How to calculate crop yield (tonnes/ha) simulated by CLM5?

[Qi Ran]

What version of the code are you using?
CESM2.2 (CLM5, CAM6-chem/MOSAIC)
Composet: FCnudged

Have you made any changes to files in the source tree?
./xmlchange CLM_BLDNML_OPTS="-bgc bgc -crop" #turn on BGC and crop model
user_nl_cam: Nudge_Model =.false.
user_nl_clm: use_ozone = .true. #Turn on ozone damage

Describe every step you took leading up to the problem:

• I output the variable GRAINC_TO_FOOD at monthly frequency
• I then post-process the output to calculate annual crop yield (tonnes/ha)

My calculation steps are:
1. Convert monthly mean flux (GRAINC_TO_FOOD) to annual accumulated grain carbon:

• For each month:
  

Δtm is seconds in month, and crop_frac represents the fraction (%) of cropland within the grid cell. "0.01" is unit conversion factor from g/m2 to tonnes/ha.
According to the CLM documentation (Section 2.26.2.4.4, Harvest), crop yield is calculated as:

I think the unit for this calculation method is g/m2.

Describe your problem or question:

1. Should crop fraction (PCT_LANDUNIT) be applied in yield calculation?
   • Or is GRAINC_TO_FOOD already normalized over crop area?
2. When I compare my annual crop yield (left panel) with that from Lawrence et al., (2019) (the right panel):

The spatial pattern matches well, but the magnitude is much smaller (significantly underestimated).

I would appreciate any comments on the discrepancy in my results. Could this be related to an issue in how I calculated crop yield (tonnes/ha)?

 

[oleson]

@samrabin , I think you have a document on how to calculate yield somewhere?

 

[samrabin]

I don't, unfortunately, especially not for versions before CTSM 5.3 or so.

The figure in Lawrence et al. (2019) is probably tonnes per hectare of cropland. I think if you saved crop yield at gridcell level, you actually need to multiply by 100/crop_frac, not its inverse.

 

[Qi Ran]

I don't, unfortunately, especially not for versions before CTSM 5.3 or so.

The figure in Lawrence et al. (2019) is probably tonnes per hectare of cropland. I think if you saved crop yield at gridcell level, you actually need to multiply by 100/crop_frac, not its inverse.

Hi, Sam. Thank you for your reply. I tried multiply by 100/crop_frac, but the spatial distribution (the figure below) can be quite different with that in Lawrence et al. (2019). And it is so strange that Amazon Rainforest has a relatively high yield.
I would really appreciate it if you have any thoughts or suggestions regarding this strange result.

 

[samrabin]

Did you save at gridcell level or PFT level?

 

[Qi Ran]

Did you save at gridcell level or PFT level?

The GRAINC_TO_FOOD and PCT_LANDUNIT are output at gridcell level. And the annual mean PCT_LANDUNIT (in 2010) for crop land is shown as below.

 

[samrabin]

What does it look like if you just remove the crop_frac/100 term? I can't think of why that would be necessary.

• If non-agricultural land was getting averaged in, you'd need to multiply by 100/crop_frac
• If not, you shouldn't need to multiply by anything, because you should already be getting the mean over crop area

 

[Qi Ran]

What does it look like if you just remove the crop_frac/100 term? I can't think of why that would be necessary.

• If non-agricultural land was getting averaged in, you'd need to multiply by 100/crop_frac
• If not, you shouldn't need to multiply by anything, because you should already be getting the mean over crop area

If remove the crop_frac/100, and just use the below formula from the CLM documentation:
X 0.01 (for converting to tonnes/ha)
The annual mean crop yield is still be largely underestimated, but the spatial distribution looks good: