= Hydraulic head
{title2=$h=z+p/(\rho g)$}
{wiki}
<Hydraulic head> is the sum of elevation and pressure head, measured in metres relative to a chosen height datum. For constant <mass density> $\rho$ and <gravitational acceleration> magnitude $g$, the pressure head $p/(\rho g)$ is the height of a column of the same <fluid> supported by <fluid pressure> $p$. A water-filled open measuring tube records <hydraulic head> as the elevation of its water level when $p$ is gauge <fluid pressure>. Changing the common pressure reference or height datum adds a constant to <hydraulic head> and leaves its driving gradient unchanged.
For <water> with $\rho=1000\,\mathrm{kg\,m^{-3}}$ and $g=9.81\,\mathrm{m\,s^{-2}}$, a point at $z=2\,\mathrm m$ with gauge <fluid pressure> $p=9810\,\mathrm{Pa}$ has $h=3\,\mathrm m$. A second point at $z=0$ with gauge <fluid pressure> $29430\,\mathrm{Pa}$ also has $h=3\,\mathrm m$: these values are compatible with a stationary column under <hydrostatic pressure>. It is a difference in <hydraulic head>, rather than a difference in <fluid pressure> alone, that drives saturated <Darcy flow>. This form uses constant <mass density>; one cannot generally pull spatially varying $\rho$ through the gradient.https://books.gw-project.org/hydrogeologic-properties-of-earth-materials-and-principles-of-groundwater-flow/chapter/hydraulic-head/{ref}
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