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# Water level change with recharge

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Hey,

I have been stuck on the problem for several days and If anyone of you can help me I will really appreciate it. I was creating a model where top elevation changes from 50m to 20m and bottom elevation is 0 m. The starting head is 20m. The only input is recharge (almost 1000 mm/yr). However, when I am running the model the cell goes dry. Could any of you possibly tell what is the reason for that?

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Not knowing what your boundary conditions are, I'll assume that the cell goes dry in an area which isn't impacted by boundary conditions (even if no-flow).  Your starting head may be too different than what the final elevation is going to be.  The calculations will do an interation which may come to a solution below the bottom of the cell (think of the iterative process as a wave where the model goes up and down at smaller amplitudes until it gets to the solution).  If the low value is below the bottom of the cell and there is no rewetting, the cell is dry and won't rewet.  You might have to try starting heads that are actually lower to be closer to the final solution.

Just my initial thinking.

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I have set the boundary condition as no flow on three sides. I am confused if the problem is due to boundary condition or starting head. And also I had another question is in 3D grid how do you assign gradual top elevation change from 50m to 20 m?

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You have another BC other than three no-flow? If so, that may be an issue. If you have more than one layer you may play with the layer thicknesses. But I guess the starting head is the issue in your model.

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On 3/2/2018 at 9:45 AM, Bidisha said:

I have set the boundary condition as no flow on three sides. I am confused if the problem is due to boundary condition or starting head. And also I had another question is in 3D grid how do you assign gradual top elevation change from 50m to 20 m?

You can simply edit the cell sizes using bore holes. If you don't have you need to create boreholes, make a solid and then map it to MODFLOW.

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