iMOD User Manual version 4.4 (html)

11.6Tutorial 6: Model Simulation

This tutorial gives a short introduction in starting a groundwater flow model simulation. See for more detailed references Model Scenarios and Model Simulation.

Outline

This is what you will do:

• • Understand the content of a model configuration file, i.e. a runfile;

• • Simulating a groundwater flow model for different cell sizes and areas of interest;

• • Understand the resulting folder structure with results;

• Computing and visualizing a waterbalance of the model;

• Defining a simple model scenario and include such a configuration to an original model configuration.

• • Applying the new PKS-solver to simulated the model parallel.

Required Data

For this tutorial you need the following iMOD Data Folders:

• • BND: IDF-files that describe the boundary conditions;

• • DRN: IDF-files that describe drainage conditions;

• • KDW: IDF-files that describe the horizontal transmissivity;

• OLF: IDF-files that describe the overland flow conditions;

• RCH: IDF-files that describe the natural recharge;

• • RIV: IDF-files that describe the river conditions;

• • SHD: IDF-files that describe the starting head conditions;

• VCW: IDF-files that describe the vertical resistance;

• WEL: IPF-files that describe the wells;

• • TUT_MODEL.RUN: file that describes the model configuration and refers to the above mentioned folders;

• • SCENARIO.GEN: file that describes the area that needs to be manipulated.

All these files are located in/below the folder:{installfolder} \tutorials \TUT_Model_Simulation.

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 model computations.

Getting Started

• 1. Copy the TUT_MODEL.RUN into the . \IMOD_USER \RUNFILES folder.

• 2. Launch iMOD by double click on the iMOD executable in the Windows Explorer, and start by selecting the option Create a new iMOD Project.

• 3. Open the TUT_MODEL.RUN in a text editor (e.g. Notepad++) and observe the $DBASE$ keywords. As soon iMOD uses this runfile to perform a simulation this keyword will be replaced by the string as defined in the IMOD_INIT.PRF file which is in {installfolder}. Go to File–>Preferences... and click on to upload the content to the KEY words list on the right. Click on a KEY word to read the reference. When you change and save the contents of the IMOD_INIT.PRF file in a text editor, click on to re-read the keyword settings for the current iMOD session. For more info on Preferences see Section 2.7, for more info on available keywords see Section 9.1. For information on the iMOD folder structure ee Section 11.1 and Chapter 9.

• 4. Go to View in the menu bar and select the iMOD Manager (or use the short-key Ctrl+M).

Model Parameters

Let us first observe some model parameters and understand what this model might be up to. We use the Project Manager for that, so lets’s start that one.

• 5. Click the option Project Manager from the View menu.

iMOD simulates a groundwater flow model by means of a runfile. A runfile gives a full description of the use of all files needed for the simulation. The Project Manager is able to read the entire runfile and present the content in a treeview field.

• 6. Click the Open Runfile button (   ) and select the TUT_MODEL.RUN file from the {IMOD_
USER} \RUNFILE folder.

iMOD presents the content of the runfile in a treeview. Each branch represents a model parameter, and whenever a branch contains more information we can expand the branch to analyse its contents. Let us visualize the starting conditions for this particular model.

• 7. Make sure the iMOD Manager is also active (if not, press CTRL+M).

• 8. In the Project Manager expand the branch called (SHD) Starting Heads from the treeview field. Select this branch (not an individual IDF) and click the draw button (   ). If no image appears you may have to click the Zoom Full Extent button (   ).

As a result iMOD will open all the files of the selected branch and adds them to the iMOD Manager. In this manner it is easy to explore the available model parameters for the model. In this model the starting condition of a model simulation is equal to a result of a previous simulation. Since IDF-files are geo-referenced, they can be easily (re)used for different modules and/or packages in a model configuration.

• 9. Analyse the starting condition by creating several cross-sections (see Section 11.3) and compute the difference between the starting condition for model layer 1 and the one for model layer 2 (see Section 11.2). This gives you insight in downward and upward fluxes.

• 10. Click the branch (RIV) Rivers (Cauchy conditions) and observe that this model has two river systems. One is connected to model layer 1 and the other one is connected to model layer 2. Furthermore each river system consists of 4 input grids, CONDUCTANCE, RIVER STAGE, RIVERBOTTOM LEVEL and INFILTRATION FACTOR. Examine the content of these files; use the draw button to load all RIV files to the Project Manager.

