Opened 2 years ago

Closed 12 months ago

#828 closed enhancement (fixed)

Early senescence - quick fix

Reported by: luyssaert Owned by: somebody
Priority: minor Milestone: ORCHIDEE 4.2
Component: Anthropogenic processes Version: trunc
Keywords: Cc:

Description

Too Early senescence was observed for PFT6 over Hesse forest (Julien Aleon). Senescence could be delayed a little by changing the rm_gpp treshold for presenescence from 0.6 to 0.7. Further increasing this value results in too initial LAIs the next growing season. Even with that change in parameter values, senescence is still too early and for PFT6 which is simulated with a "cold senescence", this is triggered by the rm_gpp threshold of 1.

If rm and gpp are well simulated, a value of 1 makes a lot of sense because it is the point where the leaves respires more C than it can assimilate. From a C-economic pointr of view it is no longer useful to keep the leaf.

In reality
GPP = Rm + Rg + biomass production + BVOCs + leaching + C-subsidy to the mycorrhizae.
Or
GPP = Ra + NPP
 
Where
NPP = biomass production + BVOCs + leaching + C-subsidy to the mycorrhizae
Ra = Rm + Rg
 
In ORCHIDEE
GPP = Rm’ + Rg + biomass production.
Where
NPP = biomass production (= common approximation and a common mistake)
And thus
Ra = Rm + Rg + BVOCs + leaching + C-subsidy to the mycorrhizae
 
However we assume that
Ra = Rm + Rg (1)
 
We tune Rm such that biomass production/GPP matches the observations, we are confident with Rg and we don’t simulate BVOCs + leaching + C-subsidy to the mycorrhizae yet. Hence Rm’= Rm + BVOCs + leaching + C-subsidy to the mycorrhizae. If we want to be precise, we should say that Ra = Rm’+ Rg in ORCHIDEE.

In summary: Rm in ORCHIDEE is a not Rm but a mixture of fluxes that we don’t understand very well or are extremely difficult to measure. This implies that the threshold of rm/gpp = 1 which makes sense in the real world is not necessarily valid in ORCHIDEE.

Quick solution: set the treshold for rm_gpp to 1.05-1.15. The above reasoning justifies why in ORCHIDEE we could exceed the threshold of 1.

Change History (2)

comment:1 Changed 13 months ago by luyssaert

  • Milestone changed from ORCHIDEE 4.3 to ORCHIDEE 4.2
  • Priority changed from major to minor

comment:2 Changed 12 months ago by luyssaert

  • Resolution set to fixed
  • Status changed from new to closed

In r7928 phenology was delayed by 40 days, this resulted in a more realistic growing season and a later senescence without having to tune rm_gpp. The concern related to the calculation of Rm remains valid but is also described in #580. As the early senescence issue seems to be fixed in a better way than proposed here, this ticket can be closed.

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