### 2.27.3.1. Water and Energy Conservation[¶](#water-and-energy-conservation "Permalink to this headline")

When subgrid areas change, the water and energy states remain unchanged on a per-area basis. This can lead to changes in the total gridcell water and energy content.

For example, consider a gridcell with two columns: column 1 has a water mass of 1 kg m\-2 and column 2 has a water mass of 2 kg m\-2 for a given water state variable, where these are expressed per unit column area. If column 1 increases in area at the expense of column 2, then column 1 will still have a water mass of 1 kg m\-2, but now expressed over the new column area. This results in a decrease in the total gridcell water content.

Water and energy are conserved by summing up the total water and energy content of each gridcell before and after a change in area. Differences in liquid and ice water content are balanced by liquid and ice runoff terms, which can be either positive or negative. (Negative runoff is effectively a withdrawal of water from the ocean.) Differences in energy content are balanced by a sensible heat flux term, which again can be either positive or negative. These balancing fluxes are spread evenly throughout the following year.

There is a special case when a given crop column type newly comes into existence - for example, when temperate corn first comes into existence in a gridcell. In this case, the column’s below-ground temperature and water states are copied from the natural vegetated column in its gridcell, so that these state variables begin in a close-to-spun-up state. Other state variables (most of which spin up relatively quickly) begin at their cold start initialization values. This initialization is not necessary for the two other land unit types that currently can grow - natural vegetation and glacier: Those land unit types are always active, even when they have zero area on the gridcell, so their state variables will be spun up immediately when they come into existence. After this initialization, the conservation code described above takes effect.

