Clarity on recharge package - how to ensure the top active layer/cell receive recharge

[Larkiie]

Hi all,

I'm still pretty new to modelling, and were thrown a bit in on the deep end, so pardon if my expressions are not quite code-savvy :-).

I'm struggling a bit with understanding the recharge module(s) - specifically how to ensure that it is the top active layer (with water) that receives the recharge.

When I read the ModFlow6 documentation on recharge (from 2017), they write:
"MODFLOW has typically had three options for specifying recharge: (1) apply recharge to layer 1, (2) apply recharge to a user-defined layer, and (3) apply recharge to the highest active cell. These options have been simplified into two options in MODFLOW 6. The default in MODFLOW 6 is equivalent to option 3 apply recharge to the highest active cell."

When I read Flopy documentation I find both a rcha module and a rch module.
(I'm currently using the rcha module in my code - I got it from a supervisor. To me it is not clear what the difference between modules is).
When I read the flopy documentation, it is not clear to me what the default setup is.
For example in the rcha module there is the irch value:
"irch (integer) IRCH is the layer number that defines the layer in each vertical column where recharge is applied. If IRCH is omitted, recharge by default is applied to cells in layer 1. "

• I read this as the topmost layer, and not as the topmost active layer/topmost layer with water?

I don't see the same parameter in the rch module documentation.

So, how to ensure the topmost layer with water receive the recharge? i.e. if water level drops below the bottom of the top cell, it should be next cell that receives recharge?

I'll pass my current rcha setup, though this is also more of a general question, I suppose.

child_rch = flopy.mf6.ModflowGwfrcha(model=child_gwf,
loading_package=False,
readasarrays=True,
fixed_cell=None,
auxiliary=None,
auxmultname=None,
print_input=None,
print_flows=None,
save_flows=False,
timearrayseries=None,
observations=None,
irch=irch,
recharge=rchinp,
aux=None,
filename='{}_RCH-1.rch'.format(child_modelname),
pname='{}_RCH-1'.format(child_modelname),
#parent_file=None,
)

irch is = -1
and rchinp has my recharge values

Cheers

[vcantarella]

The way I am interpreting your code is that the recharge will be applied to the underlying cell if the current cell in layer 1 gets dry. This is because your fixed_cell keyword is set to None.
You can see this in the docs:

"""
fixed_cell (keyword) – indicates that recharge will not be reassigned to a cell underlying the cell specified in the list if the specified cell is inactive.
"""

But if you want to be sure, I would consider testing it. A simple 2 layer model with the first layer as inactive, for example.