Removed ESM15 version with hard-wired xfpleach parameter, #278

[rml599gh]

CABLE

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Description

In casa_delsoil the evaluation of casaflux%Pleach had two versions, one specific to ESM1.5 (selected via an ifdef) and the alternat for all other cases. Following the code through, the ESM1.5 version had a hard-wired value (1.e-4) for casaflux%fPleach. casaflux%fPleach is filled from xfPleach which is read in from the pftlookup file. The ESM1.5 specific code can be safely removed with no change to the output if xfPleach is set to 1.e-4 in the pftlookup file. Note that there are versions of the pftlookup with xfPleach = 5.e-4. I am not aware of when/who set it to this value and whether it is considered to be better/more realistic.

Fixes #278

Type of change

Please delete options that are not relevant.

• Code clean-up

Checklist

• The new content is accessible and located in the appropriate section.
• I have checked that links are valid and point to the intended content.
• I have checked my code/text and corrected any misspellings

Please add a reviewer when ready for review.

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📚 Documentation preview 📚: https://cable--422.org.readthedocs.build/en/422/

[rml599gh]

This change is only relevant for ESM1.5/ESM1.6.
I have tested the change using 3 one month tests.

1. Current code with ifdef ESM15
2. Code with ESM15 specific code removed and standard pftlookup including xfPleach=5.e-4
3. Code with ESM15 specific code removed and pftlookup changed so xfPleach=1.e-4

Run 3 should give the same results as Run 1. Run 2 should be different.
casaflux%Pleach is used in the calculation of dpSoillabdt which is then used for update Psoillab and Psoilsorb, the labile and sorbed phosphorus pools (stash code 880 and 881).
The figure shows the difference in the global mean labile and sorbed phosphorus pool at the end of January between the original (case 1) and cases 2 (black, green) and 3 (red, blue). The difference is shown as a proportion of the original global mean. The y axis is pft and the global means have been calculated without tile fraction weighting. As expected, case 3 shows no difference from case 1 (the red line is under the blue line). The change in parameter value makes a very small difference in the pool size though this would presumably accumulate over time and tend to a slightly different steady state to previously.

I am comfortable that the ifdef ESM15 and associated code can be safely removed.

[abhaasgoyal]

Looks good to me (although had a query on whether the comment could be put in a better place)

[rml599gh]

@abhaasgoyal - are you OK to approve this now with the documentation addition noted above?