'Gamma_get_sm' turned off in MEGANV2.1 CLM4.5

[HazelMooney]

Model: cesm2.2.0 clm 4.5 meganv2.1
Enquiry: soil moisture activity factor

Describe your problem or question:
I have been investigating activity factors for my model runs using CLM4.5. Soil moisture is constant throughout and in the fortran code VOCEmissionMod.F90 we see that the soil moisture activity factor algorithms are commented out, and the activity factor is instead specified at gamma_sm = 1.0_r8
See screenshots attached. I have been unable to find guidance online as to why the calculation has been commented out in this version of the model, can you provide any guidance or references to help me understand this?

 

[oleson]

I don't know the history of that commented out code. I do know that MEGAN was recently updated and a new drought stress algorithm was implemented per this PR:

New updates on MEGANv2.1 by HuiWangWanderInGitHub · Pull Request #2588 · ESCOMP/CTSM

Description of changes We added new features to MEGANv2.1 for simulating isoprene emissions based on three recent studies conducted at the BAI lab in the University of California, Irvine. The first...

github.com

In it, gamma_sm was been changed to a function of btran:

gamma_sm = get_gamma_SM(btran(p))

However, there has been further conversation about low isoprene emissions perhaps partly as a result of that as indicated by the discussion in this issue:

Low MEGAN emissions in CESM3 · Issue #3016 · ESCOMP/CTSM

Using the latest beta cases, we are seeing a significant underestimation of biogenic emissions, when running a F-case with -bgc sp: beta02 was still OK, for example, isoprene emissions were around ...

github.com