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Observation Well Data Vs Active Cell Data


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I have a transient MODFLOW model where I have an observation well in a cell in the center of layer 1 of a 6 layer model. When I read the observation well gw elevation at a specified timestep, I get one groundwater elevation, but when I read the gw elevation in the same cell by hovering my cursor over it, I get a different number, which is over three feet higher than the number reported by the observation well. I am assuming that the GW elev reported by the active cell is correct, but I am not sure which one to believe. Does anyone know how the elevation in the observation well is being calculated.

I have the 3D grid option set to "by layer number" in the observation coverage diaglogue box. I moved the observation well around within the cell and reran the model, and the observation well always gives me the same number, so I am pretty sure that there is not any lateral averaging going on, but when I changed the default elevation in the observation coverage (raised it by 35 ft. to the top of the layer, but still within the same layer), the gw elevation reported by the observation well increased by 3 feet. The 3 feet put it very close to the number reported by the active cell, but not quite.

I would think that there would only be one gw elevation value within any one particular cell, but it appears that it can be many different numbers depending on what elevation within the layer your obsertvation coverage is at, or whether on not you want to read the active cell directly.

I am using GMS 6.5.3. Any help deciphering these numbers would be appreciated.

Edited by Hydrology
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The computed values at observation wells are computed by MODFLOW. See this document for more details on how it is calculated: http://gmsdocs.aquaveo.com/mf2k_calibration.pdf

If you have your 3D Grid option set to "by layer number" but you are seeing changes to the computed value when you change the elevation of the point then there may be a bug. Could you send your project and the steps to reproduce the problem to support@aquaveo.com?

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