## 2.27.2. Reconciling Changes in Area[¶](#reconciling-changes-in-area "Permalink to this headline")
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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 (section [2.27.1](#transient-land-use-and-land-cover-data)), 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%.

If the total land unit area of glaciers and crops has decreased, then the natural vegetated landunit is increased to fill in the abandoned land. If the total land unit area of glaciers and crops has increased, then other land unit areas are decreased in a specified order until the total is once again 100%. The order of decrease is: natural vegetation, crop, urban medium density, urban high density, urban tall building district, wetland, lake.

These rules have two important implications:

1.  We always match CISM’s glacier areas exactly, even if that means a disagreement with prescribed crop areas. This is needed for conservation when CISM is evolving in two-way-coupled mode.
    
2.  For land units other than crop, glacier and natural vegetation, their areas can decrease (due to encroaching crops or glaciers), but can never increase. So, for example, if a grid cell starts as 5% lake, crops expand to fill the entire grid cell, then later crop area decreases, the lake area will not return: instead, the abandoned cropland will become entirely natural vegetation.
    

For all levels of the subgrid hierarchy (land unit, column and patch), we only track net changes in area, not gross transitions. So, for example, if part of a gridcell experiences an increase in glacier area while another part of that gridcell experiences an equal decrease in glacier area (in the same glacier elevation class), CLM acts as if there were no changes. As another example, consider a gridcell containing natural vegetation, crop and glacier. If there is a decrease in glacier area and an equal increase in crop area, CLM will assume that the crop expands into the old glacier area, and nothing happened to the natural vegetation area. A more realistic alternative would be that the crop expanded into natural vegetation, and natural vegetation expanded into glacier. The final areas will be correct in these cases, but the adjustments of carbon and nitrogen states (section [2.27.3.2](#transient-landcover-carbon-and-nitrogen-conservation)) will be less accurate than what would be obtained with a full tracking of gross transitions.