• 11. Explore the content of the Project Manager, e.g. plot the elevation of the existing Rivers in the first system.

In this particular model a river is discretized for two different model layers , i.e. model layer 1 and model layer 2. The number of river elements is unlimited, however, a single IDF can store one river for each cell, so you should define more IDF files in those cases you want to specify more river elements at the same location. In this particular case we specified river elements for model layer 2 that penetrate the first aquitard and connect to the first aquifer (i.e. the second model layer).

Model Simulation

Let’s start the model simulation.

• 12. Select the option Start Model Simulation from the Toolbox option on the main menu. Whenever the Start Model Simulation window does not appear, check the keyword MODFLOW in the PRF file.

• 13. Select the TUT_MODEL.RUN from the Runfiles list. You should see a hatched area of the maximum extent of the model. If you do not see that

• 14. click the Zoom to Extent button (   ). Click the Info ) to display the runfile in a texteditor.

The first 15 lines of the runfile can be manipulated in the Start Model Simulation window. The rest refers to existing model input data, let’s check whether all these data is available.

• 15. Close the editor and click the CheckRun button (   ).

iMOD will popup a summary file ({USER} \TMP \RUNFILE.LOG) of all files that cannot be found. Use step Item 6. to open a runfile to change pathnames if needed. If no files are listed, all files can be found and we can proceed.

• 16. Select the Result Folder tab and enter a name for the model results, e.g. MODEL25 and click the Start Model Simulation button.

iMOD will copy the selected runfile [TUT_MODEL.RUN] to the {IMOD_USER} \MODELS \
MODEL25 folder and renames it into IMODFLOW.RUN. Thereafter it will copy the simulation executable (e.g. iMODFLOW_V4_3_METASWAP_SVN1233_X64R.exe) to the same folder for archiving purposes, and it will start the simulation by the statement
{installfolder}\iMODFLOW_V4_3_METASWAP_SVN1233_X64R.exe IMODFLOW.RUN
in this example in the {installfolder}\IMOD_USER\MODEL25 folder. A DOS-command tool will open in which the simulation runs. You can proceed with iMOD or wait until the simulation finishes; it will take a very short time since the starting conditions are similar to the results.

The model simulation is always logged in the file IMODFLOW.list located in the subfolder mf2005_tmp, so in this example located in the folder {IMOD_USER} \MODELS \MODEL25 \mf2005_tmp. You should check this file IMODFLOW.list first whenever there is a problem with the simulation; as mentioned above it 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

• 17. When the simulation has finished, choose Quick Open from the main menu option Map.

• 18. Select the Folder [MODELS], then the variant [MODEL25], then choose the Topic [HEAD], and choose Layer [1] and then click the Open button. Make sure the option Display is selected! If everything went well the only option to be selected is [STEADY-STATE] in the dropbox Time. For transient simulations, you might be able to select a specific date.

• 19. Compare the results to the starting conditions. Use tools experienced in Section 11.1, 2 and 3 if desired.

Let us simulate this model at a different resolution.

• 20. Start the Start Model Simulation (step Item 12.) again, select the TUT_MODEL.RUN from the Runfiles list and select the Model Dimensions tab to change the Cellsize from 25 to 100m. You can select a cellsize from the dropdown menu and/or enter a different cellsize in the input field to the right of the dropdown menu.

• 21. Go to the Result Folder tab and enter an output foldername [MODEL100] and click the Start Model Simulation button.

• 22. Open the resulting phreatic heads (model layer 1) with Quick Open (see step Item 77.).

• 23. Subtract the head calculated for MODEL100 from the head of MODEL25 using the Map Calculator ) on the iMOD Manager and/or use the Cross-Section Tool ) on the main toolbar to explore the differences caused by the different simulation cellsizes.

When using the Cross-Section Tool with the Block Line option (see Section 11.3 step Item 28.), you may expect the width of 4 blocks of the MODEL25 line equal to the width of 1 block of the MODEL100 line. But this is probably not what you observe in step Item 24. and Item 27. of this tutorial, because iMOD standard reduces the number of sampling points to speed up the calculations. To get the widths as expect, you have to increase the value of Maximum number of sampling points in the Cross-Section Properties window (see Section 11.3 step Item 27. or Section 7.1 to open this window), e.g. set Maximum number of sampling points to [1000].

Let us simulate just a part of the model.

