. iMOD will use the values from this column during the interpolation.
Figure 11.53: Example of the ’Attribute Values for ...’ window.
Similarly repeat steps 9. to 18. to sketch the other polygons and lines of the surface elevation of the island. Choose the corresponding elevation values from the table and figure below. Please note that it is not necessary to add the attribute column named [Level] for each shape since this will be applicable for all shapes that are entered. Also use a [Line] feature to express the watershed on the more elevated parts of the island.
Table 11.1: Elevation of the island elements
| Island level description | type | elevation [m] |
| Hills ridge (west and east) | line | 15 |
| Hills feet (west and east) | polygon | 5 |
| Border green area | polygon | 2 |
| Island Boundary | polygon | 0 |
| Shallow water | polygon | -1 |
| Deep water | polygon | -5 |
Figure 11.54: Example of a final result sketching the surface level for the island.
19. To save the set of shapes listed in the ’GENs-tab’ window click on the Save As button (
) and save as ISLAND.GEN in the folder {installfolder} \iMOD_USER \SHAPES. Please note that it does not matter whether only some of the shapes were selected, all shapes listed in the ’GENs’-tab will be saved as one set of shapes.
Once we’ve outlined the surface level, we will interpolate the contours to a grid (IDF) with rastersize of 10 meter. This will be accurate enough for our simulation. However, gridsizes at this stage will not be determined for the final simulation scale. See section 7.9 for more information on scaling issues for model simulations.
20. Click the button GEN-Extent in the Create IDF from: Window to adjust the coordinates of the IDF extent such that the entire GEN will be included in the gridding.
21. In the boxes for the Lower Left and Upper Right coordinates of the extent (XLLC, XURC etc), please remove the centimeter values and round down or up to meters.
22. Enter a cell size of 10 meter in the field Cellsize (m).
23. Select the option [PCG (Preconditioned Conjugate Gradient)] from the Method dropdown menu.
This interpolation method will follow the given contours accurately giving a smooth representation of the entered contours.
24. Click the Apply button and save the gridded IDF-file in the folder iMOD_USER \DBASE as SURFACE_LEVEL.IDF. Probably you need to create this folder first!
Figure 11.55: Example of a resulting topography of the island.
It is completely irrelevant where files are saved actually; however, in order to keep your project organized well, it is advisable to create a clear structure in which you save all files that are related to the model.
Note: Commonly, we use the foldername DBASE to store all model files. So whenever we refer to the folder DBASE in the coming parts of this tutorial we actually denote the IMOD_USER \DBASE folder.
25. Use your experience from the Tutorials 1, 2 and 3 to create a 3D image of the topography we just created.
Figure 11.56: Example of a 3D image of your created island.
Creating the boundary conditions Okay, now we’ve outlined the uppermost boundary of our model, we will specify those areas that are part of the model simulation (active areas) and areas that have fixed values (fixed or non-active areas) for the hydraulic heads. We will use IDF-Calc and IDF-Edit that you both have used in section 11.3 (we assume you finished section 11.3 before you moved on to section 11.5). Okay, we now make an IDF with similar extent and cell size to use for the definition of boundary conditions, therefore we can copy the SURFACE_LEVEL.IDF.
25. Select the [SURFACE_LEVEL.IDF] from the iMOD Manager and Click the IDF Calculator button.
26. Make sure the entered formula for making a copy of is [C=A]. Because we only selected one file, the field Map B is empty.
27. Enter the name of the IDF-file to be created (e.g. BOUNDARY.IDF) in the field Map C on the Algebra tab on the Map Operations window. Make sure you use the same folder name as the one used for ..\DBASE\SURFACE_LEVEL.IDF.
28. From the section “Select the extent ...“ select the option Map A to create an IDF that has the same dimensions as the IDF-file mentioned by Map A (i.e. SURFACE_LEVEL.IDF).
29. Click the Compute button.
The new file BOUNDARY.IDF is drawn and listed in the iMOD Manager. Please note that BOUNDARY.IDF is a copy of the SURFACE_LEVEL.IDF. Now we are going to determine in the file BOUNDARY.IDF the active areas of the simulation (value= 1) by selecting the area with surface level values above zero. Areas with values less than zero will be fixed areas (value=-1). So let’s continue with that.
