MODFLOW-General Head Boundary Package

[Mohamed Awad]

Hi,

While we were simulating one of our models on GMS using the General Head Boundary package(GHB), we didn’t find any descriptive tutorial for this case. At the following link http://xmswiki.com/wiki/GMS:General_Head_Package, the description clarify that these are two parameters to be defined; the General head boundary and the conductance. The conductance is clarified through this link http://xmswiki.com/wiki/GMS:Conductance which defines the term “t” as the thickness of the material and this is not representing in the case of general head boundary as it should be the length from the general head boundary to the model boundary.

Also there is no clarification about the “main” definition of the general head but only specific for the drain/river. So we recommend to enhance the description of the General head boundary to include the following:

The GHB conceptually is a fixed head far from the model where we assume it as a fixed head with time (i.e.: river, head will not be affected by the model stresses with time). The purpose of using this boundary condition is to avoid unnecessarily extending the model domain outward to meet the element influencing the head in the model. As a result, the General head condition is usually assigned along the outside edged of the model domain.

The attached is also an example to simplify the two terms.

Please check/confirm that our understanding is accurate and it will give meaningful results.

Example.pdf

[Alan Lemon]

I think the explanation given is correct.

As with all boundary conditions you want to make sure that you don't introduce anything into the model that would violate your assumptions. For example, you would not want to put a well close to your general head boundary such that the boundary affects to computed draw down of the well.