• 24. Start the Start Model Simulation (step Item 12.) again, select the TUT_MODEL.RUN from the Runfiles list and select the Model Dimensions tab and click the Draw Simulation Area of Interest button. You can interactively draw the area of interest within the hatched area. Click your left mouse button to set the first corner and give a second left mouse click to specify the opposite corner. You may drag the area of interest interactively by dragging the mouse while your inside the graphical display. Reset the cellsize to 25 meter and include a buffer-zone of 1500 meter.

• 25. Go to the Output Variables tab and select the option Save Result Variable inclusive the given Buffer Size.

• 26. Go to the Result Folder tab and enter an output foldername [MODEL25PART] and click the Start Model Simulation button.

• 27. Open the resulting phreatic heads (model layer 1) with Quick Open (see step Item 77.) and subtract the results using the Map Calculator ) on the iMOD Manager and/or use the Cross-Section Tool ) on the main toolbar to explore any differences.

Water Balances

An important aspect of groundwater flow modeling is the ability to compute water balances. In iMOD you can compute these too by analysing the output variables in the different output files. It is important that you specify the appropriate output variables prior to your simulation, see tab Output Variables in the Start Model Simulation window you might have still open. In this case the defaults for the output variables were used.

• 28. Select the option Water Balance and then Generate Water Balance from the Toolbox menu.

• 29. Select the model [MODEL25] from the list at Existing folder with Results available in the Models Folder.

• 30. Click the Modflow button to select all water balance term that are relevant to Modflow (saturated groundwater), automatically. The iMOD convention is that all flux related output files start with BDG*. The content of these files is always in m$^3$/day.

31. Select the Period and Layers tab from the Compute Waterbalance window.

Here you can specify for what layers, and periods (in case of a transient model) need to be included in the water balance. For now we select all layers (which are selected by default), so we leave it like it is.

• 32. Click the Create TXT … button and save the water balance as WBAL.TXT.

• 33. iMOD will present the content of the WBAL.TXT file in a separate window. Inspect the terminology and its content; more is explained below.

In the above given example a water balance is presented for the entire model and for all model layers sequentially. In this case the water balance is given for a steady-state simulation and summed for the entire model area. There is only one zone used and the following terms are organized row wise:

flux in or out across the boundary according the boundary condition specified around the model.

• BDGFLF (FLUX LOWER FACE)
flux over the interface between model layer 1 and 2, along the z-direction downwards.

• BDGFRF (FLUX RIGHT FACE)
flux over the interface between column interfaces between cells in model layer 1. along the x-direction eastwards.

• BDGFFF (FLUX FRONT FACE)
flux over the interface between adjacent row interfaces in model layer 1, against the y-direction southwards.

• BDGWEL (WELLS)
flux in the wells.

• BDGDRN (DRAINAGE)
flux out the drainage systems.

• BDGRIV (RIVERS)
flux in or out the river systems. There are two river systems presents, so therefore two lines are presented, though the second system is not active in the current model domain.

• BDGOLF (OVERLAND FLOW)
flux in the overland flow drainage system.

• BDGRCH (RECHARGE)
flux from the recharge.

Especially the (Q_in and Q_out) percentages are interesting and can be used to observe the relationship between the different water balance terms. In the above example $>$45% of the groundwater is discharged to the surface water.

• 34. Close the water balance text file in order to continue. The file can be inspected any time by a regular text editor.

There is another, more interactive manner, to examine water balances. Let’s try that.

• 35. Select the Create CSV … button and save the water balance as WBAL.CSV.

• 36. iMOD asks to start the Waterbalance Analyser after finishing the creation of the WBAL.CSV, click Yes to start the Analyse Waterbalance window.

Now you can see that iMOD reads 8 records, 11 budget terms, 1 period, 8 layers and 1 zone. The Waterbalance Analyser can be used to aggregate budget terms and display them graphically.

• 37. Select the Budget Terms tab, here all available budget terms are listed from the WBAL.CSV.

• 38. Select the option [STEADY-STATE] form the Timesteps list.

• 39. Select the Aggregation tab, here all type of aggregation can be performed.

40. Select the Select All button from the Model Layers list.

• 41. Select the Select All button from the Zones list.

• 42. Select the Graphics Output tab, here all types of output can be selected, we leave it for now like it is.

43. Select the Generate Preview button to display the current configuration (as set in the previous tabs) in a graphical display.

The following image might appear.

