iMOD User Manual version 4.4 (html)

7.19Compute Mean Groundwaterfluctuations (GxG)

Groundwater fluctuations are indicative for the seasonal dry or wet status of an area. The so-called GxG’s are indicative of the high and low phreatic groundwater levels occurring in a period of at least 8 years. The GxG’s are used frequently in the Netherlands when defining the geohydrological conditions of an area.

The GxG’s consist of:

In general GXG’s are expressed as groundwater depths below the land surface.

Select the option Toolbox from the main menu and then choose Compute GxG to open the Compute GxG’s window.

Compute GxG’s window, Result Folder tab



Select this radio button to specify a result folder from a different location.


Select Folder
Click this button to search for a folder on disk and use that folder to list the existing budget terms. Alternatively, it is possible to enter a folder name into the string field.

Existing folder
with Results ...

Select one of the folders that appear in this list box. These are the folders in the {IMOD_USER} \MODELS folder.

Select Term

iMOD will list all sub folders under the current selected or entered folder, in fact a GxG can be computed for any type of file. In most cases the HEAD folder need to be selected. If none are selected the other tabs in this window are unavailable. Also, whenever incorrect folders (folders that doesn’t contain the correct files) are selected this happens. Each time a single term is selected, iMOD starts searching for corresponding files. For large models, this can take a while.

Found 175 correct files

Displays the number of files (175) that are correct (i.e. they fulfil the naming convention as described above and contain model layer information (_L*) and a date string (YYYYMMDD), e.g. HEAD_20111231_L1.IDF.

Create IDF …

Click this button to compute the GxG’s. In fact iMOD computes the GLG, GHG, GVG and GT files. For a correct usage of this tool, the data need to be available for the 14\(^{\rm th}\) and 28\(^{\rm th}\) of each month, if not the following message apears.

Example of a Warning Message


At the end of the computation a summary is displayed for the files that were used.

Following files were INCLUDED:
Hydrological Year 1989 1990
GxG based upon 24 measurements
1 HEAD_19890414_L1.IDF
2 HEAD_19890428_L1.IDF
3 HEAD_19890514_L1.IDF
4 HEAD_19890528_L1.IDF
5 HEAD_19890614_L1.IDF
6 HEAD_19890628_L1.IDF
7 HEAD_19890714_L1.IDF
8 HEAD_19890728_L1.IDF
9 HEAD_19890814_L1.IDF
10 HEAD_19890828_L1.IDF
11 HEAD_19890914_L1.IDF
12 HEAD_19890928_L1.IDF
13 HEAD_19891014_L1.IDF
14 HEAD_19891028_L1.IDF
15 HEAD_19891114_L1.IDF
16 HEAD_19891128_L1.IDF
17 HEAD_19891214_L1.IDF
18 HEAD_19891228_L1.IDF
19 HEAD_19900114_L1.IDF
20 HEAD_19900128_L1.IDF
21 HEAD_19900214_L1.IDF
22 HEAD_19900228_L1.IDF
23 HEAD_19900314_L1.IDF
24 HEAD_19900328_L1.IDF

Example of the GHG


Example of the GT



Click this button to start the HELP functionality


Click this button to close the Compute GxG’s window

Compute GxG’s window, Period and Layers tab



This is obsolete here.


Click this radio button to use files only that contain date information (YYYYMMDD), e.g. HEAD_20111231_L1.IDF. This option is only available whenever these files can be found in the result folder selected on the Result Folder tab.


Specify the start date (day, month and year) from which the GxG’s needs to be computed. It has been on default filled in with the earliest result file that could be found in the result folder selected in the Result Folder tab.


Specify the end date (day, month and year) to which the GxG’s to be computed. It has been on default filled in with the earliest result file that could be found in the result folder selected in the Result Folder tab.

Define Period

Enter the periods for which the water balance need to be computed solely. Each period consist of two dates delimited by a slash, e.g. dd-mm/dd-mm. In-between periods, use the “;” as a delimited. The convention is as follows:
Periods starts on the first of January and ends after the 31\(^st\) of April
Periods starts on the first of August and ends after the 31\(^st\) of December

Select one layer only

Select one of the listed model layers. This content of the list box is based on the HEAD*-files that are found in the selected result folder on the Result folder tab. Use the Ctrl-left mouse button to exclude or add individual layers, otherwise drag the mouse cursor to select layers.

Year selection

Select one or more of the listed years. The content of the list box is based on the HEAD*-files that are found in the selected result folder on the Result folder tab.

Compute GxG’s window, Apply To tab for (left) applied to selected shapes only or (right) for all non-NoDataValues within a selected IDF-file:

pictures/h5/gxg_applyto.png pictures/h5/gxg_applytonodata.png

Apply to entire
model domain

Select this option to compute the GxG’s for the entire model extent. In fact this will be that area and dimension as described by the first HEAD*.IDF to be read.

Apply within
(Selected) Polygons (*.GEN)

Select this option to specify regions of interests by polygons. By default all polygons will be used for the GxG’s, however, if the option Selected is selected, only those polygons will be used that are currently selected.

pictures/h5/image901.png pictures/h5/image902.png pictures/h5/image903.png
pictures/h5/image904.png pictures/h5/image905.png pictures/h5/image906.png

Click these buttons to draw, open, save, delete or rename a shape. More detailed information can be found in Section 4.2 (Create a GEN-file).

Apply for non-
NODATA values
within IDF

Select this option to use an IDF-file that describes the area of interest by its non-NoDataValue. For those locations of the first HEAD*.IDF read, the value within this IDF-file will be read and evaluated. A non existing IDF file name will outgrey the Create IDF … button.


Open IDF-file
Click this button to open an IDF-file. The selected file will be displayed, however, an IDF-file can be entered in the string field alternatively. If the IDF-file does not exists, the Apply button will grey out.

Compute GxG’s window, Surface Level


Surface Level

Enter the IDF file that represents the surface level. If this is empty, iMOD computes the GxG’s in absolute terms, alternatively if a IDF file is entered, the GxG’s will be relative to the values of the entered IDF file.


Click these button to open an IDF file.