Why the `o3coefg` in the PhotosynthesisMod.F90 only used for rs_z rather than gs?

[Haven]

Hi everyone,

I am doing some studies about ozone-vegetation feedback. And the ozone can damage the vegetation's stomatal conductance.

I have seen the source code in the `PhotosynthesisMod.F90` file. The ozone damage factor for stomatal conductance (`o3coefg`) only affects `rs_z` (resistance) rather than stomatal conductance (`gs`).

The source code in `PhotosynthesisMod.F90` is as follows:

rs_z(p,iv) = rs_z(p,iv) / o3coefg(p)

Does that mean the ozone damage can not affect the stomatal conductance in this version?

Thanks!

Best regards,
Haofan

 

[slevis]

@Haven, I have not looked at this section of code as much as you have, but I have a question that may help answer your question:

Am I wrong if I say that stomatal conductance equals the inverse of stomatal resistance?

 

[Haven]

Hi @slevis,

Many thanks for your quick response.

You are right!!!

However, you can find the file named `PhotosynthesisMod.F90` in `components/clm/src/biogeophy`.

Line 1715-1717:
gs = gs_mol(p,iv) / cf
rs_z(p,iv) = min(1._r8/gs, rsmax0)
rs_z(p,iv) = rs_z(p,iv) / o3coefg(p)

and Line 3367-3372:

gs = gs_mol_sun(p,iv) / cf
rs_z_sun(p,iv) = min(1._r8/gs, rsmax0)
rs_z_sun(p,iv) = rs_z_sun(p,iv) / o3coefg_sun(p)
gs = gs_mol_sha(p,iv) / cf
rs_z_sha(p,iv) = min(1._r8/gs, rsmax0)
rs_z_sha(p,iv) = rs_z_sha(p,iv) / o3coefg_sha(p)

This code indicates that the `o3coefg` only affects the stomatal resistance rather than stomatal conductance. Does that mean the ozone damage code cannot affect any other process (such as vegetation transpiration) via stomatal conductance?

If I try to change the code as follows:

gs_mol(p,iv) = gs_mol(p,iv) * o3coefg(p)
gs = gs_mol(p,iv) / cf
rs_z(p,iv) = min(1._r8/gs, rsmax0)

Does that make sense?

Best regards,
Haofan Wang

 

[slevis]

From a very quick look it makes sense. If you want to be certain, you will need to test without and with your code change to see if the model behaves as you expect.