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General Questions

Question: Could you give me a brief run down of the type of data I would require to run an aquifer through Modflow.

Ans: Depening on the type of the aquifer (confined or unconfined) and the type of simulation (steady-state or transient) you will need different aquifer parameters. Here are the required aquifer parameters:

1. Steady-state flow simulation

1. Confined (2D):
   Transmissivity.
2. Confined (3D):
   Transmissivity.
   Vertical leakance
3. Unconfined (2D):
   Geometry of the aquifer (bottom elevation of the aquifer)
   Hydraulic conductivity
4. Confined+unconfined (3D):
   Geometry of the aquifer (bottom elevation of the first layer and the top and bottom elevation of all other underlying layers).
   Horizontal hydraulic conductivity (Transmissivity for confined layers can be obtained by K*thickness)
   Vertical hydraulic conductivity or vertical leakance (vertical leakance can be obtained by using Kv and the layer thickness).

2. Transient flow simulation

1. Confined (2D):
   Transmissivity
   Storage coefficient
2. Confined (3D):
   Transmissivity
   Vertical leakance
   Storage coefficient
3. Unconfined (2D):
   Geometry of the aquifer (bottom elevation of the aquifer)
   Hydraulic conductivity
   Specific yield
4. Confined+unconfined (3D):
   Geometry of the aquifer (bottom elevation of the first layer and the top and bottom elevation of all other underlying layers).
   Horizontal hydraulic conductivity (Transmissivity for confined layers can be obtained by K*thickness)
   Vertical hydraulic conductivity or vertical leakance (vertical leakance can be obtained by using Kv and the layer thickness).
   Specific yield for unconfined layers.
   Specific storage or storage coefficient for confined layers (storage coefficient can be obtained by specific storage times layer thickness)
   

Aquifer geometry is required by modeling solute transport for all cases.

Question: we would need more informations about a parameter: "specific yield". Can you give us a definition about this parameter and the difference between "effective porosity" and "specific yield" in unconfined layer.

Ans: Here are the basic definitions of porosity and specific yield used in the groundwater hydraulics (after Freeze & cherry, Groundwater; and Bear & Verruijt, Modeling groundwater flow and pollution):

1. Porosity:
   If the total unit volumn V of a soil rock is divided into the volume of the solid portion Vs and the volume of viods Vv, the porosity n is defined as n=Vv/V. Sometimes, even in the flow of a single homogeneus fluid, part of the fluid in the pore space is immobile (or partially so). This may occur when the flow takes place in a fine textured medium where adhesion (i.e., the attraction to the solid surface of the porous matrix of the fluid molecules adjacent to it) is important. The effective porosity with respect to flow through the medium is generally less than the porosity.
2. Specific Yield Sy:
   It is defined as the volumn of water that an unconfined aquifer releases from storage pre unit surface area of aquifer per unit decline in the water table. One should be careful not to identify the specific yield with the porosity of a phreatic aquifer. As water is being drained from the interstices of the soil, the drainage is never a complete one. A certain amound of water is retained in the soild against gravity by capillary forces. After drainage has stopped, the volume of water retained in an aquifer per unit (horizontal) area and unit drop of the water table is called specific retention Sr. Thus
   Sy + Sr = n
   For this reason, Sy ( Relationship between specific yield, porosity and specific retention can be found in text books, e.g., Dynamics of fluids in porous media (Jacob Bear).

PMWIN Specific Questions

Question: I have exisitng MODFLOW files (ascii), and I want to import them into PMWIN. Can this be done, or do they have to be loaded into PMWIN by hand?

Ans: You need to load the model into PMWIN by hand. Fortunately, you can split most of the MODFLOW input files into ASCII matrices and use Value>Matrix>Browse of the Data Editor to load each of these matrices into the Model. Before you can do this, you have to create a new model with the same number of cells in all three directions and specify the cell sizes in both x and y directions by hand.

Question: Is it possible to export the x and y cell matrices?

Ans: PMWIN always saves the grid data in a grid specification file with the extension .GRD. This file is saved in ASCII and has the following format:

NY, NX
X0, Y0, A
dx(i), i=1 ... NX
dy(j), j=1 ... NY

where NX is the number of columns and NY is the number of rows. (X0, Y0) are the coordinates of the left-upper corner of the model grid. A is the rotation angle of the grid, expressed in degrees and measured counterclockwise from the positive x-direction. dx(i) is the width of column i and dy(j) is the width of row j.

Question: Is it possible to save an imported base map along with model data, contours, etc. I try saving contoured model data to a graphics file (dxf format) for import into a grpahics program but only seem to get the data without the map.

Ans: No, PMWIN will not save the base map along with model data (except you save them in BMP files). The basic idea was that if one has a DXF map and saved other model data maps, he/she will be able to merge these maps with graphics processors (such as CorelDraw or AutoCAD), so it is no need to save the base maps (once again) in a file.