Figure 11.57: Example of assigned active (=1) and fixed head cells (=-1).
30. Select the [BOUNDARY.IDF] from the iMOD Manager and click the Map option from the main menu, select IDF Options and then IDF Edit option to start the IDF Edit window.
31. Click the Select button to start the IDF Edit Select window.
32. Select the option [\(>\)=] from the dropdown menu Logic in the groupbox Evaluate IDF A.
33. Click the Get Selection button to get a selection of all cells that have values greater and equal zero.
Figure 11.58: Example of the selection of cells with values greater or equal to zero.
34. Click the option Close to return to the IDF Edit window again.
35. Click the Calculate option from the IDF Edit window to start the IDF Edit Calculation window.
36. Select the option New Value in the group Define Values by and enter the value [1] so it says New Value [=] [1.0].
37. Click the Calculate button.
38. Click the Close button and click Yes on the appearing window asking you to be sure to leave this Edit environment.
Repeat steps 31. upto 38. to adjust all values that have values less than zero and calculate those values to become -1.0.
Creating the well file Last thing we need to do is to create an IPF file (iMOD Point File) to represent the well in our model. We have a single well situated in the centre of the island, in the following steps we will create this simple file inside iMOD; however, it can be easily modified/created outside iMOD. For large datasets it is often more convenient to process these types of data in another program.
39. Use the CTRL-left-mouse to deselect the BOUNDARY.IDF in the iMOD Manager.
40. Click the Refresh button (
) to refresh the graphical canvas.
41. Make sure you’ve switched the background image on, if not press Show Background Image (
) again.
42. Select the option Edit from the main menu, then select the option Create Feature and then IPF’s to start the Create IPF’s window.
43. Click the Draw button (
) and click your left mouse button when the cursor is on the location of the well.
44. Click your right mouse button to return to the Create IPF’s window.
45. Click the Information (
) button to open a window displaying the content of the file.
iMOD already presents the first free data column of the IPF, named ID. The two non-free columns (X-coordinate and Y-coordinate) are not displayed because editing is not permitted. However, for a standard ipf representing a well, the first free column should contain the abstraction rate Q. So we will change the name of the column and fill in the abstraction rate.
46. Click in the field with value 1 to select the column.
47. Click the Rename Shape button (
) and enter the label [Q] in the Input window that arises.
48. Click the OK button to return to the Attribute Values for... window. Observe that the column name has been changed.
49. Enter a value of [-500.0] for the extraction rate in the data field. Rates are entered in m\({}^{3}\)/day.
Figure 11.59: Content of the IPF file before and after editing.
50. Click the Apply button to close this window and return to the Create IPFs window.
51. Click the Save As button (
) to save this file in . \DBASE \WELL.IPF.
Conceptual model - summary For this hypothetical model, these three files ..\DBASE \SURFACE_LEVEL.IDF, ..\
DBASE \BOUNDARY.IDF and ..\DBASE \WELL.IPF are the only spatial varying data sources. The other model input are constant values throughout the model domain and we will assign those values by creating a modeling project in the following section. In the next figure we’ve given a sketch of the subsoil and flow patterns that might occur.
Figure 11.60: Sketch of a estimated flow pattern that might occur in our island model.
We simulate this model with three model layers. The first model layer has a thickness of 1.0 meter (almost no horizontal flow in that model layer) to intercept the recharge. From there water will migrate to the deeper layers 2 and 3. The third model layer is the actual aquifer from which water is extracted via the well screen.
Schematic, the model can be represented by the following figure:
Figure 11.61: Schematic representation of the model.
Creating a Modeling Project and defining a Runfile iMOD arranges a model project by a project file, a so called PRJ file. This file stores all parameter files (IDF, IPF, GEN etc.) that are assigned to particular phenomena in the model. From a project file (*.PRJ) you can generate a runfile (*.RUN) that will be used eventually to simulate groundwater heads. You can imagine that with the same set of parameter files you can simulate many different configuration or different scenarios. For
instance steady-state simulation versus transient simulations, a scenario with 50% abstraction, an increase of aquifer permeability or a simulation for a smaller subwindow.
