Salt budget

The conservation law for salt in a reservoir: the rate of change of its salt content equals inflowing minus outflowing salt transport and any explicit salt sources. Freshwater addition changes salinity without directly adding salt. For a well-mixed fixed-volume reservoir, salt content is proportional to its volume times salinity.

The salt forcing used to represent freshwater exchange while retaining a fixed reference volume. Removing freshwater at volume flux $E$ is represented by a salt input $E{S}_0$, and adding freshwater by the opposite salt forcing, with reference salinity $S_0$. Replacing the actual salinity in a compensating-flow salt budget by $S_0$ is an approximation when salinity contrasts are small.