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Michal

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Posts posted by Michal

  1. Hi Lalith,

    this is because you used layer range. If you use Screen Top/Bot GMS will use different algorithm and the amount specified will be total for entire well. Or you could just leave it as Layer range and set the Q value as 1/3 of the total rate (if the well is set in 3 layers). I belive this is intentional GMS feature.

  2. Hi,

    well, I think this feature that is not working properly.

    Here is an excerpt from MT3D manual, that describes how it handles units:

    "The MT3DMS code uses any consistent units for input and output variables. In the input file to the Basic Transport Package, the user decides the units for time, length, and mass. Then, any input variable or constant should be entered in units consistent with the three basic units. ..."  [chapter 6.2 of Zheng, Wang (1998): MT3DMS A modular three-dimensional multispecies transport model for simulation of advection, dispersion and chemical reactions of contaminants in groundwater systems (Release DoD_3.50.A).]

    There is more in the chapter, so please read it. You can contact technical support for GMS and point them to the problem. I would treat the units specifed in GMS as purely informative for the user. I think the value for a given property you specify goes directly into the input files of MT3D. And therefore if you specify values of a mass rate in terms of different units compared to what your input concentrations are, your outputs will most likely be wrong (and cannot simply be rescaled). Also if there is any nonlinear process involved in the simulation the units must be consistent otherwise the results will be wrong.

  3. Hi,

    I agree it is poorly documented. I had to dig into the source codes to figure it out. It seems to be an excess infiltration volume rejected by the model. It can be routed to other packages as described in the ModelMuse documentation:

    "If the applied infiltration rate is larger than the vertical hydraulic conductivity, the actual infiltration will be reduced to the vertical hydraulic conductivity. Any remaining potential infiltration will be available to the MVR package as rejected infiltration."

    So I guess if you lower your infiltration rate this term should approach zero.

    I have no idea why the text description in the budget file states "HORT+DUNN". Maybe somebody else could clarify that.

  4. Hi Bruce, it is hard to tell. I suggest you to compare the HK and Sy arrays of the HDF5 and native MF version of your model. See if they are different. They should be exactly same.

    You could read the HDF file directly using Python for instance, see the discussion here.

    Good luck

    Michal

  5. Hi,

    I think what you are trying to achieve could be done only aproximately, because you only have to assume what the head in the model will be. Anyway, for a given cell
     

    Qb = cb (hb - h), where
    
    Qb is the flow into the cell from the source
    cb is the conductance between external source and the cell
    hb is the head assigned to the external source and
    h is the model calculated head in the reference cell

    You problably have measured hb, so I would calculate cb this way:

    cb = Qb / (hb-h)

    But as I said, you can only assume what the value of h will be.

    Maybe calibrating the model against the measured flow could be a better practice.

  6. Hi, it is actually quite simple. Each stress period is divided into one or more time steps. The time steps within a stress period does not have to be the same length, but can increase exponentialy in length in order to capture system response to change in stress more accurately. So by specifying a multiplier f, each following step is f multiple of the previous step. So if you have stress period of lets say 10 days and you want to solve it in 3 time steps with 4.0 as multiplier, modflow would calculate the following step lengths:
     {0.47619, 1.90476, 7.61905}

    It is not so hard to figure the realtion as for timesteps 0 to n:
    step_length_i=(perlen*f^i)/Sum[f^i, {i, 0, n - 1}]

     

  7. Hi,

    I think you could use the conceptual model approach. You need to think of your cross section model as a normal layered aquifer model, that has only a single row. Therefore everything has to be treated layer-wise. You need to follow the MODFLOW logic, there is no special case for cross section models. And you obviously need to add Y coordinate to your elevation dataset.

  8. Hi Jonathan,

    I was dealing with similar issue some time ago. There is not a satisfying way how to do it in GMS. The solution depends on the complexity of the task, but it involves grid level approach. You can isolate cells by material, select boundary cells by intersecting with boundary Arc (from Top view, Orto off) and adding a Sink/Source term. In some situtions it may be convenient to map BC form conceptual model to all layers and delete from grid where appropriate. This works if the boundaries do not overlap. Or you can modify the Package outside GMS with some script.

  9. Hi Rachel,

    The problem is probably that the concentration observation file is not generated after your model finishes. Therefore PEST complains that it cannot find it. The reasons for this could be many. I suggest you to study how model equivalent concentration observations are generated with VM. Alternatively, you could do it manualy with MOD2OBS program from PEST utility suit for instance. When not sure what to do, read PEST manual.

  10. Hi, I think that what you describe could be achieved by general head boundary. Without specifying head at the boundary modflow cannot calculate groundwater gradient between the cell and the external region. So I think the problem is unsolvable the way you try to aproach it. I suggest using GHB package. If you have no information about heads outside the model domain, you could for instace specify GHB head the same as some initial model result head and together with lower conductance the head in the boundary condition cells will vary based on pumping/injection in the model domain. For higher conductance values the boundary will resemble constant head boundary while for lower values it will act more as no flow BC. The key is to find the right value which could be achieved through parameter estimation process.

  11. Hello,

    I think that MTD3MS was released long before MODFLOW-NWT. MODFLOW-NWT does not change IBOUND value to 0 if cell goes dry and this is probably the cause of problems you are facing. I think MT3D-USGS has improved handling of such situation:

    MT3D-USGS also includes the capability to route a solute through dry 
    cells that may occur in the Newton-Raphson formulation of MODFLOW (that is,
    MODFLOW-NWT).
  12. Hi, I think it is not directly posible through MODFLOW, but maybe you could create the second drawdown dataset in postprocessing using GMS data calculator. Just compute difference in head between the first timestep of the first simulation and the transient head dataset of the second sim. That should be the drawdown you are looking for.

  13. Hi bokuhata,

    I assume you have maped a general head boundary (GHB) at the coastline. When you select an arc representing this boundary you see, in the tooltip area of GMS, a flow across this boundary to/from an external reservoir. On the other hand, the Zone flow in Zonebudget is the flow between the two zones in the model. From this you see it has to be different.

  14. Hi Aly,

    I think there is no straight forward way to do this in GMS. I would suggest the following procedure:

    1. Create the profile as a regular 3d grid with high enough resolution to carry the material information (1 row, many columns, many layers)

    2. Export your grid into a text file with the following format:

    Cell_id		X		Y		Z

    Y coordinate should be constant for all cells.

    3. Swap Y<->Z

    4. Import into GIS

    5. Digitize the material polygons in GIS in X-Y space

    6. Select the cell centers according to the material polygon and asign material ID into a new column in the atribute table (spatial join)

    7. Export the atribute table into a text file

    8. Copy the material column into a new file and format the new file as GMS 3d dataset format (see GMS wiki for details)

    9. Import the new material dataset to the existing 3d grid

    10. Select cells in GMS according to the material dataset and specify material properties as you like

     

  15. Yes, but if the model is steady, than the best approach is to set the initial heads high, because during the consecutive aproximation of the solution the chance of making cell inactive as a result of undershoot is lower. It can happen anyway, but in your case it looks fine.

    Btw. I think the cells never get rewetted because they are initialy dry.

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