Question about BiogenCO2 Module Parameters and Urban CO₂ Flux Trends

[ZhouxhTHU]

Dear developers,

I am using the BiogenCO2 module in SUEWS to calculate CO₂ fluxes in Shanghai. Since I do not have the Shanghai-specific parameters (e.g., alpha_enh, beta_enh, resp_a, resp_b) in the SUEWS_BiogenCO2.txt file, I used the values provided in the Helsinki test example.

For input variables, I found corresponding values for temp, lai, and kdown. However, I did not have specific vegetation data for Fr_EveTr and other fractions, so I approximated them using NDVI (average allocation among vegetation types）. Additionally, Fr_Bsoil data was unavailable and thus omitted.

We validated the model results using real observations from a single site, and the results are correct. However, I am somewhat confused about the overall urban trend:

In summer, urban CO₂ flux is higher than that of forests (with respiration being positive and photosynthesis negative), which seems reasonable.
However, in other seasons, the model suggests that urban CO₂ flux is lower than that of forests. Based on the model outputs, this seems to be because urban areas have less vegetation, leading to lower CO₂ emissions, while forests, despite having strong photosynthesis, exhibit even stronger respiration.
I am unsure whether this result is realistic. Could you provide any insights on whether this seasonal trend aligns with expectations, or if there might be an issue with my parameter assumptions?

Thank you for your time and help!

Best regards,
xuanhong
@sunt05 @biglimp

[biglimp]

I think also @ljarvi should be included in this discussion. Personally, I have not so much experience of this part of the model.

[ljarvi]

Hi, Thanks Xuanhong for your question. I'm not sure I understand exactly your issue. When you say that in summer the CO2 flux is higher do you mean that the net biogenic flux (photosynthesis + respiration) is less negative? Traditionally forests exhibit larger CO2 sinks than urban areas (see eg Havu et al. 2024, https://www.sciencedirect.com/science/article/pii/S1618866724000591) but this is not always the case. For example in Beijing the forested areas have lower temperatures and thus less efficient photosynthesis, thus they do not exhibit as large sinks than more built-up areas. You also write that " in other seasons, the model suggests that urban CO₂ flux is lower than that of forests.". If you mean that fluxes less negative in urban areas than in forests, this is more realistic than that in summer the fluxes would be less negative. Likely your forests dry in summer which are limiting your photosynthesis which is not necessarily true. This can be helped by modifying the G2-G6 parameters or maximum conductance. Or increasing your soil depth to 1 m, keeping the maximum volumetric water content fixed (ie if now the maximum amount of water would be 300 mm in 500 mm depth, in 1 m it could be more than 600 mm).

[ZhouxhTHU]

Thank you for your reply, as I am a beginner and may not have explained many aspects clearly, sorry.
I refer to co2_biogen (https://suews.readthedocs.io/en/2020a/_static/html/namespaceco2__module.html) from co2_module. I have been confused about this for a long time, so there are some questions that I would like to ask you, and I am really very much in trouble with you!
We are using the branch (emissionsmethod >= 31 .AND. emissionsmethod <= 36). After thinking about it yesterday, I think there is a problem with the parameters : alpha beta theta alpha_enh beta_enh resp_a resp_b min_respi . These parameters we are using the Helsinki test from SUEWS_BiogenCO2.txt, which is causing the error. We don't have these parameters, and as a novice I don't know where to start, I was wondering if you could give me the parameters for Shanghai, or a rough reference range.
Really thank you especially for your reply! Your reply is very useful to me! @ljarvi

[ljarvi]

The only recommended method to be used in 45. The emission methods you are now using is the one used in Bellucco et al, but this is something which has not been tested properly and should be avoided to be used. When you use emission method 45 you can eg test those parameters with have been used in Helsinki, Beijing or Zurich.