move negative melt_on_glacier to snowfall

[lilianschuster]

In hydrological projections, negative melt_on_glacier values occurred in the past. Although the absolute values were mostly relatively small, negative melt_on_glacier (and consequently negative glacier runoff) does not makes sense.

With this pull request, no negative melt_on_glacier values should exist anymore. The previously negative values are now clipped to zero, and the negative values are added as absolute values to the snowfall_on_glacier (for mass-conservation issues).

• There is one line in a test that still fails:

  oggm/oggm/tests/test_models.py

  Line 4311 in 6952200

  assert_allclose(odf['tot_prcp'], odf['tot_prcp'].iloc[0])

  . This is because the total precipitation changes even in a constant climate (because of adding the negative values as absolute values to the snowfall_on_glacier variable). I am not sure if this needs to stay constant. @fmaussion, what do you think?

• I could have a quick check if this makes a difference in the run_with_hydro output (by doing projections for one basin). And test again if the melt_on_glacier is now everywhere non-negative.

This PR is part of the ToDo-List of #1582

• Tests added/passed (except for one line)

[fmaussion]

Thanks lily - I didn't check the maths, but its probably worth checking on one region or two if it changes the results

[fmaussion]

Yes it's ok to remove the first time step - maybe just check that tot_prcp is constant after the first year

[lilianschuster]

ok, I only check now that tot_prcp is constant after the first year. This works, but I don't know if it always works. If melt_on_glacier is negative after the first year, then the tot_prcp wouldn't be constant anymore. But I guess in this idealised case, due to the constant climate, the melt_on_glacier also does not get negative?

[lilianschuster]

I checked whether the results changed between the versions.

• as expected, melt_on_glacier is now always non-negative (checked in RGI reg 11, 06, 15)
• the new melt_on_glacier is thus always equal or larger than the old one, and the new snowfall_on_glacier is also equal or larger than the old one
• (same for the monthly values)

For single glaciers and years, the runoff can be visibly different. Over the sum of an entire RGI region, the differences are almost not visible (although the relative differences can be substantial due to sometimes small absolute numbers, specifically when comparing the mean monthly cycle of melt_on_glacier and snowfall_on_glacier).

(checks are in https://nbviewer.org/urls/cluster.klima.uni-bremen.de/~lschuster/runs_oggm_v16/compare_negative_melt_on_glacier_snowfall_on_glacier.ipynb )