From here you can zoom in or zoom out in the image, as well as select a different combination of model layer and zone number. If you have multiply zones $nz$ and multiply layers $nl$, the list in the drop down menu is as long as $nz \times nl$. Let us display another graphical presentation of the water balance.

• 44. Click the Close button to close the Graph window.

• 45. Select another Output Type, the option Graphical Representation.

The water balance is presented as follows:

There will be a sequence of 8 figures passing by, since we have selected 8 model layers, so repeat the following step 8 times to close all the repeating windows.

• 46. Click the Close button to close the graphical image window.

Let’s try a more complicated CSV file in this tool.

• 47. Click the Close button to close the Graph window.

• 48. Select the CSV-File tab from the Analyse Waterbalance window.

• 49. Click the Open CSV-File button (   ) and select the file{installfolder} \tutorials \TUT_Model_Simulation \DELTARES1994.CSV. It’s a pretty big water balance file and holds the results of a daily model simulation (365 periods) for 29 budget terms, 19 layers and 17 zones.

• 50. Select the Budget Terms tab.

• 51. Click the Select All button from the Timesteps list.

• 52. Select the Aggregation tab.

• 53. Select the option Months, a single value per month, starting at the first month of the series. Although the original data is for a daily base, the Waterbalance Analyser can aggregate the budget terms on a monthly base automatically.

• 54. Click the Select All button from the Model layers list.

• 55. Select the option Sum Selected Layers from the Layer Aggregation input field. In this way, all fluxes as summed over the selected layers.

• 56. Click the zones 8, 9 and 13 from the Zones list. Use the Ctrl-Left mouse button to make that selection.

• 57. Select the Graphics Output tab.

• 58. Click the Generate Preview, make sure the option Time Series is still selected. The Graph window will appear.

• 59. Select the option Layer [sum]; Zone 13 from the drop down list in the Graph window.

The following image appears (colours of the FluxTerms may differ).

There is a lot to try in this tool, all kind of different settings can be combined, see section Section 7.18.2 for more detailed information. Feel free to experiment a bit more with all the possibilities. If you’re done, we would like to compute a water balance for a specific region.

• 60. Click the Close button on the Analyse Waterbalance window to close it.

• 61. If the Compute Waterbalance window is removed, restart it again by following the steps Item 28. up to Item 31..

• 62. Go to the Apply To tab and select the option Apply within selected polygons (*.gen). Select the pencil (   ) and start drawing a polygon on the graphical canvas (see Section 4.4 for more information about the specific functionalities that you can use while drawing a polygon).

• 63. Click the Create TXT button and enter the name WBAL_PART.TXT and inspect the resulting water balance file.

In the iMOD Manager you will notice the $\langle$water balance name$\rangle$.IDF. This file reflects the position of the given polygon. You can reuse this file (e.g. after editing) in another water balance computation, therefore choose the option (Apply for non-NoData values within IDF).

• 64. Close the water balance text file in order to continue. The file can be inspected any time by a regular text editor.

Scenario Simulation

Let’s build a scenario in which we will increase a river stage from the current model configuration. We can do that in two manners. One manner is to adjust the appropriate IDF-files that discretize the river system, e.g. RIV \RIV_STAGE_L1.IDF and RIV \RIV_STAGE_L2.IDF by means of IDF Edit (see Section 6.7.4).

• 65. Click the Close button on the Model Simulation window, if needed.

• 66. Select the file RIV \RIV_STAGE_L1.IDF in the iMOD Manager and click the Redraw button (   ) to (re)draw it.

Make a copy of RIV_STAGE_L1.IDF and RIV_STAGE_L2.IDF in the folder ..IMOD_USER \DBASE by using the Map Operation option and the following instructions:

• 67. Click on the iMOD Calculator button (   ) on the Maps tab of the iMOD Manager to enter the Map Operations window.

• 68. fill in on tab Algebra in field Map C the new IDF name RIV_STAGE_L1_0.5.IDF. Don’t forget the right FULL pathname, e.g. C:\IMOD\IMOD_USER \DBASE \RIV_STAGE_L1_0.5.IDF.

• 69. (You can also create a new file name by a click on the (   ) button, find the right folder and type the new IDF name.)

• 70. fill in ‘C=1.0*A’in the field Formulae

• 71. select Map A and click on Compute....

Now a copy of the selected file is made.

