Question about land unit change in the landuse.timeseries file

Hello everyone. In the CLM Technical Note (section 2.27.2 Reconciling Changes in Area), I read the following description:

"In the first time step of January 1, changes in land unit weights can potentially come from two sources: Changes in the area of the crop land unit come from the landuse.timeseries dataset, and changes in the area of the glacier land unit come from the ice sheet model. The areas of other land units are then adjusted so that the total land unit area remains 100%."

My question is: why does the change in land-unit weights not include changes in the vegetated land unit? For example, if the landuse.timeseries file shows an increase in the vegetated land unit and a decrease in the wetland land unit, while crop area remains unchanged, will CLM recognize this type of land-unit change?

Thanks in advance!

This may happen in nature, but the model does not represent changes in wetland area, so this cannot happen in the model. However, in post-clm5 versions we allow changes in urban and lake area with time, as well, so we will need to update the above comment.

slevis said:

This may happen in nature, but the model does not represent changes in wetland area, so this cannot happen in the model. However, in post-clm5 versions we allow changes in urban and lake area with time, as well, so we will need to update the above comment.

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Dear slevis, thank you for your reply. We wish to use a scientifically supported version, hence our selection of CESM2.1.5. Through this post, I discovered that this version cannot be coupled with CTSM5.3 or later versions, which characterize wetland land units. However, my primary concern is changes in vegetation land units. Since the CLM Technical Note only mentions that alterations in the crop land unit within the landuse.timeseries file affect land unit weights, I want to know if the CLM recognizes changes in other land units when the crop remains unchanged. For example, if vegetation land unit increases, lake land unit decreases, but crop land unit stays constant.

The vegetated landunit isn't on the landuse file (PCT_NATVEG). Only the pfts on the landuse file change (PCT_NAT_PFT). The other landunits on the file are urban, crop, and lake. With release-cesm2.1.5, urban and lake are not dynamic and therefore they are not on the landuse file. So if crop stays constant then the only thing that will be dynamic are the pfts.

oleson said:

The vegetated landunit isn't on the landuse file (PCT_NATVEG). Only the pfts on the landuse file change (PCT_NAT_PFT). The other landunits on the file are urban, crop, and lake. With release-cesm2.1.5, urban and lake are not dynamic and therefore they are not on the landuse file. So if crop stays constant then the only thing that will be dynamic are the pfts.

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Dear oleson, thank you for your reply! I understand now.