That kind of configurations or scenarios can be initiated by the Project-Manager. Well, probably it is better just to start with it.
52. Select the option View from the main menu and select the option Project Manager to start the Project Manager window.
This window shows all available packages that are supported by iMOD. Still many will come in future though. Okay, we have to fill in this project manager with our model configuration. In the table, shown below, we have outlined the requirements for this particular three-layered model.
Table 11.2: Model requirements for a confined, steady-state three layered model.
| Parameter | Model layer | IDF file / Constant Value |
| (BND) Boundary | 1 | . \DBASE \BOUNDARY.IDF |
| 2,3 | 1 | |
| (SHD) Starting Heads | 1,2,3 | 0.0 m+MSL |
| (TOP) Top Elevation | 1 | . \DBASE \SURFACE_LEVEL.IDF |
| 2 | ’SURFACE_LEVEL.IDF’ with Addition Value -1.0 m | |
| 3 | -15.0 m+MSL | |
| (BOT) Bottom Elevation | 1 | ’SURFACE_LEVEL.IDF’ with Addition Value -1.0 m |
| 2 | -15.0 m+MSL | |
| 3 | -20.0 m+MSL | |
| (KHV) Horizontal Permeability | 1,2,3 | 25.0 m/day |
| (KVA) Vertical Anisotropy | 1,2,3 | 1.0 |
| (KVV) Vertical Permeability | 1,2 | 25.0 m/day |
| (WEL) Wells | 3 | . \DBASE \WELL.IPF |
| (RCH) Recharge | 1 | 0.5 [standard unit is mm/day] |
Okay, let us fill in the boundary conditions in the Project Manager.
53. Select the option [(BND) Boundary Conditions] in the Project Definition list.
54. Click the Properties button (
) to start the Define Characteristics for window.
In the current window you can specify how the package (in this case the Boundary Condition) needs to be configured. Let us fill this dialog for the boundary condition for model layer 1.
55. Because we enter data for model layer 1, the field Assign Parameter to model layer …must be set to value [1], if this is not the case by default.
56. Specify a Parameter Multiplication Factor of [1.0], if that is not the case by default. Any parameter can be multiplied with the associated factor during runtime. You can use this factor to easily perform some sensitivity analyses on parameters and their effect on the distribution of the groundwater head.
57. Specify a Parameter Addition Value of [0.0], if that is not the case by default. Any value can be added to or subtracted from a parameter.
58. Select the option Assign Values from a File and click the Open File button. Select the file
. \DBASE \BOUNDARY.IDF from the appropriate folder. This file we’ve created in step 38., remember?
Figure 11.62: Example of the ’Define Characteristics for:’ window, filled in for Boundary Condition (BND).
59. Click the Add System button and this will return you to the Project Manager window. You’ll notice that the option [(BND) Boundary Conditions] has been altered. You can select the “plus” sign to expand the tree view. You’ll notice the entered fields in the presented string.
Let us fill in the boundary conditions for the remaining 2 layers.
60. Select the option [(BND) Boundary Conditions] in the Project Definition list.
61. Click the Properties button (
) to start the Define Characteristics for window.
62. This time we enter data for model layer 2, so the field Assign Parameter to model layer …must be set to value [2].
63. The Boundary Condition for layer 2 is value 1.0 so select the option Assign a Single Value to all cells and specify the value [1.0]
64. Repeat the last 4 steps to add the Boundary Condition for layer 3.
Now let us fill in the remaining parameters from table 11.2.
65. Repeat the steps 53. upto 59. for the remaining parameters. Take a look at the following suggestions and figures for support.
Take care to select the parameter name in the Project Definition list each time you want to open the Define Characteristics for window to enter NEW parameters.
Figure 11.63: Example of selecting a parameter in the ’Project Manager’ window: in this example first ’(BOT) Bottom Elevation’ is selected to expand the treeview by clicking the ’+’-sign.
