This tutorial shows a simulation using two models, demonstrating how to use exchanges and exchange subpackages.
Introduction to Multiple ModelsMODFLOW-6 simulations can contain multiple models, which can be linked through the groundwater exchange package, which can contain mover and ghost node correction subpackages.
The following code sets up a basic simulation.
# package import from tempfile import TemporaryDirectory
# set up where simulation workspace will be stored temp_dir = TemporaryDirectory() workspace = temp_dir.name name = "tutorial09_mf6_data"
# create the FloPy simulation and tdis objects
sim = flopy.mf6.MFSimulation(
sim_name=name, exe_name="mf6", version="mf6", sim_ws=workspace
)
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)],
)
Groundwater Flow Model Setup
We will start by setting up two groundwater flow models that are part of the same simulation.
# set up first groundwater flow model
name_1 = "ex_1_mod_1"
model_nam_file = f"{name_1}.nam"
gwf = flopy.mf6.ModflowGwf(sim, modelname=name_1, model_nam_file=model_nam_file)
# create the discretization package
bot = [-10.0, -50.0, -200.0]
delrow = delcol = 4.0
nlay = 3
nrow = 10
ncol = 10
dis = flopy.mf6.modflow.mfgwfdis.ModflowGwfdis(
gwf,
pname="dis-1",
nogrb=True,
nlay=nlay,
nrow=nrow,
ncol=ncol,
delr=delrow,
delc=delcol,
top=0.0,
botm=bot,
)
# create npf package
npf = flopy.mf6.ModflowGwfnpf(
gwf,
pname="npf-1",
save_flows=True,
icelltype=[1, 1, 1],
k=10.0,
k33=5.0,
xt3doptions="xt3d rhs",
# rewet_record="REWET WETFCT 1.0 IWETIT 1 IHDWET 0",
)
# create ic package
ic_package = flopy.mf6.modflow.mfgwfic.ModflowGwfic(gwf, strt=0.0)
# create ghb package
ghb_spd = {0: [((0, 0, 0), -1.0, 1000.0)]}
ghb = flopy.mf6.modflow.mfgwfghb.ModflowGwfghb(
gwf,
print_input=True,
print_flows=True,
pname="ghb-1",
maxbound=1,
stress_period_data=ghb_spd,
)
# create wel package
welspd = {0: [((0, 5, ncol - 1), -100.0)]}
wel = flopy.mf6.ModflowGwfwel(
gwf,
print_input=True,
print_flows=True,
mover=True,
stress_period_data=welspd,
save_flows=False,
pname="WEL-1",
)
# set up second groundwater flow model with a finer grid
name_1 = "ex_1_mod_2"
model_nam_file = f"{name_1}.nam"
gwf_2 = flopy.mf6.ModflowGwf(sim, modelname=name_1, model_nam_file=model_nam_file)
# create the flopy iterative model solver (ims) package object
# by default flopy will register both models with the ims package.
ims = flopy.mf6.modflow.mfims.ModflowIms(
sim, pname="ims", complexity="SIMPLE", linear_acceleration="BICGSTAB"
)
# no need to create a new ims package. flopy will automatically register
# create the discretization package
bot = [-10.0, -50.0, -200.0]
dis_2 = flopy.mf6.modflow.mfgwfdis.ModflowGwfdis(
gwf_2,
pname="dis-2",
nogrb=True,
nlay=nlay,
nrow=nrow * 2,
ncol=ncol * 2,
delr=delrow / 2.0,
delc=delcol / 2.0,
top=0.0,
botm=bot,
)
# create npf package
npf_2 = flopy.mf6.ModflowGwfnpf(
gwf_2,
pname="npf-2",
save_flows=True,
icelltype=[1, 1, 1],
k=10.0,
k33=5.0,
xt3doptions="xt3d rhs",
# rewet_record="REWET WETFCT 1.0 IWETIT 1 IHDWET 0",
)
# create ic package
ic_package_2 = flopy.mf6.modflow.mfgwfic.ModflowGwfic(gwf_2, strt=0.0)
# create ghb package
ghb_spd = {0: [((0, 0, 19), -10.0, 1000.0)]}
ghb_2 = flopy.mf6.modflow.mfgwfghb.ModflowGwfghb(
gwf_2,
print_input=True,
print_flows=True,
pname="ghb-2",
maxbound=1,
stress_period_data=ghb_spd,
)
# create lak package
lakpd = [(0, -2.0, 1)]
lakecn = [(0, 0, (0, 5, 0), "HORIZONTAL", 1.0, -5.0, 0.0, 10.0, 10.0)]
lak_2 = flopy.mf6.ModflowGwflak(
gwf_2,
pname="lak-2",
print_input=True,
mover=True,
nlakes=1,
noutlets=0,
ntables=0,
packagedata=lakpd,
connectiondata=lakecn,
)
Connecting the Flow Models
The two groundwater flow models created above are now part of the same simulation, but they are not connected. To connect them we will use the gwfgwf package and two of its subpackages, gnc and mvr.
