The amount of work that must be done per unit quantity of pure water in order to transport reversibly and isothermally an infinitesimal quantity of water, identical in composition with the soil water, from a pool at the elevation and the external gas pressure of the point under consideration, to the soil water. This potential component is to be considered only when external gas pressure differs from atmospheric pressure as in a pressure membrane apparatus. A specific term and definition are not given. It may be identified with the matric potential defined above. It may be identified with the osmotic potential defined above.
Total pressure is therefore equal to the sum of soil water pressure and osmotic pressure.
Total pressure may also be derived from the measurement of the partial pressure of the water vapor in equilibrium with the soil water. It may be identified with the total potential defined above when gravitational and external gas pressure potentials can be neglected. The elevation with respect to a specified reference level at which water stands in a piezometer connected to the point in question in the soil. Its definition can be extended to soil above the water table if the piezometer is replaced by a tensiometer. The hydraulic head in systems under atmospheric pressure may be identified with a potential expressed in terms of the height of a water column.