Whenever you select the expression under an expanded branch in the treeview in the Project Manager list, you’ll be able to edit an existing entered parameter; see the example below.
Figure 11.64: In this example the exisiting (BOT) parameter set of layer 1 is selected. Click on the ’Properties’ button to open the ’Define Characteristics for:’ window to edit the Bottom Elevation parameters.
The figure below is an example of the Project Manager window after filling in all parameters.
66. Check if each module contains layer definitions for layer 1, 2 and 3 (see the orange circle). If not, layer definitions can be changed with the Properties button.
67. Check if your parameters refer to the right layers and if the Addition Value for TOP layer 2 and BOT layer 1 is -1. (see the blue circles).
Figure 11.65: Example of Project Manager window after filling in a model configuration.
For the meaning and explanation of the available buttons on the Project Manager window go to section section 5.5.
Now the model parameters are added, we need to add also settings of the solver. iMOD uses the Preconditioned Conjugate Gradient method (PCG), the same solver used in MODFLOW.
68. In the Project Manager select the Project Definition "(PCG) Preconditioned-Conjugate-Gradient" and click on the Properties button (
).
69. In the PCG Settings window click on Apply to add the default solver settings.
70. Click the Save As button (
) to save this model configuration in a PRJ file. This file may be loaded again whenever we need to modify this project.
The next step will be to define and start a model run.
71. Click the Start Simulation Manager button (
) and the Simulation Manager window will open (see figure 11.66).
Figure 11.66: The Simulation Manager window, define the modeltype and modelname
Within the Simulation Manager window it is possible to configure runs of all supported simulators (e.g MODFLOW2005, MODFLOW6, SEAWAT). On the different tabs several options are available, such as defining the location and raster size of (sub)models, differing time-discretisations, (de)activating of packages, (de)selection of outputs etc. iMOD will automatically fill all tabs with the information available from the Project Manager.
For the full description of this window and the different tabs, see section 5.5.5.
72. Click on the tab Layers / Packages. See in the left table the available topics. From the right tables we read that we loaded a three-layered model.
73. Click on the tab Space dim. and see that the default cell size is set to 25m. Let’s keep it that way.
74. Click on the tab Time dim.. We are not able to create a transient model run since we do not have any transient data. We will generate a runfile for a three-layered model.
In a few moments we will run the model. To be able to analyse the heads afterwards iMODFLOW must export an IDF for each layer. Later on in this TUTORIAL we will configure a flowpath analyses. Therefore we need an export of the flow field.
Let’s set these output variables.
76. In the list of Results Variables select the topic (SHD) Starting Heads. See that all layers are selected. This means that iMODFLOW will save all calculated HEADS. This is the default.
77. To save the flux between cells (the flowfield), select the topic (BND) Boundary Condition. In the right list select all layers 1 to 3 with Ctrl+left mouse button (see figure 11.67). With selecting the topic BND, iMODFLOW will internally select all flux topics (FLF, FRF, FFF, BND) as a group. Read more about output settings in the section describing the keyword ILSAVE in section 10.9.
Figure 11.67: Select output for (BND) Boundary Conditions for all layers
78. Return to the Main tab.
79. Type the name you want for your model in the field Enter or Select Output Folder:, e.g. ’ISLANDQ500’.
80. Click the START... button and iMOD will show a window with a check if you really want to (over)write the *.RUN file you just typed.
81. Click Yes. NOTE: the following 2 windows might appear:
1. An error window like below.
Figure 11.68: Error window after running a model simulation.
This is an indication that there was an error in running the MODFLOW model.
In most cases the cause can be found in the input parameters, sometimes in the window size. A recurrent cause is that a parameter definition for one of the layers is missing. See the example in figure 11.65. Please check the topics in the Project Manager window (BND, TOP, BOT, KHV, KVA, KVV and SHD). Don’t miss out layers and prevent for layers appearing in the list twice.
For more insight in the Modflow run, check the echo of MODFLOW while running or the MODFLOW output file *.LIST. For some help on that, read the Intermezzo on the next page (p.787).
