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Michal

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Posts posted by Michal


  1. 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.


  2. 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.


  3. 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.


  4. 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).

  5. 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.


  6. 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.


  7. 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

     


  8. 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.


  9. I would say that MF-USG is very different in terms of how the solution to the flow equation is aproached compared to the traditional structured modflow. However it is usually not too difficult to set up a MF-USG simulation in GMS. There are some tutorials out there that could help. I would recommend reading the MF-USG manual as well to grasp the internal differences.

    Regarding your second question. I believe there are other people here that could provide more accurate answer. The problem comes from the way MODFLOW solves the groundwater flow equation, that is by numerical approximation. During these approximations (in MF-2005) if head in a cell falls bellow the cell bottom the cell is assumed dry and is inactivated (becomes no-flow cell). However in subsequent approximation the cell could become wet and active again if certain criteria are met. For instace if head in the cell below (i,j,k-1) is 0.1 m above the cell (ijk) bottom. The 0.1 m would be the user specified threshold for conversion from dry to wet. This whole process brings instability into the solution and may cause convergence problems especialy if poor initial head values and low threshold values are specified.

    However MF-NWT adressed this issue in a brilliant way. It does not deactivate dry cells, but instead it uses upstream weighting approach to limit flow from dry cell to a neighboring partialy saturated cell. It also uses additional smoothing for conductance and storage functions which allows the use of newton numerical method for outer iterations. It therefore provides a continuous solution for all unconfined groundwater flow conditions and is very robust.

    However if you have steep aquifer your problem is different. Consider two neighboring cells of 10 m thickness. Lets say, under extremely steep conditions they share only 1 m thickness. However structured modflow will assume 10 m anyway and would not modify transmissivity accordingly. Such a solution would be incorrect. That is why I recommend using MF-USG in your case as mentioned earlier in my previous post.

    For literature I suggest reading the manuals published by USGS for modflow 2000, 2005, NWT and USG. There are plenty of further references there too.

     


  10. Hi, MODFLOW assumes flat continuous aquifer. You could probably overcome the drying/wetting problem by using Newton linearization implemented in MODFLOW-NWT. However, if there are large not negligible differences in elevation between two horizontaly adjacent cells MODFLOW results will be incorrect. Fortunately the finite volume formulation in MODFLOW-USG adressed this issue - it involves true interface area into the calculation. If I remember it right, this is supplied for each cell connection in the DISU file somewhere. If you use GMS, this should be done automaticaly for you. You could use the upstream weighting (LPF package) and newton linearization (SMS package) for a more robust solution. With this setup the model should converge to a correct solution.


  11. Hi Mina,

    the output control file should look like this:

    HEAD PRINT FORMAT 15
    HEAD SAVE FORMAT (20F10.3) LABEL
    HEAD SAVE UNIT 30
    COMPACT BUDGET FILES
    DRAWDOWN PRINT FORMAT 14
    PERIOD 1 STEP 1
         PRINT HEAD 2 6
         PRINT DRAWDOWN
         PRINT BUDGET
         SAVE BUDGET
         SAVE HEAD
     
    PERIOD 1 STEP 7
         SAVE HEAD 1 3 5
         PRINT DRAWDOWN
         SAVE BUDGET
    
    PERIOD 2 STEP 5
         PRINT HEAD
         PRINT BUDGET
         SAVE BUDGET
         SAVE HEAD

    See this page for detailed input instructions.


  12. Hi T,

    I didnt understand what you do, so forget my last comment :). I dont know much about this topic. It would be nice if GMS would allow us to do "Interpolate to modflow layers" command with RIV package (to prepare the river bottom/elev dataset and head), but i think only Head in GHB package could be distributed this way. If you decide to model it with RIV, which would be my choice, you need to prepare the package outside GMS.


  13. Hello,

    what kind of errors are there during the check? If you were able to load the simulation into gms, you should also be able to read the solution. Try right clicking on the simulation (MODFLOW in the project tree) and select Read Solution command. Then just select the modflow name file and the binary data should load.


  14. Hi Kanchana,

    its realy hard to tell what may be causing the troubles. I guess it will be something in the reaction module related to the reaction rate. Try to run the simulation for lets say a minute with time steps in the order of seconds or fractions of a second. Also increasing grid resolution may improve the solution.


  15. I guess it can. For instance if the reaction module causes abrupt changes in concentration of some specie. Does it run ok with MT3DMS? I mean without the reactions only as conservative transport?


  16. Hi Kanchana, on the stress period dialog, try to increase the Max. trans. steps variable. It is the maximum number of transport steps allowed during each time step of a flow solution. Give it a high value like 1e6 and see what happens.  While performing transport simulation with MT3D based model, time steps are further divided into transport steps. When you leave 0.0 as transport step size, the model calculates transport steps size itself according to certain criteria based on the flow solution. These stability criteria relates among others to the Courant number, you can find more about these in the MT3DMS manual (Zheng, Wang, 1999).


  17. Hi, I would like to see a functionality in GMS, that would make working with pumping data more easy. Let me make an example to explain what I mean. Consider we would like to import pumping schedule for a single well into a transient model with three periods with a length of 30 days each. The well Q for first period was -100, for second the well was off and for third it was -200. The pumping schedule extracted from a database has this format:
     

    well_name        time         Q
    
    well1              1         -100
    
    well1             61         -200

    I would like GMS to ask me during import, how do I want to treat the stress periods without any pumping data, in this case the second period. I would like to have an option to choose, that when there are no data for a given period GMS would add 0 for that period automaticaly, e. g. the well is off for that period.

    This would be realy useful, because otherwise one easily forgets to add the zeroes manualy and ends up with incorrect schedule.

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