ihc input for disu

[RyanConway91]

The documentation for the ihc array states:
ihc : [integer]
is an index array indicating the direction between node n and all of its m
connections. if ihc = 0 then cell n and cell m are connected in the vertical
direction. cell n overlies cell m if the cell number for n is less than m;
cell m overlies cell n if the cell number for m is less than n. if ihc = 1
then cell n and cell m are connected in the horizontal direction. if ihc = 2
then cell n and cell m are connected in the horizontal direction, and the
connection is vertically staggered. a vertically staggered connection is one
in which a cell is horizontally connected to more than one cell in a horizontal
connection.

However, when I use gridgen to build a 10-layer octree grid and use get_gridprops_disu6():

  gridprops = g.get_gridprops_disu6()
   disu = flopy.mf6.ModflowGwfdisu(model = gwf,**gridprops) 

the ihc array in the disu file looks like this ( I added the ....):

 ihc
    INTERNAL  FACTOR  1
    0  1  1  0
    0  1  1  1  0
    0  1  1  1  0
    0  1  1  1  0
    0  1  1  1  0
    0  1  1  1  0
    0  1  1  1  0
    0  1  1  1  0
    0  1  1  1  0
    0  1  1  1  0
    0  1  1  1  0
    0  1  1  1  0
    0  1  1  1  0
	....
	....
    9  0  1  1  1  1  1  1
    9  0  1  1  1  1  1
    9  0  1  1  1  1
    9  0  1  1  1  1  1
    9  0  1  1  1  1  1
    9  0  1  1  1  1  1
    9  0  1  1  1  1  1
    9  0  1  1  1  1
    9  0  1  1  1  1
    9  0  1  1  1  1  1  1
    9  0  1  1  1  1  1
    9  0  1  1  1  1
    9  0  1  1  1  1  1
    9  0  1  1  1  1
    9  0  1  1  1  1
    9  0  1  1  1  1
    9  0  1  1  1  1  1  1
    9  0  1  1  1  1
    9  0  1  1  1
    9  0  1  1
    9  0  1  1  1
    9  0  1  1  1
    9  0  1  1  1
    9  0  1  1  1
    9  0  1  1  1
    9  0  1  1  1
    9  0  1  1  1
    9  0  1  1  1
    9  0  1  1  1
    9  0  1  1  1  1
    9  0  1  1  1  1

You can see each row in the ihc array starts with a 0-based layer number. The model runs with this ihc array, but this syntax does not reflect the documentation and nothing else in mf6 is zero-based. What's going on here?

[christianlangevin]

This is a subtle flopy trick operating under the hood. Those first numbers in the IHC array are not used by MODFLOW, even though they are required. FloPy uses this slot to include the zero-based layer number. This allows flopy to plot an unstructured grid with the PlotMapView class, for example, and show specific layers.

[RyanConway91]

Thanks! Maybe add something about this to the MF6 documentation so others aren't confused in the future?