Questions about the "TOTSOMC" magnitude from CLM5BGC-CROP

[liliyao]

Hi,

I have questions about the "TOTSOMC" value (total soil organic matter carbon) from our CONUS-scale CLM5 BGC-CROP simulation using ctsm5.1.dev118. Our simulation utilized the default surface dataset and landuse timeseries file from 1980-2014, along with our own climate data (TGW-WRF). We divided the CONUS into 7 different regions for the simulation and then stitched the output back together for further analysis.

When comparing our simulated "TOTSOMC" with the benchmark in the ILAMB (benchmark derived from the Harmonized World Soil Database (HWSD) v1.2 dataset), we noticed that our soil carbon values are significantly higher than the benchmark in the Midwestern region, as shown in the figure below. When inspecting our final spinup results for this region, we observed that the TOTSOMC is much higher than TOTVEGC in this region, as shown in the attached final spinup result.

We would like to inquire if you have any insights into why the TOTSOMC is so much higher than TOTVEGC in this region during the spinup and how we can address it. We are also curious if this disparity might be contributing to the unreasonably high TOTSOMC in this region during the simulation . Any feedback would be greatly appreciated. Thank you very much!

 

[slevis]

First I want to make sure that you followed the spin-up phase (AD) with a post-AD phase. If you did, I don't have an answer for why the simulated is greater than the observed, but I have a question. Have you done a literature survey on the topic and have found that other researchers show the same behavior? Or do others not see the same behavior, in which case it is unique to your simulations? In the former case (other researchers encountered the same behavior) you will likely find insights about that in their articles. In the latter case (your results differ from results by others), then you should think about how your methods differ from the methods of other researchers that you would end up with this result.

 

[evasinha]

@slevis I work with @liliyao and was investigating the cause of higher soil carbon with her. The likely cause of higher `TOTSOMC` (soil carbon) is because we spun the model to 1980 conditions. In the 1980 surface dataset cropland fraction in the US is quite large in the US-Midwest while natural vegetation is low. This is likely causing higher soil carbon and low vegetation carbon in our spun-up model.

My follow up questions are:

1. Why is cropland fraction not resulting in increase in vegetation carbon and decrease in soil carbon?
2. Is there a way that we can equilibrate soil carbon pools in cropland fraction?

 

[oleson]

1. Likely because we don't have all of the realistic management practices (e.g., tillage, residue removal) that would result in that behavior? We do have tillage and residue removal now in ctsm5.1.dev165 (it's on by default for ctsm5.1 but uses some global default setup).
2. If you are following the spinup procedure, then soil carbon pools should be equilibrating in the cropland fraction as well as the entire gridcell.

 

[liliyao]

@selvis @oleson Thanks for the responses! Regarding the spinup procedure, we did it following the user's guide 1.5.5. Spinup of CLM5.0-BGC-Crop — ctsm release-clm5.0 documentation
we first conducted an AD spinup and followed by a final spinup. Attached are the AD spinup and final spinup results for this region.

 

[oleson]

Not sure if this is helpful or not but here is a comparison of CLM5 and CTSM51 results for soil carbon to HWSD in ILAMB:

https://webext.cgd.ucar.edu/I20TR/clm50_ctsm10d089_2deg_GSWP3V1_luna3_hist/lnd/_build_CLM50CTSM51BGC/EcosystemandCarbonCycle/SoilCarbon/HWSD/HWSD.html

I don't think I see such high model values of soil carbon in that region. However, I'll note that we use TOTSOMC_1m (soil carbon in the top 1m) to compare with HWSD.