pictures/imod_logo.png

iMOD User Manual version 5.2 (html)


11.10Tutorial 9: Lake Package

This tutorial gives an introduction to a transient implementation of the Lake package (LAK), see section 12.29.


Outline

This is what you will do:


Required Data

For this tutorial you need the following iMOD Data Files/folders in {path of tutorialfolder} \TUT_LAK:


Getting Started


Create the boundary conditions

We start to create our first IDF file.

Now we copy the geometry of this BND file to a to be created TOP file.

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.

We will assign the IBOUND value ’-1’ (fixed heads) to all cells of columns and rows 1 and 34. There are a number of possibilities to select the appropriate cells; we will now select them by dragging the mouse of the cells, similar to step 14. of Tutorial 8: Surface Flow Routing (SFR) and Flow Head Boundary (FHB) Package:

Now we need to define the active area in the boundary conditions so we need to do another selection.

Now, we will create the surface level. The surface level declines gradually from west to east, starting at 160 m and ending at 140 m.

pictures/tutorial11/select_top.png

Figure 11.141: 34 cells selected after clicking the ’Get selection’ button.

Repeat the steps above to compute the right side in a similar manner and give this east side of the model the value 140.0.

Note: It is easy to just move the drawn polygon to the right and than follow the steps again.

Let’s interpolate the surface level.

pictures/Ch-imod-tutorials/LAKE_pcg_settings.png

Figure 11.142: The Solver Settings window.

Let’s see how the interpolation looks like.

pictures/tutorial11/top.png

Figure 11.143: The interpolated surface level.


Create the Lake

We would like to introduce a lake in the middle of our model. We need an IDF file that describes the maximal extent of the lake, and another IDF file that describes the bathymetry of the lake.

I’m sure you know by now how IDF Edit works, so:

You should have the following IDF files created:

pictures/tutorial11/lake_id.png

Figure 11.144: Lake Identification.

You can configure your IDF files to display the actual IDF values as follows:

pictures/tutorial11/lake_bathymetry.png

Figure 11.145: Lake Bathymetry.


Add the Lake to the Modelling Project

Now, we will add the parameters for the Lake Package in our modelling project.

The entire model has been filled in already. Notice that this MODEL.PRJ file refers to the prepared IDF files given with the iMOD install in the folder {path of tutorialfolder}\TUT_LAK\DBASE instead of the files you created yourself and saved in the folder{installfolder} \IMOD_USER \DBASE \TUT_LAK. With the Define Characteristics button on the Project Manager window you can change the file reference if you like (see also section 5.5.2).

You may inspect the model for a while and you will notice that it is a model with 5 model layers. The BND.IDF and TOP.IDF are filled in as well. The BND.IDF is used to define the boundary types for all model layers and the TOP.IDF is used at the module (TOP) for the first model layer. That file is also used for the Surface Elevation used by the (EVT) module. We will now enter the parameters for the (LAK) module.

pictures/tutorial11/projectmanager_lak.png

Figure 11.146: Example of the ’Define Characteristisc for: (LAK) Lake Package’ window; the part ’Parameter Assignment’ contains a pull-down Parameter list which should be parameterized according to the values given in the table below.

Table 11.7: Modeling Parameters for the Lake Package.

- Parameter Entry Units
1 Lake Identifications LAK_ID.IDF -
2 Lake Bathymetry LAK_BATHYMETRY.IDF m+MSL
3 Initial Lake Levels 115.0 m+MSl
4 Minimal Lake Levels 97.0 m+MSl
5 Maximal Lake Levels 145.0 m+MSL
6 Lakebed Resistance 10.0 days
7 Precipitation at surface Lake 0.0116 m/d
8 Evaporation at surface Lake 0.0103 m/d
9 Overland runoff 0.0 m\(^3\)/d
10 Lake Withdrawall 0.0 m\(^3\)/d

So, let’s first save our configuration in a new project file.


Start the model simulation

With this project file we can generate a Runfile and/or a standard MODFLOW2005 model. As the LAK-package is not supported by a Runfile we need to create standard MODFLOW2005 files, let’s do that.

pictures/tutorial11/configuration.png

Figure 11.147: Example of the iMOD Define Simulation Configuration window.

So, we will create a model that is transient, starts at the 1\(^{\rm st}\) of December 2016 00:00:00 and ends at the 1\(^{\rm st}\) of December 2050 00:00:00. The model will generate output after each year.

iMOD will now first create the necessary MODFLOW2005 files. As the model is tiny, this will be finished rapidly. Then the simulation will start immediately. You’ll see that the model start in a separate DOS-command window and it will echo the simulation progress. As it is a transient simulation with 35 stress periods, it will consume probably 30 seconds to accomplish.