PEST Specific Questions

Question:The template file BCFTPL.DAT is not writing all of the parameters indicated in the parameter list. The parameter list was writen in PMWIN using the zone definitions. How can I make sure that the preprocessor will include all of the inidicated parameters within the parameters list in the template file?

Ans: There are at least two situations that not all parameters are written to BCFTPL.DAT. The second situation is more complicated.
1. A zone is declared as a parameter but the parameter is not Active (The active flag of the corresponding parameter is No.)
2. Depend on the settings of layer-types and the simulation types (steady state or transient), not all user-specified data will be written to the BCF and BCFTPL files. To make sure which parameters will be included in these files, please refer to the input instructions of the Block-Centered Flow 2 Package (p. A-11 of the PMWIN manual).

A common mistake is described as follows: One specifies that transmissivity of a layer should be calculated by using the layer thickness and hydraulic conductivity (please refer to Layer Type of p. 3-10 to 3-12) and then defines a zone of Transmissivity as an estimated parameter. In this case, the zone of transmissivity will not be written to BCFTPL.DAT, because PMWIN will always use the hydraulic conductivity and the layer thickness to calculate the transmissivity.

Finally, you can make sure "What parameters are used during the estimation process" by examing the run record file of PEST. A run record file has the extension .REC.

Question: If you have a number of observation bores (say n) and you are doing a transient MODFLOW run and you have less than n readings for one particular output time, then the instruction set for that output time seems to be incorrect. To read each borehole water level from the MODBORE putput file MODELOUT.DAT, you begin the instruction with "l1". However if there is no reading pertaining to a particular bore at a certain time, then you still being the instruction line for the next bore with "l1" when, in fact, it should be "l2" because of the missing reading.

Ans: If the observation (reading) of a particular bore at a certain time is not available. You can give any value to the reading and set the weight to zero. This reading will be ignored by PEST.

STR1 Package Questions

Question: I have a question regarding the processing of the input file STR1.DAT for MODFLOW with the software "Processing Modflow for Windows, version 4.0". My problem is thus: The first line of STR1.dat is made up of constants. The 6th constant (CONST) is related to the stream flow. If the units being used in the model for length are meters and for time are seconds, this number should be 1. Does this value make a difference in the interpretation of the stream input? The input values for the model I am working on are all based on the units of feet and days. CONST should be 128390.4 (the conversion factor from cubic meters per second to cubic feet per day). Why doesn't the software write the proper CONST value? and how can I remedy this problem (besides editing STR1.dat by hand before each model run)?

Ans: The STR1 package calculates the depth in each reach by using equation 3.14 (please see page 3-21 of the PMWIN manual). Since the manning's roughness coefficient (n) and the "constant" (C) are always used at the same time, we need only to specify the value n/C to the program (denoted as Manning's roughness coeff./C. in Fig. 3.13). The unit conversion factor is already given in the value n/C and the 6th constant will always be 1. The C values given in equation 3.12 of the manual of PMWIN are incorrect!! The C value should be 128390.4 for units of cubic feet per day and 86400 for units for cubic meter per day.

How to model lakes

Question: I have been using your package to model natural and artifical lakes. Do you have any examples of modelling lakes? For the initial model, I assumed constant heads for each of the lakes. This is okay for a general representation of the groundwater flow, but I would like to be able to model the combined inflow/outflow based on recharge and evapotranspiration; then let the model tell me the predicted lake levels. I tried the river package, but the lakes are larger than individual cells, so that the reaches don't make sense. I also tried the drain package, but that only handles outflow. Any suggestions??

Ans: There are at least two possibilities to model lakes and reservoirs. The first one is to use the new Reservoir Package which is implemented in the PMWIN 4.1.
The other possibility is to assign the following parameters for the lake-cells:

1. Specific Yield (and/or effective porosity) =1
2. Hydraulic conductivities (both vertical and horizontal): 1 m/s (or large enough so that the flow resistance in the lake-cells can be ignored).
3. The influence of confined storage coefficient can normally be ignored. All other parameters do not need to be changed.
   

How to model multi-layered wells

Question: I am currently interested in numerical modeling of pumping tests in multi-layered aquifers. An important issue for this kind of modeling is how to handle the vertical flow movement through the well bore in the pumping and observation wells which penetrate two or more aquifers.

Ans: If you are using a FD-model, such as MODFLOW, you could use the following approach: Set a very large vertical hydraulic conductivity (or vertical "leakance"), say 1 m/s or 0.1 l/s, to cells of the well. The pumping rate for each penetrated layer can be equal to the total rate divided by the number of the penetrated layers. For layers, which are not screened, you can use the Horizontal Flow Barriers (a package of MODFLOW) to confine the well, so that no discharge will occur in those layers. In this way, the flow rates into the screened sections will be calculated by MODFLOW implicitly. The exact flow rates can be obtained by calculating the water budgets for the wells. The other possibility is to use the Multiaquifer Well Package. This package has been developed in 80's but has never been published. If you need the code, contact Wen-Hsing Chiang.