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RE: ISG Attributes windows cross-sections tab

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Dear community,

I have a problem with the cross-sections tab of the ISG Attributes window. The problem occurs with iMOD versions 3.4, 3.6 and 4.0. However, the problem is different in version 4.0.

description of the ISG:
I have several segments in my ISG file. These segments have different amounts of cross-sections and vary in length. I use 1-D type cross-sections.

description of the problem in iMOD versions 3.4 and 3.6:
The problem occurs in the "MRC" column of the cross-sections tab. Generally speaking, the last two rows contain values that are not reasonable, for instance -15245.83.
I then manually change those values to the desired MRC value of 0.03. After that I save those changes (both in the ISG attributes window and the ISG Edit window) and then leave the editing mode of ISG Edit.
When I open the cross-sections tab of the same segment in ISG Edit again, the values of the last two rows of the "MRC" column have changed again to those unreasonable values.

difference of the problem with iMOD version 4.0.
In iMOD version 4.0 I cannot manually change those unreasonable MRC values without the program giving an error message (see attachment 3).

I just encountered, that this problem is bound to negative z-values and does not occur as long as z is >= 0. This raised the question: what exactly is meant by z?

Morevoer, I would like to know, how to handle water levels that are below mean sea level, e.g. in the cross-sections tab.

Thanks in advance for your help.

Best regards,



Attachments: isg attributes cross-sections tab_after saving.PNG (120.0k), isg attributes cross-sections tab_prior to saving.PNG (135.8k), isg_error message_iMODv4.PNG (29.4k)
RE: ISG Attributes windows cross-sections tab
Answer (Unmark)
11/10/17 2:22 PMas a reply to Christian Haars.
Dear Christian

Your experience is that in case the z-values are positive, the problem does not occur. That is also my experience.
In my ISG's the Z is always positive because the z-value is defined as relative vertical position.
In an ISG it is relative to the bottom level of the ISG, defined in the tab "waterlevels".
An answer to your other question (how to handle water levels that are below mean sea level) is that you can force that by giving waterlevel and bottomlevel the (low) value you prefer.
Use the table in the tab "Waterlevels"; type your values or upload a csv file.

I tried to reproduce your issue with automatic low values in the MRC column (-15245.83).
Only once I saw that value appear in the table but were not able to reproduce it another time.

Hopefully making z > 0 will fix the problem for now.
In case you can reproduce this issue with one of your ISG files, please send that ISG file (attached to a post or by email).

Your issue is rather young and I do not remember recently a similar issue that is fixed.
Anyway, perhaps in the new iMOD version you will not face the error anymore.
Download iMOD 4.1 from

Kind regards,
RE: ISG Attributes windows cross-sections tab
isg conductance resistance
Answer (Unmark)
11/17/17 11:48 AMas a reply to Frans Roelofsen.
Thanks a lot for your answer, Frans. Making z > 0 fixed my problem and I was able to rasterize my ISGs properly. If you are still interested in a not-so-good working ISG I can send it to you via E-Mail.

However, I still have difficultuies dealing with conductance as m²/day instead of m/d or m/s. I suppose there is a formula to calculate conductance values from the defined parameters in the ISG. As this formula is not stated in the manual, it is a bit more difficult to compare model results in iMOD with Modflow models.
Moreover, the conductance values seem to be a lot more inconsistent compared to my conductance values in the Modflow model, whereupon my iMOD model is based. The changes between two neighbouring cells are often quite high.
When rasterizing my ISG, it is stated that conductance is computed based on waterdepth, intersection length and resistance. Which resistance is taken into account? There is a resistance column in the waterlevels tab and MRC in the cross-section tab.

General remark:
I think it would be helpful for everyone if you stated the definition of z in the iMOD user manual.

Best regards,
Christian Haars
RE: ISG Attributes windows cross-sections tab
Answer (Unmark)
1/22/18 10:40 AMas a reply to Christian Haars.
Hi Christian,

We followed your suggestion; in the present manual an explanation of parameter z is added.

I hope you found out about the river conductance based on the ISG module in the iMOD manual.
Chapter 12 describes the "Theoretical Background" of iMOD packages.
In paragraph 27, the ISG iMOD Segment package is explained, including the conductance.

Kind regards,