The convention is:
```
SnowIce  = snow->swq - snow->pack_water - snow->surf_water;
PackSwq = SnowIce - SurfaceSwq;
Ice = SnowIce + GlacierIce;
```
So when SnowMelt occurs, we are basically melting the snow pack layer and updating all of these quantities.

These variables are defined at line ~208.