• 72. Repeat the previous step for RIV_STAGE_L2_0.5.IDF

• 73. Zoom in for the desired river segment at the coordinates [x=145000.0] and [y=448100]. You can use the option GotoXY from the View menu (see Section 5.2) and use Zoom(m)=[1500m].

• 74. Select RIV_STAGE_L1_0.5.IDF in the iMOD Manager and select the option IDF Edit from the menu Map, IDF-options or with the right mouse click on the map.

• 75. Click the Open GEN button (   ) and open the file SCENARIO.GEN that is located at{installfolder} \tutorials \TUT_MODELSIMULATION.

We’ve created a shape (polygon) to specify the area in which we will change the river stage. Let’s assign the measure to be attached to the polygon.

• 76. Select [1] from the list (see figure below).

• 77. Click the Select… button at the Selection tab of the IDF Edit window.

• 78. The IDF Edit Select window will be opened. Notice that the option Select for Polygon is checked!

• 79. Select ‘All’in the Logic dropdown menu and click on Get Selection at the bottom of the window. Now all river cells in the polygon ‘Shape 1’are selected. The number of selected cells is 203 (see lower left corner of the window).

• 80. Click on Close to close the IDF Edit Select window. All cells stay selected and are available for calculation.

• 81. Click on Calculate… on the IDF Edit window.

• 82. Select the option New Value on the IDF Edit Calculation window and choose ‘+’from the dropdown menu.

• 83. Fill in ‘0.5’in the field behind the dropdown menu and click on Calculate.

• 84. Click on Close. iMOD will asks if you are sure to save the changes. Click the ’Yes’ button.

• 85. Repeat step Item 74.-Item 83. for RIV_STAGE_L2_0.5.IDF.

So, we’ve created a scenario definition that raises by 0.50m the stage of all river systems that penetrate model layer 1 and 2 inside the current polygon (SHAPE1), by making use of the iMOD Edit option.

Okay, let’s use this scenario definition in a model simulation.

• 86. Select the option Start Model Simulation from the Toolbox option on the main menu. Select the TUT_MODEL.RUN from the Runfiles list. (Note: if TUT_Model.run is not available in the list copy the runfile from the Tutorial folder to your IMOD_USER/RUNFILE folder.)

• 87. Click on the Info button (   ) to open the runfile in the inbuild texteditor.

• 88. Find the lines with the filenames of the river stages:
1,1.0,0.0,C: \iMOD \tutorials \TUT_MODEL_SIMULATION \RIV \RIV_STAGE_L1.IDF
2,1.0,0.0,C: \iMOD \tutorials \TUT_MODEL_SIMULATION \RIV \RIV_STAGE_L2.IDF.

• 89. Change the file names so they refer to the IDF files your created earlier, e.g.
C:\IMOD\IMOD_USER \DBASE \RIV_STAGE_L1_0.5.IDF.
C:\IMOD\IMOD_USER \DBASE \RIV_STAGE_L2_0.5.IDF.

• 90. Click on the save button and close the text editor window.

As you might observe, the area of interest (within the shape) is smaller than the total extent of our model. Let’s decrease the size of our model (in order to speed up our simulation).

• 91. Select the Model Dimensions tab and click the Draw Simulation Area of Interest button.

• 92. Left click your mouse approximately 1,000m west of the south west corner of the polygon (SHAPE1) and left click on approximately 1,000m east of the north east corner of the polygon. This will be our area of interest. You can increase or decrease it by moving your mouse in the neighbourhood of the boundaries and drag your mouse as soon as the mouse cursor changes in and .

• 93. Select or enter a buffersize (Include a Buffer-zone of) of [1500m].

• 94. Select the Output Variables tab and select the option Save Result Variable inclusive the given Buffer Size.

A buffer zone prevents that model results are affected by boundary conditions on the lateral model boundary. It depends on the scenario configuration, model configuration itself and the geohydrological subsoil what this buffersize should be. It is hard to determine beforehand, so it is wise to analyse the effects near the model boundaries to decide whether your simulation is affected by the lateral boundary conditions too.

• 95. Go back to iMOD and select the Result Folder tab on the Start Model Simulation window.

• 96. Give a name for the new output folder (e.g. RIVER_STAGE) and click the Start Model Simulation button to confirm the operation. Results of scenario computations will be stored in the folder.

• 97. After the simulation ended, open the phreatic heads (HEAD_STEADY-STATE_L1.IDF) with Quick Open (see step Item 77.).