Use exchangedata to define how the two models are connected. In this example we are connecting the right edge of the first model to the left edge of the second model. The second model is 2x the discretization, so each cell in the first model is connected to two cells in the second model.
gwfgwf_data = []
row_2 = 0
for row in range(0, nrow):
gwfgwf_data.append([(0, ncol - 1, row), (0, 0, row_2), 1, 2.03, 1.01, 2.0])
row_2 += 1
gwfgwf_data.append([(0, ncol - 1, row), (0, 0, row_2), 1, 2.03, 1.01, 2.0])
row_2 += 1
# create the gwfgwf package
gwfgwf = flopy.mf6.ModflowGwfgwf(
sim,
exgtype="GWF6-GWF6",
nexg=len(gwfgwf_data),
exgmnamea=gwf.name,
exgmnameb=gwf_2.name,
exchangedata=gwfgwf_data,
filename="mod1_mod2.gwfgwf",
)
Due to the two model’s different cell sizes, the cell centers of the first model do not align with the cell centers in the second model. To correct for this we will use the ghost node correction package (gnc).
gnc_data = []
col_2 = 0
weight_close = 1.0 / 1.25
weight_far = 0.25 / 1.25
for col in range(0, ncol):
if col == 0:
gnc_data.append(
(
(0, nrow - 1, col),
(0, 0, col_2),
(0, nrow - 1, col),
(0, nrow - 1, 0),
1.00,
0.0,
)
)
else:
gnc_data.append(
(
(0, nrow - 1, col),
(0, 0, col_2),
(0, nrow - 1, col),
(0, nrow - 1, col - 1),
weight_close,
weight_far,
)
)
col_2 += 1
if col == ncol - 1:
gnc_data.append(
(
(0, nrow - 1, col),
(0, 0, col_2),
(0, nrow - 1, col),
(0, nrow - 1, 0),
1.00,
0.0,
)
)
else:
gnc_data.append(
(
(0, nrow - 1, col),
(0, 0, col_2),
(0, nrow - 1, col),
(0, nrow - 1, col + 1),
weight_close,
weight_far,
)
)
col_2 += 1
# set up gnc package
fname = "gwfgwf.input.gnc"
gwfgwf.gnc.initialize(
filename=fname,
print_input=True,
print_flows=True,
numgnc=ncol * 2,
numalphaj=2,
gncdata=gnc_data,
)
The extraction well at the right-hand side of the first model is pumping the water it extracts into a nearby lake at the left-hand side of the second model. Using the mover (mvr) package, water extracted from the first model’s wel package is moved to the second model’s lak package.
package_data = [(gwf.name, "WEL-1"), (gwf_2.name, "lak-2")]
period_data = [(gwf.name, "WEL-1", 0, gwf_2.name, "lak-2", 0, "FACTOR", 1.0)]
fname = "gwfgwf.input.mvr"
gwfgwf.mvr.initialize(
filename=fname,
modelnames=True,
print_input=True,
print_flows=True,
maxpackages=2,
maxmvr=1,
packages=package_data,
perioddata=period_data,
)
sim.write_simulation() sim.run_simulation()
writing simulation...
writing simulation name file...
writing simulation tdis package...
writing solution package ims...
writing package mod1_mod2.gwfgwf...
writing package gwfgwf.input.gnc...
writing package gwfgwf.input.mvr...
writing model ex_1_mod_1...
writing model name file...
writing package dis-1...
writing package npf-1...
writing package ic...
writing package ghb-1...
writing package wel-1...
INFORMATION: maxbound in ('gwf6', 'wel', 'dimensions') changed to 1 based on size of stress_period_data
writing model ex_1_mod_2...
writing model name file...
writing package dis-2...
writing package npf-2...
writing package ic...
writing package ghb-2...
writing package lak-2...
FloPy is using the following executable to run the model: ../../home/runner/work/flopy/flopy/modflow6/bin/mf6
MODFLOW 6
U.S. GEOLOGICAL SURVEY MODULAR HYDROLOGIC MODEL
VERSION 6.7.0.dev2 (preliminary) 05/12/2025
***DEVELOP MODE***
MODFLOW 6 compiled Aug 15 2025 15:35:02 with GCC version 13.3.0
This software is preliminary or provisional and is subject to
revision. It is being provided to meet the need for timely best
science. The software has not received final approval by the U.S.
Geological Survey (USGS). No warranty, expressed or implied, is made
by the USGS or the U.S. Government as to the functionality of the
software and related material nor shall the fact of release
constitute any such warranty. The software is provided on the
condition that neither the USGS nor the U.S. Government shall be held
liable for any damages resulting from the authorized or unauthorized
use of the software.
MODFLOW runs in SEQUENTIAL mode
Run start date and time (yyyy/mm/dd hh:mm:ss): 2025/08/15 15:36:12
Writing simulation list file: mfsim.lst
Using Simulation name file: mfsim.nam
Solving: Stress period: 1 Time step: 1
Solving: Stress period: 2 Time step: 1
Run end date and time (yyyy/mm/dd hh:mm:ss): 2025/08/15 15:36:12
Elapsed run time: 0.212 Seconds
Normal termination of simulation.
try:
temp_dir.cleanup()
except PermissionError:
# can occur on windows: https://docs.python.org/3/library/tempfile.html#tempfile.TemporaryDirectory
pass
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