River/Drain allocation: Option for negative layer missing

[JoerivanEngelen]

We currently only check for layer=0, but not for layer<0. In this case iMOD5 does something different.

From the iMOD5 docs:

⋄ LAYER > 0: in this case the layer number is a model layer to which
the package is assigned to;
⋄ LAYER < 0: in this case the package is assigned to the upper-most
active model layers;
⋄ LAYER = 0: in this case the package can be assigned to multiple
layer whereby the method is two-folded based on a) its elevation
and/or b) intersection:
⋄
Elevation
This is appropriate only for packages that have a single elevation, these are the CHD,DRN and GHB packages. Based on the
elevation the package entry is assigned to the layer in which it is;
⋄
Intersection
This is appropriate to the package RIV, ISG and WEL which assign the package entry in those model layers which intersect the
given river-elevation and river-bottom or the top- and bottom of
the well-screen. The original conductance of the RIV and ISG
package is distributed upon the length of intersection of the corresponding model layers as the original well rate of the WELpackage is distributed upon the transmissivity of intersection of
the corresponding model layers. Additional to this, for parts that
intersect a model layer partly, the mismatch with the centre of the
model cell is corrected as the fractional difference of the thickness of the model layer and the intersection length. For the MNW
package this option let the MODFLOW2005 MNW2 implementation itself distribute the well rates among layers in between the
top- and bottom elevation of screen.
All will be assigned to the first model layer in case the elevation(s) are
above the top of model layer 1. Elevations that are beneath the bottom of
the lowest model layers are discarded. In case the package is assigned
to an aquitard (part in between 2 model layers, the package is assigned
eventually to the aquifer of that model layer. This holds for quasi-3D
models only.