Inspect the result of the Lake simulation

The lake levels will be part of the saved hydraulic heads, so we only have to open, e.g. the hydraulic of the first stress-period to generate time series. The lake exchange with groundwater will be saved in a separate budget file, we will open that as well.

iMOD will load all selected results files into the iMOD Manager and displays the result on the graphical canvas. Use your experience learned from the previous Tutorials (e.g. LineColor and LineStyle "BlockLines") to display the computed heads as time series as shown in the following figure.

pictures/tutorial11/results.png

Figure 11.148: Time Series of lake levels.

The lake package simulates the exchange of groundwater and surface water such that the water balance of the lake equals (more-or-less). So, in the end it finds a water level of 133.47 m+MSL. At this lake level there is an equilibrium between the nett recharge of the lake (precipitation minus evaporation) and the drainage to the lake.

Note: The total volumes can be found, per stress-period, in the list file after the simulation. You can find the file here:{installfolder} \IMOD_USER \MODELS \TUT_LAK \TUT_LAK.LIST. To get this water balance, search for a part of the string “HYDROLOGIC BUDGET SUMMARIES FOR SIMULATED LAKES”. For the first year, the total inflow to the lake is 1.9388E+07 m\(^3\)/year (we have time step lengths of one year). This is equal to -53118 m\(^3\)/d, that is the sum of all fluxes from the BDGLAK-files. You can find the total fluxes per BDGLAK-file via the Map Info option (  pictures/h72-end/image995.png ) and then select the Statistic button (  pictures/h72-end/image996.png ) you can read the Sum of the individual flux file.

Let’s look at the lake spatial exchange volumes.

pictures/tutorial11/results_fluxes.png

Figure 11.149: Computed spatial Lake fluxes.

The fluxes to- and from the lake are given by the BDGLAK-files. These fluxes are stored within the first model cell next to the lake. So whenever you hoover your mouse, you’ll notice that the fluxes are all zero at the location of the lake for the first model layer. As the lake connects for only a part to the second model layer, you’ll notice that lake fluxes appear in the second layer only underneath the lake and even for model layer 3, the bottom of the lake. Try to understand the pattern of the fluxes. Having a look at the head differences between layer 1 and 5 in a cross-section (utilizing the Cross-Section Tool) may also shed some extra light on how the flux pattern looks like.


Connect the Lake with the SFR package

In this final step, we will connect the lake with the surface water model as described by the SFR package. The implementation of this package is explained in section 11.9. We have created the SFR layout and stored the file in {path of tutorialfolder} \TUT_LAK \DBASE \SFR.ISG. Let us open the file.

pictures/tutorial11/layout_lak_sfr.png

Figure 11.150: Current layout of the SFR and LAK maps.

In the figure you can see that the streams are connected to the lake on the south and north sides. From the south, the stream feeds the lake, from the north, it drains the lake.

To connect the lake (LAK) to the stream (SFR), we need to specify a negative lake number in the SFR.ISG, let’s do that.

Now we need to connect the upstream segment 2, such that it can receive water from the lake.

That’s all, we need to add this ISG file to our project.


Simulate the enhanced model

As we use the LAK-package in combination with the SFR we need to create standard MODFLOW2005 files, let’s do that.


Inspect the result of the enhanced model simulation

Use the steps from 98. to open the result files from the the folder{installfolder} \IMOD_USER \MODELS \TUT_LAK_SFR. Use yor skills to observe that the steady-state lake level becomes 112.10 m+MSL and the stream stage up- and downstream of the lake are 118.63 and 111.57, respectively.

Note: Use ISG Edit to explore the results of the results of the SFR package, see section 11.9, step 156. onwards.

pictures/tutorial11/results_lak_sfr.png

Figure 11.151: Current result of the groundwater levels for 31\(^{\rm st}\) of December 2037.

Use the{installfolder} \IMOD_USER \MODELS \TUT_LAK \TUT_LAK_SFR.LIST to explore the water balance for the lake at the last stress period.

It follows that:

Table 11.8: Summary of Lake Water balance.

Parameter Value Unit
Lake Stage 110.64 m+MSL
Lake Volume 1.726118E+07 m\(^3\)
Precipitation 1.300215E+07 m\(^3\)/year
Evaporation 1.154501E+05 m\(^3\)/year
Groundwater Inflow 5.7897E+07 m\(^3\)/year
Groundwater Outflow 0.0000E+00 m\(^3\)/year
Surface Water Inflow 2.5738E+08 m\(^3\)/year
Surface Water Outflow 3.1528E+08 m\(^3\)/year

Okay, please feel free to experiment more with several parameters for the LAK package.