2. Depending on the level of the safety settings on your computer a Windows Security Alert might pop up (see figure 11.69). Just click the Allow Access button and this window will probably not appear in the future.
Figure 11.69: Fire Wall message appearing before running a model simulation.
The actual model simulation is carried out by the iMODFLOW executable as referred to in the *.PRF file (see section 2.7). It will run in the DOS box attached to iMOD. Please check whether you can find this window and examine the results, it will look more or less like this:
Figure 11.70: Example of a DOS window showing the echo of the model simulation.
Intermezzo
Result folder
What will iMOD do when you start a model:
• Your RUN file is saved as \IMOD_USER \RUNFILES\ISLANDQ500.RUN.
• A RESULT folder is created: \IMOD_USER \MODELS\ISLANDQ500
• In the RESULT folder a copy of the RUN file is saved.
• In the RESULT folder a batch script (RUN.BAT) is saved to re-run the model.
In these Tutorials we will work purely within the iMOD GUI. But you can re-run this MODFLOW model outside iMOD by double clicking this batch script (RUN.BAT) from Windows Explorer or Total Commander. This can be very convenient whenever some trial-and-error computations have to be carried out.
Logfile *.LIST
This logfile is very helpful to track down issues when there is an error in your run. In case your model did not run successfully, only the folder ...\ISLANDQ500 \mf2005_tmp is available. In that case, check the standard MODFLOW output file IMODFLOW.list in this folder. In short, this iMODFLOW standard output file contains info on:
• the model discretization
• the model time and length units
• the processed input packages
• the solver used and how the iteration process progressed
• the volumetric budget for the entire model, including the percent discrepancy
• elapsed run time
Your simulation ended correct if the last lines of the LIST file should look like this:
Run end date and time (yyyy/mm/dd hh:mm:ss): 2018/10/25 11:31:05
Elapsed run time: 4.261 Seconds
If this is not the case, the last file mentioned might be the cause.
At the end of the LIST file the overall volumetric budget is printed and can be checked for the resulting water balance error (IN - OUT):
Figure 11.71: Example of the volumetric water balance as printed by MODFLOW in the iMODFLOW.list-file.
Model results Let’s have a look at some model results.
3. Close the Model Simulation window by selecting the Close button.
4. Select the option Map and then the option Quick Open to start the Quick Open window, see section 6.2. With this window it is easy to open and view results from a model simulation.
5. Select the option [ISLANDQ500] from the Variant dropdown menu.
6. Select the option [HEAD] from the Topic dropdown menu. If only the Topic ’mf2005_tmp’ is available, your model did not run successfully! In that case, check the Result Folder and the *.LIST file (see next paragraph).
7. Select the options [1], [2] and [3] from the Layer list menu.
8. Click the Open button.
iMOD will load the selected results files (HEAD for model layers 1, 2 and 3) into the iMOD Manager and displays the result on the graphical canvas. Use your experience learned from the previous Tutorials to display the computed heads as shown in the example on the next page. To show the IDF by contourlines, open the Legend window and click the Contourlines button (
).
Figure 11.72: Isolines of the computed hydraulic heads of the island.
As we can see by the computed hydraulic head, the gradient towards the well in the centre of the island tends to be such that no water is extracted from the ocean. To illustrate this even more we can compute pathlines that show the actual path through the subsoil that groundwater follows from the location of infiltration towards the location of extraction. We call that particle tracking which is explained in the next section.
Create Startpoints for a Pathline Simulation
9. Select the option Toolbox from the main menu, then Pathline simulation and then the option Define Startpoints to open the Open/Create a Startpoint Definition window, see section 7.13 for more detailed information.
10. Enter the name [ISLAND] in the input field.
11. Click the Open / Create.. button to open the Start Point Definition window.
iMOD offers the possibility to define startpoints for any particle tracking independently of a model, modelsize and or cellsize. Startpoints will be defined by means of a polygon a line and/or points and startpoints are distributed within the limits of that/those polygon(s)/lines. So, let us define startpoints on the island.
12. Click the Draw button (
) and start drawing a polygon wide around the island, so we can observe whether seawater is flowing to the well too. We’ve created a polygon before (see step 8.) so you’ll manage to get this done.
