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# Simple question about Boundary condition

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Dear all

I have trouble modeling a square site with different BC.

at one side I have river (I put constant head as BC) and no problem in that for modeling.

now,

1) at one side I have rock (no flow coming in or out from site at this side) and

2) at other side of square area I have no BC (I mean the water can flow out of area etc)

what is the difference in modeling number 1 and 2? how to difine them in my model? my model currently have active cells in coverage so I think it means all sides I have rock, right?

Thank you,

Zara

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At the edge of a model, if you have no boundary condition, it is considered a "no-flow boundary condition," and as you noted, water does not enter or leave there. For water to leave the model, there needs to be some sort of boundary condition (e.g., river, stream, constant head, general head, drain, etc.).

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Thank you Sean. sounds like I need to know lots of details to get it work.

- can I define drain for those arcs to let them have flow out of my domain if it's necessary.

- how people model an area? they always are aware of all boundary around their farm/ field/ wells/ etc?

is there any source I can see multiple type of fields modeled in Mod-flow?

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Yes - drains are certainly a way to do it (often used when simulating wetlands), but a constant/changing/general head boundary is probably better (drains only take water out - they don't add water to maintain a certain elevation). Estimate what your head should be in that area and set the head at that elevation. You can adjust it if your initial estimate doesn't produce the results you expect.

You typically try to set up the model domain (extents) to reach some sort of boundary condition (river, lake, wetland, top of a hill). Having the upgradient and downgradient conditions set makes life a lot easier. Often times we don't have good information and you have to do as I just suggested above with boundary conditions. The more data you have, usually the better model you can design.

If you go to the GMS help, you will see the references for the various MODFLOW user's guides. Those are where you should look for information.

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Perfect. will check it out for more information. Thank you again for helpful insight.

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• 7 years later...

Sir i have flow model covered with solid rocks on three side and 4th side is continous . How i define boundary conditions  . It has assumed that water can not enter or leave the system in rock covered 3 sides. Pl help

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The 3 sides with rock don't need anything.  The 4th side needs some condition to allow water to leave (or enter), such as a constant or general head boundary...river/stream, etc.

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