Hydraulic Head, Salt water related hydraulic head measures

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Hydraulic head, Fresh water head, Environmental head, Saltwater head, pressure head

Hydraulic head (total head) = pressure head + elevation head = constant for a static water column.

Equivalent Fresh water head in saltwater problems is similar to hydraulic head. It determines the flow of water.

Environmental head (Modflow-surface, HGL): computed from the equivalent freshwater head at any location, by subtracting the effects of density from that point up to the surface. Note that this an integrated process, concentration may varying from that point up to the surface. In the real world, this can be water level measured from an open hole well (opens from the point to the surface)

Saltwater head, native head (SEAWAT, USGS): computed from the equivalent freshwater head at any location, by subtracting the effects of density from that point only. Note that this is not an integrated process, concentration is a point value. In the real world, this can be water level measured from an well with a short screen opening.

Previous version of SEAWAT, input and output were expressed in terms of equivalent freshwater head.
SEAWAT-2000 uses equivalent freshwater head as the dependent variable in thevariable-density ground-water flow equation. But input and output are expressed in terms of the head of the native aquifer water (saltwater head)

So, for a given point, usually Equvalent Fresh water head >= Saltwater Head, native head (SEAWAT) >= Environmental Head (Modflow-surfact)