# Circle (transport)#

This example illustrates how to setup a very simple unstructured groundwater transport model using the `imod` package and associated packages.

In overview, we’ll set the following steps:

• Setting up the flow model, just like in the circle.py example

• set up the transport model

• Run the simulation.

• Visualize the results.

```from datetime import date, timedelta

import matplotlib.pyplot as plt
import meshzoo
import numpy as np
import xarray as xr
import xugrid as xu

import imod

# helper function for creating iterable with all dates between two dates
def daterange(date1, date2):
for n in range(int((date2 - date1).days) + 1):
yield date1 + timedelta(n)
```

## Create a mesh#

As explained in circle.py we first generate a grid and a hydraulic conductivity array

```nodes, triangles = meshzoo.disk(6, 6)
nodes *= 1000.0
grid = xu.Ugrid2d(*nodes.T, -1, triangles)

nface = len(triangles)
nlayer = 2

idomain = xu.UgridDataArray(
xr.DataArray(
np.ones((nlayer, nface), dtype=np.int32),
coords={"layer": [1, 2]},
dims=["layer", grid.face_dimension],
),
grid=grid,
)
icelltype = xu.full_like(idomain, 0)
k = xu.full_like(idomain, 1.0, dtype=float)
k33 = k.copy()

bottom = idomain * xr.DataArray([5.0, 0.0], dims=["layer"])
```

## Recharge#

We need a recharge rate for the fluid and a recharge rate for the solute. The fluid recharge rate is volumetric and per unit area, so the unit is length/time. The solute recharge rate is the concentration of solute in the recharge, and has concentration units.

```rch_rate = xu.full_like(idomain.sel(layer=1), 0.001, dtype=float)
rch_concentration = xu.full_like(rch_rate, 1.0)
rch_concentration = rch_concentration.expand_dims(species=["salinity"])
```

Unlike a recharge boundary, with a prescribed head boundary we don’t know a priori whether water will flow in over the boundary or leave across the boundary. If water flows into the model over the boundary, it carries a prescribed solute concentration. If it leaves, it leaves with the concentration that was computed for the cell.

In this example we set the prescribed head value to 1 and the external concentration to 1.0 as well. The boundary only operates on the top layer.

```chd_location = xu.zeros_like(idomain.sel(layer=1), dtype=bool).ugrid.binary_dilation(
border_value=True
)
constant_concentration = constant_concentration.expand_dims(species=["salinity"])
```

## Add flow model to simulation#

```gwf_model = imod.mf6.GroundwaterFlowModel()
gwf_model["disv"] = imod.mf6.VerticesDiscretization(
top=10.0, bottom=bottom, idomain=idomain
)
concentration=constant_concentration,
print_input=True,
print_flows=True,
save_flows=True,
)
gwf_model["npf"] = imod.mf6.NodePropertyFlow(
icelltype=icelltype,
k=k,
k33=k33,
save_flows=True,
)
gwf_model["sto"] = imod.mf6.SpecificStorage(
specific_storage=1.0e-5,
specific_yield=0.15,
transient=False,
convertible=0,
)
gwf_model["rch"] = imod.mf6.Recharge(
rch_rate, concentration=rch_concentration, print_flows=True, save_flows=True
)

simulation = imod.mf6.Modflow6Simulation("circle")
simulation["GWF_1"] = gwf_model
simulation["flow_solver"] = imod.mf6.Solution(
modelnames=["GWF_1"],
print_option="summary",
csv_output=False,
no_ptc=True,
outer_dvclose=1.0e-4,
outer_maximum=500,
under_relaxation=None,
inner_dvclose=1.0e-4,
inner_rclose=0.001,
inner_maximum=100,
linear_acceleration="bicgstab",
scaling_method=None,
reordering_method=None,
relaxation_factor=0.97,
)
simtimes = daterange(date(2000, 1, 1), date(2000, 10, 10))
```

## Add transport model to simulation#

The transport model requires the flow field inside the domain computed by the flow model. It also needs the fluxes over the boundary. It uses the same discretization as the flow model. Here we create a transport model for salinity, derive sources and sinks based from the flow model, and tell it to use the same discretization.

```transport_model = imod.mf6.GroundwaterTransportModel()
transport_model["ssm"] = imod.mf6.SourceSinkMixing.from_flow_model(
gwf_model, "salinity"
)
transport_model["disv"] = gwf_model["disv"]
```

Now we define some transport packages for simulating the physical processes of advection, mechanical dispersion, and molecular diffusion dispersion. This example is transient, and the volume available for storage is the porosity, in this case 0.3.

```transport_model["dsp"] = imod.mf6.Dispersion(
1e-4, 1.0, 10.0, 1.0, 2.0, 3.0, False, False
)
transport_model["mst"] = imod.mf6.MobileStorageTransfer(0.3)
```

Now we define initial conditions (0.0) and output options for the transport simulation, and assign the transport model to the simulation as well.

```transport_model["ic"] = imod.mf6.InitialConditions(start=0.0)
transport_model["oc"] = imod.mf6.OutputControl(
save_concentration="all", save_budget="last"
)

simulation["GWT_1"] = transport_model
simulation["transport_solver"] = imod.mf6.Solution(
modelnames=["GWT_1"],
print_option="summary",
csv_output=False,
no_ptc=True,
outer_dvclose=1.0e-4,
outer_maximum=500,
under_relaxation=None,
inner_dvclose=1.0e-4,
inner_rclose=0.001,
inner_maximum=100,
linear_acceleration="bicgstab",
scaling_method=None,
reordering_method=None,
relaxation_factor=0.97,
)
```

We’ll create a new directory in which we will write and run the model.

```modeldir = imod.util.temporary_directory()
simulation.write(modeldir, binary=False)
```

## Run the model#

Note

The following lines assume the `mf6` executable is available on your PATH. The examples introduction shortly describes how to add it to yours.

```simulation.run()
```

## Open the results#

```sim_concentration = imod.mf6.out.open_conc(
modeldir / "GWT_1/GWT_1.ucn",
modeldir / "GWF_1/disv.disv.grb",
)
```

## Visualize the results#

We can quickly and easily visualize the output with the plotting functions provided by xarray and xugrid:

```fig, ax = plt.subplots()
sim_concentration.isel(time=33, layer=0).ugrid.plot(ax=ax)
ax.set_aspect(1)
``` We observe the initial water (without solute) slowly being flushed out by water coming in from the recharge with a concentration of 1.0.

Total running time of the script: ( 0 minutes 3.446 seconds)

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