Potential energy of soil water due to the weight of water (hydrostatic pressure) on the point of interest. Pressure head is expressed as energy per weight and is equal to the height of water between the point of interest and the free water surface (water table), hp, also termed the piezometric head. Pressure potential is expressed as energy per unit volume and equals the piezometric head, Hp, the water density, ρ, and the gravitations constange, g (ρghp.