Fix misleading name of "gddplant"

[samsrabin]

Description of changes

1. Rename gddplant_patch to hui_patch (and GDDPLANT to HUI).
2. Add new variable gddaccum_patch (output GDDACCUM).

Specific notes

CTSM Issues Fixed: #1655

Are answers expected to change (and if so in what way)? No.

Any User Interface Changes (namelist or namelist defaults changes)? Yes. Output variable GDDPLANT (not included by default) is now HUI, and new output GDDACCUM has been added to track actual accumulation of GDDs since planting without any modifications such as occurs here:

CTSM/src/biogeochem/CNPhenologyMod.F90

Lines 2044 to 2046 in d9ae4b4

	if (peaklai(p) >= 1) then
	hui(p) = max(hui(p),huigrain(p))
	endif

Testing performed

As expected, GDDACCUM was never greater than HUI in any patch.

Looking at 30 patches with discrepancies between GDDACCUM and HUI, the time series look as expected. E.g.:

Diffs when testing with SMS_D_Ld10_P72x1.f10_f10_mg37.IHistClm51BgcCrop.cheyenne_intel.clm-ciso_decStart.C.0311-123808ch and SMS_P72x1_Lm25.f10_f10_mg37.I2000Clm51BgcCrop.cheyenne_intel.clm-monthly.C.0311-123808ch are due to the rename of GDDPLANT to HUI.

[billsacks]

Thank you for taking on this cleanup. I'm happy with the changes you're making here – though I'd also like to make sure that @danicalombardozzi is happy with this. I do have some specific comments below, the most substantial of which is that I think we need to consider backwards compatibility with existing restart files (unless I'm misunderstanding things and in fact that's not needed).

• One other thing that's needed here is changing the following testmod to remove GDDPLANT and add HUI and GDDACCUM in its place:

CTSM/cime_config/testdefs/testmods_dirs/clm/crop/user_nl_clm

Line 3 in bf958ac

'GDDPLANT', 'GDDTSOI', 'A5TMIN', 'A10TMIN'

[billsacks]

Alright, this is looking really close now! I have a couple of small follow-ups.

[billsacks]

Looks great now, thanks! I just want to get approval from Danica when she is available, and then we can do final testing of this PR.

[danicalombardozzi]

Thanks for your work on this @samsrabin! I very much appreciate the separation of HUI, used in the phenology calculations, and GDDACCUM, which accumulates the actual GDD. Two minor comments (questions, really).

[danicalombardozzi]

Does GDDACCUM also need to be added here? It seems that HUI and GDDACCUM are both added in most other places throughout the code, so I want to make sure it's not an oversight to leave it out here.

[billsacks]

I'll answer this one, since this is related to something I asked @samsrabin to change: GDDACCUM does not need to be updated here. However, the reason for this is subtle, and the fact that its absence caught your eye (thank you for your careful look that led to that!) suggests that there should probably be a comment here explaining why this is needed for HUI. Something like: "HUI needs this special update (which is non-standard for accumulation fields) because it can be changed outside of the accumulation routines (in CropPhenology), so the accumulation buffer needs to be reset when that happens." @samsrabin what do you think?

[samsrabin]

Good idea, done (66abdbc).

[danicalombardozzi]

Is HUI accumulated over a certain time period? GDD specifies that it's accumulated since planting, and I think it's the same for HUI. If so, does it make sense to add this description? Would something like "patch accumulated heat unit index (growing degree-days) since planting" be correct?

[samsrabin]

You're right, a bit more info would be helpful. These descriptions now (10032f1) read (new text in bold), "crop patch heat unit index (growing degree-days); set to 0 at sowing and accumulated until harvest".

[samsrabin]

Thanks, both!

[danicalombardozzi]

Thanks @samsrabin! I think these changes look great and I'm happy to sign off on this!