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MODFLOW 6: Working with MODFLOW Scalar Data

This tutorial shows how to view, access, and change the underlying data variables for MODFLOW 6 objects in FloPy. Interaction with a FloPy MODFLOW 6 model is different from other models, such as MODFLOW-2005, MT3D, and SEAWAT, for example.

FloPy stores model data in data objects (MFDataArray, MFDataList, MFDataScalar objects) that are accessible from packages Data can be added to a package by using the appropriate parameters when the package is constructed and through package attributes.

The MODFLOW 6 simulation structure is arranged in the following generalized way:

Simulation --> Package --> DATA

Simulation --> Model --> Package (--> Package) --> DATA

This tutorial focuses on MODFLOW Data that is a single integer or string, or consists of boolean flag(s). These data are stored by FloPy in a MFScalar object and are referred to as MODFLOW scalar data.

Introduction to MODFLOW Scalar Data

MODFLOW single integer, strings, or boolean flag(s) are stored by FloPy as scalar data in MFScalar objects. The different types of scalar data are described below.

1. Single integer values. Examples include nrow, ncol, nlay, and nper.

2. Single string values. Examples include time_units and length_units.

3. Boolean flags. These can be found in the options section of most packages. These include perched, nogrb, print_input, and save_flows.

4. Boolean flags with an additional optional flag. These include newton under_relaxation and xt3d rhs.

In the following all four types of scalar data will be added to a model. Before adding data to your model first create a simulation (MFSimulation) and a model (MFModel) object in FloPy.

# package import
import os
from pathlib import Path
from tempfile import TemporaryDirectory

import numpy as np

import flopy

# set up where simulation workspace will be stored
temp_dir = TemporaryDirectory()
workspace = temp_dir.name
name = "tutorial05_mf6_data"

# create the flopy simulation object
sim = flopy.mf6.MFSimulation(
    sim_name=name, exe_name="mf6", version="mf6", sim_ws=workspace
)

# create the flopy groundwater flow (gwf) model object
model_nam_file = f"{name}.nam"
gwf = flopy.mf6.ModflowGwf(sim, modelname=name, model_nam_file=model_nam_file)
# create the flopy iterative model solver (ims) package object
# (both pname and complexity are scalar data)
ims = flopy.mf6.modflow.mfims.ModflowIms(sim, pname="ims", complexity="SIMPLE")

Adding MODFLOW Single Integer and String Values

Single integer and string values can be assigned on construction of the MFScalar data object, and can be assigned or changed after construction.

Below, a TDIS package is constructed with the time_units and nper parameters being assigned “DAYS” and “2”, respectively.

# create the FloPy temporal discretization object
tdis = flopy.mf6.modflow.mftdis.ModflowTdis(
    sim,
    pname="tdis",
    time_units="DAYS",
    nper=2,
    perioddata=[(1.0, 1, 1.0), (1.0, 1, 1.0)],
)

Next, time_units is reassigned a value after construction by using TDIS’s time_units attribute.

tdis.time_units = "MONTHS"

Setting MODFLOW Boolean Flags

Boolean flags can be assigned a True or False value. In the example below nogrb is assigned a value of True and then changed to false.

For this example, first some values are first defined for the discretization package

nlay = 3
h = 50.0
length = 400.0
n = 10
bot = np.linspace(-h / nlay, -h, nlay)
delrow = delcol = length / (n - 1)

Below the discretization package is created. The MODFLOW nogrb option assigned a value of True, switching this option on.

dis = flopy.mf6.modflow.mfgwfdis.ModflowGwfdis(
    gwf,
    pname="dis",
    nogrb=True,
    nlay=nlay,
    nrow=n,
    ncol=n,
    delr=delrow,
    delc=delcol,
    top=0.0,
    botm=bot,
)

The nogrb option is then switched off by setting the DIS package’s nogrb attribute to False.

dis.nogrb = False

Boolean flags with an additional optional flag can either be specified by:

1. Specifying the entire line as it would be displayed in the package file as a string (xt3doptions="xt3d rhs")

2. Specifying each flag name in a list (xt3doptions=["xt3d", "rhs"])

To turn off both flags use an empty string (xt3doptions="") or an empty list (xt3doptions=[]).

First, an NPF package is created. xt3doptions can either be turned on or off, and if it is on rhs can optionally be turned on. xt3doptions is set to the string “xt3d rhs”, turning both options on.

# create the node property flow package with xt3doptions as single
npf = flopy.mf6.modflow.mfgwfnpf.ModflowGwfnpf(
    gwf,
    rewet_record="REWET WETFCT 1.0 IWETIT 1 IHDWET 0",
    pname="npf",
    icelltype=1,
    k=1.0,
    save_flows=True,
    xt3doptions="xt3d rhs",
)

<flopy.mf6.data.mfstructure.MFDataItemStructure object at 0x7f1d543a2800>
<flopy.mf6.data.mfstructure.MFDataItemStructure object at 0x7f1d543a2830>

Next, the rhs option is turned off by setting xt3doptions to the string “xt3d”.

npf.xt3doptions = "xt3d"

Finally, both xt3d and rhs are turned off by setting xt3doptions to an empty string.

npf.xt3doptions = ""

Retrieving MODFLOW Scalar Data

MODFLOW scalar data can be retrieved with get_data, repr/str, or get_file_entry.

Retrieval Method	Description
get_data	Returns scalar value
repr/str	Returns string with a header describing how data is stored (internal, external) with a string representation of the data on the next line
get_file_entry	Returns string with the scalar keyword (if any) followed by a space and a string representation of the scalar value (if any). This is the format used by the MODFLOW-6 package file. This is the format used by the MODFLOW-6 package file.

The IMS package’s complexity option and the NPF package’s xt3doptions are printed below using the different data retrieval methods highlighted above.

First the complexity data is printed using the get_data method.

print(ims.complexity.get_data())

simple

The xt3doptions data can also be printed with get_data.

print(npf.xt3doptions.get_data())

[]

The complexity data is then printed with repr

print(repr(ims.complexity))

{internal}
('simple')

The xt3doptions data is printed with repr

print(str(npf.xt3doptions))

{internal}
([])

The complexity data is printed as it would appear in a MODFLOW 6 file using the get_file_entry method.

print(ims.complexity.get_file_entry())

  COMPLEXITY  simple

The xt3doptions data is printed as it would appear in a MODFLOW 6 file using the get_file_entry method.

print(npf.xt3doptions.get_file_entry())

try:
    temp_dir.cleanup()
except PermissionError:
    # can occur on windows: https://docs.python.org/3/library/tempfile.html#tempfile.TemporaryDirectory
    pass