13. Select the right mouse button to stop drawing a polygon.
14. Select the Definition tab and enter a cellsize of [50] meter for the Distance X (m) and Distance Y (m), so each 50 meter we will have a particle starting.
15. Select the Open IDF button (
) and select the computed hydraulic head of layer 1 from \IMOD_USER\Models\ISLANDQ500\head\head_steady-state_l1.idf. This hydraulic head is used as Top Level, use the same IDF for the Bottom Level.
16. Click the Preview button to get an idea of the density of the particles to be started.
Figure 11.73: The ’Start Points Definition’ window including a Preview of the spatial distribution.
17. Click the Close button to close this Start Point Definition window. Whenever iMOD asks to overwrite the current [ISLAND.ISD], do so.
So now we’re finished creating startpoints, let’s use them in the pathline simulator.
Running the Pathline Simulator
18. Select the option Toolbox from the main menu, then the option Pathline simulation and then the option Start Pathline Simulation. This will start the Pathlines Simulation window; see section 7.14 for more detailed information.
19. Select the model [ISLANDQ500] in the list at Existing Results under Models.
iMOD will search in the appropriate folders to see whether all necessary files are available. For a particle tracking you need at least the budget terms in the x, y and z direction, these files are stored in the . \BDGFRF, . \BDGFFF and . \BDGFLF, respectively. iMOD will display the availability of those files whenever you select a model result.
In case no budget terms are found, run the model again from step 75. and make sure budget files are saved.
The example shown here will only highlight the most important steps to perform the particle simulation; however, it is difficult to explain the results whenever one cannot fully understand the technique behind it. So please, read some documentation on particle tracking, done by D.W. Pollock (USGS OpenFile Report 94-464).
Figure 11.74: The ’Pathline Simulation’ window; tab Model and tab Input.
20. Click the Input tab on the Pathlines Simulation window.
On this tab we need to tell iMOD the specific information that is needed for the particle simulation. Most important are the top- and bottom interfaces for the two model layers, see section 7.14 for more detailed information on this topic. Let’s fill it in quickly.
Important note "do not close this window before you saved your input otherwise you lose all your work"!
21. Click the Properties button (
) next to the Boundary Settings to display the Input Properties window.
22. Enter the Full Path of your file [..\DBASE \BOUNDARY.IDF] in the first row and enter [1] for the second and third row as you can see in figure 11.75. The particle tracking algorithm will use this information to exclude areas where the values in the IDF-files are less or equal to zero.
Figure 11.75: The ’Input Properties’ window for the Boundary Conditions.
23. Click the Apply button to use the entered values as input.
24. We are not ready yet filling in all input but click the Save As button (
) to save this part of the input to an IPS file.
Before we enter the Properties for the Top- and bottom Files we need to create another IDF file. Remember our conceptual model of the Islans model? For the bottom of layer 1 (and the top of layer 2) we need an IDF which has an elevation 1 meter below the surface level IDF; you can create such an IDF in the iMOD manager without closing the Pathlines Simulation window.
25. Go the the iMOD manager and select the file DBASE\SURFACE_LEVEL.IDF
26. Click on the IDF Calculator (
) button, type the Formula ’C=-1+A’ and assign e.g. the name
DBASE\SURFACE_LEVEL_MINUS_ONE.IDF to Map C. In the next steps we will refer to this file.
27. Return to the Pathlines Simulation window and the tab Input.
28. Click the Properties button (
) next to the Top- and bottom Files field to display the Input Properties window.
29. Specify the tops and bottoms (IDF names or single value) according to the table on page 787; the specification for the top- and bottom files could look similar to the figure below.
Figure 11.76: The ’Input Properties’ window for the Top- and bottom Files.
30. Click the Apply button to use the entered values; iMOD will check the existence of the specified files.
31. Select the Properties button next to the Porosity for aquitards/aquifers field and fill in the Input Properties appropriately. Use the configuration as mentioned in the table on page 787 and adjust the following 2 parameters.
