Tutorial 1: Confined Steady-State Flow Model

This tutorial demonstrates use of FloPy to develop a simple MODFLOW model. And I mean really simple. Note that you can access this tutorial python script from here.

Getting Started

If FloPy has been properly installed, then it can be imported as follows:

import flopy

However, you may have a custom version of flopy, or maybe flopy hasn’t been officially “installed”. In this case, you can tell python where flopy is by adding the flopy path to sys.path. In this case, we use the following commands to set the path correctly:

import os
import sys
import numpy as np
flopypth = os.path.join('..', '..', 'flopy')
if flopypth not in sys.path:
    sys.path.append(flopypth)
import flopy

The os.path.join function is used here to create the path so that it will work on any operating system. A relative path is used here to indicate that the flopy package is contained two folders up and down into the flopy.svn folder.

Now that we can import flopy, we begin creating our simple MODFLOW model.

Creating the MODFLOW Model

One of the nice things about creating models in python is that it is very easy to change one or two things and completely change the grid resolution for your model. So in this example, we will design our python script so that the number of layers, columns, and rows can be easily changed.

We can create a very simple MODFLOW model that has a basic package (BAS), discretization input file (DIS), layer-property flow LPF) package, output control (OC), and preconditioned conjugate gradient (PCG) solver. Each one of these has its own input file, which will be created automatically by flopy, provided that pass flopy the correct information.

Discretization

We start by creating our flopy model object as follows:

# Assign name and create modflow model object
modelname = 'tutorial1'
mf = flopy.modflow.Modflow(modelname, exe_name='mf2005')

Next, let’s proceed by defining our model domain and creating a MODFLOW grid to span the domain:

# Model domain and grid definition
Lx = 1000.
Ly = 1000.
ztop = 0.
zbot = -50.
nlay = 1
nrow = 10
ncol = 10
delr = Lx/ncol
delc = Ly/nrow
delv = (ztop - zbot) / nlay
botm = np.linspace(ztop, zbot, nlay + 1)

With this information, we can now create the flopy discretization object by entering the following:

# Create the discretization object
dis = flopy.modflow.ModflowDis(mf, nlay, nrow, ncol, delr=delr, delc=delc,
                               top=ztop, botm=botm[1:])

The obvious question at this point is, how do I know which arguments are required by this strange thing called flopy.modflow.ModflowDis? Fortunately, there is an online help page for each one of the model objects. The page for the DIS input file is located at flopy.modflow.mfdis.

Basic Package

Next we can create a flopy object that represents the MODFLOW Basic Package. Details on the flopy BAS class are at: flopy.modflow.mfbas. For this simple model, we will assign constant head values of 10. and 0. to the first and last model columns (in all layers), respectively. The python code for doing this is:

# Variables for the BAS package
ibound = np.ones((nlay, nrow, ncol), dtype=np.int32)
ibound[:, :, 0] = -1
ibound[:, :, -1] = -1
strt = np.ones((nlay, nrow, ncol), dtype=np.float32)
strt[:, :, 0] = 10.
strt[:, :, -1] = 0.
bas = flopy.modflow.ModflowBas(mf, ibound=ibound, strt=strt)

Layer-Property Flow Package

Details on the flopy LPF class are at: flopy.modflow.mflpf. Values of 10. are assigned for the horizontal and vertical hydraulic conductivity:

# Add LPF package to the MODFLOW model
lpf = flopy.modflow.ModflowLpf(mf, hk=10., vka=10.)

Because we did not specify a value for laytyp, Flopy will use the default value of 0, which means that this model will be confined.

Output Control

Details on the flopy OC class are at: flopy.modflow.mfoc. Here we can use the default OC settings by specifying the following:

# Add OC package to the MODFLOW model
oc = flopy.modflow.ModflowOc(mf)

Preconditioned Conjugate Gradient Package

Details on the flopy PCG class are at: flopy.modflow.mfpcg. The default settings used by flopy will be used by specifying the following commands:

# Add PCG package to the MODFLOW model
pcg = flopy.modflow.ModflowPcg(mf)

Writing the MODFLOW Data Files

The MODFLOW input data files are written by simply issuing the following:

# Write the MODFLOW model input files
mf.write_input()

Running the Modeling

Flopy can also be used to run the model. The model object (mf in this example) has an attached method that will run the model. For this to work, the MODFLOW program must be located somewhere within the system path, or within the working directory. In this example, we have specified that the name of the executable program is ‘mf2005’. Issue the following to run the model:

# Run the MODFLOW model
success, buff = mf.run_model()

Here we have used run_model, and we could also have specified values for the optional keywords silent, pause, and report.

Post-Processing the Results

Now that we have successfully built and run our MODFLOW model, we can look at the results. MODFLOW writes the simulated heads to a binary data output file. We cannot look at these heads with a text editor, but flopy has a binary utility that can be used to read the heads. The following statements will read the binary head file and create a plot of simulated heads for layer 1:

import matplotlib.pyplot as plt
import flopy.utils.binaryfile as bf
plt.subplot(1,1,1,aspect='equal')
hds = bf.HeadFile(modelname+'.hds')
head = hds.get_data(totim=1.0)
levels = np.arange(1,10,1)
extent = (delr/2., Lx - delr/2., Ly - delc/2., delc/2.)
plt.contour(head[0, :, :], levels=levels, extent=extent)
plt.show()

If everything has worked properly, you should see the following head contours.

head contours in first layer