About the GHB

[riad arefin]

Dear Friends and Modflow Team,

I am developing the modflow model using GMS modflow. I am getting confusion for assigning the General Head Boundary (GHB). My study area contains 5 layers. Where, 5th layer is the aquifer, has well screen and giving the General Head Boundary (GHB) information. Well screen depth 50 to 60 m. 5th layer thickness is 20m. And 5th Aquifer top and bottom depth 45m to 65m. Where, water table depth about 3m. Which is existing to the first layer. And all mentioned depths are from ground surface. 

I am not understanding which layer should I need to assign for the General Head Boundary, first or 5th or all layers?

Thanks in advance.

Regards

Riad

[Richard Winston]

What physical feature does the GHB represent? GHB is not typically used to represent wells. Layer 1 is the top layer. Layer 5 is the bottom layer. 

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[franklin]

Hello

If your wells are installed in the 5th layer, then you should assign the GHB to only the 5th layer. However, if the water level extends to maybe the 3rd layer, then you should assign your GHB from layer 3 to layer 5.

I hope this helps

Regards.

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[Jakab Andras - Gmail]

Assigning GHB or any other lateral boundary condition to low permeability units is not required if one can assume predominantly vertical flows within these units.

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[Mohammad Shaibaz Khan]

Hello Everyone. 
I have a question as well regarding the same. 
  *** Warning *** GHB elevation below layer bottom at (85,4,1)
 There are many more such warnings. But I checked that this cell has it's layer 1 top elevation as 210.3925 and bottom elevation is 204.8925 and Water level at that cell is 129.739, and for layer 2 at that cell, top elevation is 204.8925 and bottom elevation is 157.8877 and for layer 3, top elevation is 157.8877 and bottom elevation is 152.3877 and for layer 4, top elevation is 152.3877 and bottom elevation is -214.8. And the 4th layer is kind of a confining layer. The bottom elevaton of 4th layer is -214 everywhere (perks of Leapfrog). I have many such warnings. Example  *** Warning *** GHB elevation below layer bottom at (175,210,1). 
I am using the Groundwater vista. I used a shapefile to import the heads at cells. When i run the model (steady state) while applying GHB to layers other than 4, it gives error. So i tried applying GHB only in Layer 4, the model successfully run. But in mass balance i dont see any volume as inflow or outflow due to GHB. I opened the .ghb file , there the conductance is 1. while during import in Groundwater vistas , i myself put it 400. 
Can i get any direction, advise or instructions to resolve this. 
Youtube tutorial on Groundwater Vistas are also rare. 
Thanks in Advance.