bug: Better error support for reading UCN file from modflow 6

[hansonmcoombs]

Describe the bug
I'm finally getting around to transitioning my workflows from mt3d to mf6 transport.

I expected loading the GWT concentration file to be supported by the flopy.utils.UcnFile, I now know that I was wrong and that it should be loaded by flopy.utils.HeadFile(path, text="concentration").

The error produced was NOT helpful : EOFError

To Reproduce

ucn_file = 'path/to/mf6/concentration.ucn'
flopy.utils.UcnFile(ucn_file)

Expected behavior
Raise a better error message when a user tries to read a mf6 transport concentration file:

e.g.,

'If this is a mf6.GWT file, please use flopy.utils.HeadFile(path, text="concentration") instead.'

[christianlangevin]

Hi @hansonmcoombs, you are right. That error message is not helpful, nor is it intuitive to access concentrations for the GWT model using HeadFile.

We have been moving away from the low-level HeadFile approach for the standard use case and instead recommend the .output approach in FloPy. So in your case, the easiest way to access concentrations is with the following syntax:

gwf = sim.gwf[0] 
gwt = sim.gwt[0]
headall = gwf.output.head().get_alldata()
concall = gwt.output.concentration().get_alldata()

Note that gwt.output.concentration() creates an instance of HeadFile behind the scenes with the appropriate instantiation arguments. Does that work for you?

[hansonmcoombs]

Hi @christianlangevin, I understand I'm doing a bit of edge case stuff, but I will say the low level access is super helpful for some use cases. I often save (just) the output files from suites of model runs and then inevitably be asked to pull something different from the data. Recreating a model object just to read data would be a bummer.

As it's lower level, I expect some friction as I get into it, but might as well make it easier for the next guy.

Another option is a wrapper class e.g. something like:

class Mf6UcnFile(HeadFile):

    def __init__(filename, text='concentration', precision, verbose **kwargs)

           super().__init__(filename, text, precision, verbose **kwargs)