Vague instructions on NetCDF dimension writing method

[kasra-keshavarz]

summa/docs/input_output/SUMMA_input.md

Line 217 in 19a7f9c

* <a id="forcing_file_dimensions">Forcing dimensions</a>: SUMMA expects the dimensions of the forcing NetCDF file as `(hru,time)`. Note that different programming languages interact with NetCDF dimensions in different ways. For example, a NetCDF file with dimensions `(hru,time)` generated by Python, will be read by Fortran as a file with dimensions `(time,hru)`.

Basically the issue is the line above. Do we know which Python library messes with the order of dimensions in writing NetCDF files specifically?

I can send a PR clarifying this point or update the writing if you are aware which library (@ specific version) causes/used to cause this issue.

[wknoben]

I'm unsure which specific library prompted this (probably xarray or netcdf4 directly) but this is not a library issue. It is a difference in conventions used for the memory layout of arrays between Python (C-based) and Fortran. The netcdf convention docs provides a brief explanation (section "most rapidly varying dimension"; https://cfconventions.org/cf-conventions/cf-conventions.html).

If I recall correctly, SUMMA has the dimension order hard-coded when it reads the data, and to get the files to read properly and also account for these different conventions, you need to swap the dimension order depending on the language being used. So SUMMA expects the order to be (hru,time), and to get a Python-written file to be read like that by Fortran, you need to put the dimensions in as (time,hru).

The current description is vague because I am only vaguely aware of how this all actually works. If you can clarify things beyond what we currently have that would be very welcome.

Other related info:

• https://stackoverflow.com/questions/28275442/netcdf4-python-dimensions-reordered
• https://stackoverflow.com/questions/28275442/netcdf4-python-dimensions-reordered
  o Reordering of 2D data dimensions when writing to file Unidata/netcdf4-python#337 (comment)
  o Reordering of 2D data dimensions when writing to file Unidata/netcdf4-python#337 (comment)