unit fix on excess respiration mass balance checking

[rgknox]

Description:

Excess respiration is activated when a plant is nutrient limited but continues to accumulate carbon. When stores are full, the plant will respire excess carbon. For book-keeping purposes, we make sure that carbon fluxes match the change in the carbon stocks. This excess respiration term had a unit error when calculating these balance check fluxes. There was no error in how this term actually affected NPP, which drives plant allocations, just the checking process. The unit error was fixed and descriptive text was updated.

Also, it was noticed that unit conversions of fluxes that are per year and per day, used a global variable "hlm_days_per_year". This is an integer, but was being used in multiply and divides on real numbers. Compilers should be converting this integer to a float for us, but it is better to pre-convert it to reduce warning messages and we shouldn't assume that the compilers will fix things for us. It is better practice to specify type conversions ourselves, so I applied the conversions.

Fixes: #1354

To verify that this fix I ran the following two ELM land tests:

SMS_D_Ld20.f45_f45.IELMFATES.pm-cpu_gnu.elm-fates_rd
SMS_Ld60.f45_f45.IELMFATES.pm-cpu_gnu.elm-fates_eca

Note that the second test is typically a 20 day test, but I wanted to run it longer to give it more time to trigger the error. Both tests now pass.

Collaborators:

@glemieux @ckoven

Expectation of Answer Changes:

This should fix failing tests in ELM.
It should not affect answers. If it does, that means the integer conversion was long overdue. :/

Checklist

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All checklist items must be checked to enable merging this pull request:

Contributor

• The in-code documentation has been updated with descriptive comments
• The documentation has been assessed to determine if updates are necessary

Integrator

• FATES PASS/FAIL regression tests were run
• Evaluation of test results for answer changes was performed and results provided

Documentation

• Technical Note update:
• User's Guide update:

Test Results:

CTSM (or) E3SM (specify which) test hash-tag:

CTSM (or) E3SM (specify which) baseline hash-tag:

FATES baseline hash-tag:

Test Output:

[glemieux]

Do we need to update the calculation here in history as well? _acc_hold variables are kgC/indiv/year and resp_excess_hold is now kgC/indiv/day, correct?

fates/main/FatesHistoryInterfaceMod.F90

Lines 2839 to 2841 in 626f5e2

	hio_aresp_si(io_si) = hio_aresp_si(io_si) + &
	(ccohort%resp_g_acc_hold + ccohort%resp_m_acc_hold + &
	ccohort%resp_excess_hold) * n_perm2 / days_per_year / sec_per_day