Note
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Model time discretization#
iMOD Python provides nice functionality to discretize your models into stress
periods, depending on the timesteps you assigned your boundary conditions. This
functionality is activated with the create_time_discretization()
method.
Basics#
To demonstrate the create_time_discretization()
method, we first have to
create a Model object. In this case we’ll use a Modflow 6 simulation, but the
imod.wq.SeawatModel
and imod.flow.ImodflowModel also support this.
Note
The simulation created in this example misses some mandatory packes, so is
not able to write a functional simulation. It is purely intended to describe
the workings of the time_discretization
method. For a fully functional
Modflow 6 model, see the examples in MODFLOW6.
Wel’ll start off with the usual imports:
import numpy as np
import pandas as pd
import xarray as xr
import imod
We can discretize a simulation as follows, this creates a
TimeDiscretization object under the key "time_discretization"
.
simulation = imod.mf6.Modflow6Simulation("example")
simulation.create_time_discretization(
additional_times=["2000-01-01", "2000-01-02", "2000-01-04"]
)
simulation["time_discretization"]
To view the data inside TimeDiscretization object:
simulation["time_discretization"].dataset
Notice that even though we specified three points in time, only two
timesteps are included in the time
coordinate, this is because Modflow
requires a start time and a duration of each stress period. iMOD Python
therefore uses three points in time to compute two stress periods with a
duration.
simulation["time_discretization"].dataset["timestep_duration"]
These two stress periods use their respective start time in their time
coordinate.
Boundary Conditions#
The create_time_discretization
method becomes especially useful if we add boundary
conditions to our groundwater model. We’ll first still have to initialize a
groundwater flow model though:
gwf_model = imod.mf6.GroundwaterFlowModel()
We’ll create a dummy grid, which is used to provide an appropriate grid to the boundary condition packages.
template_data = np.ones((2, 1, 2, 2))
layer = [1]
y = [1.5, 0.5]
x = [0.5, 1.5]
dims = ("time", "layer", "y", "x")
Next, we can assign a Constant Head boundary with two timesteps:
chd_times = pd.to_datetime(["2000-01-01", "2000-01-04"])
chd_data = xr.DataArray(
data=template_data,
dims=dims,
coords={"time": chd_times, "layer": layer, "y": y, "x": x},
)
gwf_model["chd"] = imod.mf6.ConstantHead(head=chd_data)
gwf_model["chd"].dataset
We’ll also assign a Recharge boundary with two timesteps, which differ from the ConstantHead boundary:
rch_times = pd.to_datetime(["2000-01-01", "2000-01-02"])
rch_data = xr.DataArray(
data=template_data,
dims=dims,
coords={"time": rch_times, "layer": layer, "y": y, "x": x},
)
gwf_model["rch"] = imod.mf6.Recharge(rate=rch_data)
gwf_model["rch"].dataset
We can now let iMOD Python figure out how the simulation’s time should be discretized. It is important that we provide an endtime, otherwise the duration of the last stress period cannot be determined:
endtime = pd.to_datetime(["2000-01-06"])
simulation_bc = imod.mf6.Modflow6Simulation("example_bc")
simulation_bc["gwf_1"] = gwf_model
simulation_bc.create_time_discretization(additional_times=endtime)
simulation_bc["time_discretization"].dataset
Notice that iMOD Python figured out that the two boundary conditions, both with two timesteps, should lead to three stress periods!
Specifying extra settings#
The TimeDiscretization
package also supports other settings, like the
amount of timesteps which Modflow should use within a stress period, as well
as a timestep multiplier, to gradually increase the timesteps modflow uses
within a stress period. This can be useful when boundary conditions change
very abruptly between stress periods. These settings are set by accessing
their respective variables in the dataset
.
times = simulation_bc["time_discretization"].dataset.coords["time"]
simulation_bc["time_discretization"].dataset["timestep_multiplier"] = 1.5
simulation_bc["time_discretization"].dataset["n_timesteps"] = xr.DataArray(
data=[2, 4, 4], dims=("time",), coords={"time": times}
)
simulation_bc["time_discretization"].dataset
Total running time of the script: (0 minutes 0.141 seconds)