Water balance of a region (a column) [per unit area]


Storage terms
W: atmospheric Water vapor content ("precipitatble water")
S:terrestrial water Storage (surface water+snow+soil moisture+groundwater)
Flux terms
C: Convergence of water vapor transport;  P: Precipitation;
E: Evaporation;  R: Runoff (divergence of net freshwater transport)
For steady state (long-term mean), C = R.