98. Compute the differences in phreatic heads between the . \MODELS \MODEL25 and this scenario . \MODELS \RIVER_STAGE. Use step Item 21. and forward from Section 11.2.

• 99. Analyse the differences in head for all model layers to observe whether the chosen buffersize was sufficient. Use Quick Open to load all files in the iMOD Manager.

In the example above it is clear that the boundaries of our submodel have been chosen appropriately since the change in head is not affected by the model boundary. You can also make a cross-section of the computed effect to judge whether the boundary has been chosen right.

• 100. Finally try to answer the question: “Why are the head differences more than 0.50m at some locations (0.55 meter), although river stages are increased by 0.50 meter only?”

Model simulation with the Parallel Krylov Solver (PKS) package

So far we only used the single core PCG solver, now let’s switch to the multi core Parallel Krylov Solver. This requires that you have correctly installed the MPI software, see the iMOD Installation Instructions (Section 2.3).

The PKS package solver settings can be configured in two ways:

• 1. In the iMOD-GUI: in the ’Solver Settings’ tab of the ’Model Simulation’ window of the ’Tools’ main menu option (see the figure below); we will practise this in a minute.

• 2. Manually: by editing Data Set 5 of a runfile according the specifications given in Section 10.6; an example is given in Section 10.20.7.

Note: The PKS package is not yet available in the Project Manager.

When using the PKS package, the model domain is divided in sub-domains automatically; the number of sub-domains is always equal to the number of computational cores the user assigns in the ’Solver Settings’ window.

The overall computational model performance depends among others on how long it takes to solve each individual sub-domain; an iteration for the whole model domain can only be completed when all individual sub-domains have been solved. This means that load balancing is very important for the overall parallel performance. Ideally the actual work/load should be distributed as equally as possible over the multiple computational cores. PKS now supports two methods sub-domain partitioning methods:

• 1. Uniform sub-domain partitioning in x,y-direction; when e.g. using four CPU’s the model domain will be divided into four equally sized sub-domains.

• 2. The Recursive Coordinate Bisection (RCB) method. The RCB method incrementally partitions the model domain step by step and alternates the partitioning in the x- and y-direction until the number of sub-domains is equal to the number of assigned CPU’s. Simultaneously the sub-domains are automatically being re-sized such that ultimately the load of each sub-domain is the same. The load of a sub-domain is the summation of the user-defined weights (or load) of the model cells within the boundaries of that particular sub-domain.

Figure Figure 12.18 in Section 12.32.2 shows an example of both methods for the Netherlands Hydrological Model ((DeLange14)) and 128 sub-domains.

When assigning two CPU’s and selecting the uniform partitioning method, the model domain will be divided into two equal sub-domains.

When using the RCB method, the user has to specify per model cell a weight representing an estimate of how much each cell contributes to the computational effort to be made to solve the set of equations. There is no partitioning in the z-direction, so ideally the specified weights should also take variations of the total number of active cells per x,y-location (a particular vertical column) into account. One should be aware of the fact that even with the RCB and irregular boundaries, finding an optimal weight distribution can be difficult and subject to trial-and-error. The spatial weight distribution depends on for example differences in the complexity of boundary conditions (stresses) and coupling concepts.

In this tutorial we will exercise the use of the RCB method: you will run the groundwater flow model using two CPU’s applying a load balancing grid. This can be done by the following steps.

• 3. Select the option Toolbox from the main menu and then the option Start Model Simulation to start the Start Model Simulation window.

• 4. Select the TUT_MODEL.RUN from the Runfiles list.

• 5. Select the Solver Settings tab.

6. Within this tab, select 2 for Preferred number of cpu’s to be used.

• 7. For Preferred method of subdomain partition select Recursive Coordinate Bisection.

The following figure shows the specified loads of the LOAD.IDF grid: in the left part of the grid all cells have the value $1$ and in the right part all cells have a value $2$. So in this example we assign twice as much weight to approximately 20% of the model cells (note that this grid is just illustrative since for this model a uniform load of $1$ for each computational cell would be most optimal).

• 9. Turn on the checkbox for Merge IDF output files of subdomain.

• 10. Select the Result Folder tab and enter a name for the model results, e.g. MODEL25_PKS and click the Start Model Simulation button; the model will be run using two computational cores.