Table 11.3: Adjust the following 2 parameters
| Parameter | model layer | IDF/Constant Value |
| Porosity Aquifer | 1,2,3 | 0.3 |
| Porosity Aquitard | 1,2,3 | 0.1 |
32. Click the Save As button (
) to save the input properties to an IPS file. Next time you can use the Load button (
) to read the input properties from disk. Whenever you save the settings as [. \IMOD_USER \SETTINGS \IMODPATH.IPS], iMOD will read this file automatically each time you start the Particle Simulation window.
We now need to specify how the starting points for the simulation are defined.
33. Click the Open button at the right of the input field which states Defined via a Startpoint Definition File (ISD).
34. Select the [ISLAND.ISD] from the folder . \IMOD_USER \STARTPOINTS.
35. Click the Open button to file and return from the File Selector window.
This is the starting definition file that we’ve created earlier.
For now we will skip most of the configuration setting in the other tabs, but feel free to have a look in more detail at section 7.14.
36. Click the Result tab and make sure the Trace Direction is [Forward]. We compute the particles from the groundwater elevation upto the discharge location (a well and/or the ocean).
37. Select the option Save Flowpath so we will get a file that describes the entire flowpath of each particle.
38. Enter a name for the yielding flowfile in the white rectangle (or select an existing one from the menu to overwrite), e.g. [ISLAND]. iMOD will save Flowlines in an *.IFF file (End- and Startpoints are saved as *.IPF file).
Figure 11.77: The ’Result’ tab on the Pathline Simulation window.
39. Click the Start button. This can take a while, especially whenever you have a lot of particles.
Bear in mind that whenever you have a lot of particles to examine, but you’re not actually interested in their paths but only in their age at interception, consider the IPF files as alternative to flowlines. Those files are much-much smaller and can be examined quicker.
40. Click the OK button on the Information summary that will be displayed after the particle tracking is completed.
iMOD will load the computed IFF file and presents it like black lines. So let’s colour it by their age, which makes more sense.
41. Click the Map option from the main menu, select the option IFF Options and then the option IFF Configure to display the IFF Configure window. See section 6.9.1 for more detailed information about the functionalities on this window.
42. In the section Apply colouring to chose the option Apply to and select the [TIME (YEARS)] from the dropdown menu.
One of the other items to be plotted is the velocity, that is computed as the flux (m\({}^{3}\)/day) divided by the porosity (-) divided by the area (m\({}^{2}\); width*model layer thickness). A change in porosity will change the velocity linearly and therefore the age of the flowline, but will, however, not affect the shape of the pathline.
43. Click the Close button to redraw the IFF with the assigned adjustments.
Figure 11.78: Example of a two-dimensional image of pathlines.
44. Zoom to the extent of the IFF file with the special zoom button.
45. Go to the menu Toolbox and select the option 3D tool.
From the 3D image below, it is clear that most of the water penetrates vertically to the deeper subsoil and then flows to the well. Given that the material is highly permeable (25m/day) and homogeneous. Please note that the vertical scale is always very much exaggerated! In the next section 11.6we will enhance this model to include more resistance in the subsoil which will affect the pathline behaviour.
Finally, our major question is still unanswered.
Figure 11.79: Example of a three-dimensional image of pathlines near the well.
Sensitivities and sustainable yield
So, now we know that an extraction of 500m\({}^{3}\)/day will be sustainable, we’re still wondering what the maximum will be. It will be your task as hydraulic engineer to give an answer to that question by simulating a variety of model simulations for different extraction rates for the well. You could vary the permeability too, since this parameter is often uncertain. It will be interesting to illustrate the accuracy of your sustainable yield estimation with a bandwidth that expresses the inaccuracy of the permeability too.
As stated above, after each model simulation you should check the total water balance of the model in the iMODFLOW.list file located in the model directory ..\MODELS \ISLANDQ500 \mf2005_tmp), it shows the total summary of the model simulation. If you scroll down, you’ll see the total water balance for the model.
In the example above shows the quantity of water flowing in from the sea is close to zero while the amount of water flowing out to the sea is 4081m\({}^{3}\)/day. You could use these as evaluation criteria as well!