Output heads are much higher than initial heads in steady-state MODFLOW model

[Ajaykumar Venkatarao]

Hello everyone,

I am facing an issue in my steady-state MODFLOW model developed in Visual MODFLOW Flex.
My initial heads are around 740- 850 (MSL), but after running the simulation, the output heads are unrealistically high — in the order of thousands of meters (750- 22000 m).  Could anyone please suggest possible reasons for this large difference between the initial and simulated heads, and how to fix it?

[Dawood Muhammad]

Please check the hydraulic conductivity and recharge values

Sent from my iPhone

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[Dr. Jagadeesh G]

Hi Ajaykumar,

The issue with output heads being much higher than initial heads is common in MODFLOW models. It often indicates a problem with boundary conditions or hydraulic conductivity values.

Please check the following:

Boundary Conditions: Ensure your boundary conditions are set up correctly. For constant head boundaries, verify the head values are realistic and consistent with your initial heads. For general head boundaries (GHB), confirm the conductance values are appropriate.

Hydraulic Conductivity: Review your hydraulic conductivity (K) values. They should be within a reasonable range for your aquifer material. Extremely low K values or isolated areas with zero/very low K can impede flow and cause large head discrepancies.

Initial Heads: Confirm your initial heads are a suitable starting point for a steady-state solution. If they are significantly different from the expected steady-state solution and your boundary conditions are not robust, large discrepancies can occur.

Model Discretization: While less common for such extreme differences, ensure your model grid is appropriate.

Source/Sink Terms: Verify the rates and locations of any wells or other source/sink terms.

Solver Settings: You could try adjusting relaxation parameters or using a different solver, though the issue is likely with the model's physical setup.

A good troubleshooting approach is to simplify your model. Start with a basic setup (e.g., two constant head boundaries and homogeneous hydraulic conductivity) to ensure reasonable heads. Then, gradually add complexity to identify where the problem is introduced.

I hope these suggestions help you troubleshoot the issue.

Regards,

Jagadeesh, 

+91-8074840503

Groundwater modeling consultant and founder, 

smartbhujal.com

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[Ajaykumar Venkatarao]

Thank you Jagadeesh. Its more clear explanation.

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[Dr. Obasi Arisi Ikenna]

Ensure that you import your wells and HOBs appropriately as points. To import them as points, your latitudes and longitudes must be in UTM, if not, you may be having irregular heads.

Ikenna 

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[anandconsultants2009]

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