Similar to a serial computations you can view the head results with Quick Open from the main menu option Map. We turned on the checkbox Merge IDF output files of subdomain: after the model-run the sub-domain-IDF’s will be merged to IDF’s covering the total model domain and the sub-domain-IDF’s are deleted. Of course we are also curious about how the total model domain was partitioned in two sub-domains automatically using our weight distribution grid. To see the partitioning-result of RCB method we will re-run the model in parallel mode, however, now without turning on the Merge IDF output files of subdomain-option:

• 11. Select the Solver Settings tab.

• 12. Within this tab, turn off the checkbox for Merge IDF output files of subdomain.

• 13. Select the Result Folder tab and enter a name for the model results, e.g. MODEL25_PKS2 and click the Start Model Simulation button.

• 14. When the simulation is done, go to View in the menu bar and select the iMOD Manager (or use the shortcut Ctrl+M). Select the Open Map button (   ) and click the button Open. Navigate to \IMOD_USER \MODELS \MODEL25_PKS2 \head and select the files head_steady-state_l1_p000.idf and head_steady-state_l1_p001.idf for the computed heads for the first model layer. Click the button Open.

• 15. Select the Map option from the main menu, choose the option Current Zoom Level and then choose the option Percentiles.

• 16. Select the View option from the main menu, choose the option Show IDF Features and then choose the option IDF Extent.

These steps result in figure Figure 11.102, where the left sub-domain is clearly larger than the right sub-domain due to the specified weights. As you may have noticed the partitioning is not equal to the weight distribution of the LOAD.IDF grid, in other words, by specifying this pointer grid, you are not enforcing a particular partitioning of the model domain. This is caused by the RCB method which results in two sub-domains that each have an equal computational load (based on your estimated weight distribution); as mentioned above, the load of each sub-domain is calculated as the sum of the user-assigned weights of all cells lying within the boundaries of that sub-domain. Suppose we would have taken the LOAD.IDF grid as a basis for partitioning, this would have resulted in a relative load for the left part of ’80’ and for the right part ’2 x 20 = 40’. The RCB automatically shifts the boundary between the sub-domains such that the two resulting sub-domains each have a fifty-fifty (50-50) computational burden; that’s why the right sub-domain also contains part of the model domain having cells with weight values equal to $1$.

For your own model, experiment with different weight distributions for finding optimal load balancing. Provided your machine has more than two CPU’s available experiment with using (almost) all of them and compare overall performance.

Additional background questions The following questions are meant for extra training and get more insight in the concept of groundwater modeling.

• 1. Make a second scenario: increase the stationary groundwater recharge (RCH_L1.IDF) in the entire model area by a factor 1.2 (Note: be sure that you use the equation C=1.2*A in the iMOD calculator), thus simulating a possible future climatic change. Follow the procedure analog to increasing the rivers stages. Again, compare computed groundwater levels with those in the default situation (’MODEL25’).

• 2. Explain the spatial pattern of the increase in groundwater levels. In which areas is it more and in which areas less? Examine this by exploring other IDFs e.g. RIV, DRN.

• 3. Why is a converged model not necessarily a correct model?

• 4. Consider a drain pipe, ending in a surface water channel. Will the drain pipe keep draining groundwater into the channel if the surface water stage rises above the drain pipe elevation?

• 5. In MODFLOW, each package treats an inflow or outflow, resulting from a boundary condition, as an external source or sink (Q_ext). It does not consider possible interactions with other packages / boundary conditions. Knowing this, what will happen in MODFLOW if the surface water stage (RIV) rises above the drain pipe elevation (DRN)?

• 6. How should DRN_LEVEL_L1.IDF be adjusted to prevent this?

• 7. OLF_L1.IDF represents the surface elevation, referenced to sea level. Using this OLF file and the simulated heads, calculate the (steady state) groundwater depth in the MODEL25 situation.

• 8. Subtract the drainage elevation from the surface elevation, and compare the resulting drainage depth map to e.g. Google Maps for the area South East of the city of Utrecht. What is the drainage depth that occurs most often in built-up areas?

• 9. Compare the drainage elevation in the built-up areas in the north east of the model to the groundwater head. What is the general picture?

• 10. Compare the drainage elevation in the built-up areas in the (south)west of the model to the groundwater head. What is the general picture?

• 11. With which of the statements below do you agree most? Motivate.

• • This model is suitable to determine which model cells in built up areas have too high groundwater levels.

• • This model is suitable to determine which towns and villages are dependent on drainage systems to prevent too high groundwater levels.