USDA LTAR Common Experiment measurement: Soil water potential and matric potential v1
Authors: M O'Reilly Andrew , Baffaut Claire 
Published: September 2024

The potential of subsurface water measures the energy associated with position or internal conditions, thus representing the potential energy of a parcel of water (Hillel, 1998). Given the typically small flow velocity of subsurface water, its kinetic energy commonly can be assumed negligible; thus, the total potential represents the total energy status of the water parcel. Potential energy due to position depends on the location of the water parcel within the Earth’s gravitational field, and potential energy due to internal conditions depends on the temperature and solute concentration of the water parcel. Because all systems equilibrate by moving from higher to lower energy states, a difference in potential energy from one point to another drives water moving between those points. The absolute value of the potential energy of a water parcel at any given point is irrelevant to its movement. Therefore, the potential of a water parcel is defined by the specific potential energy and may be expressed in three common forms: Energy per unit mass (Φ), [L2T–2]; Energy per unit volume, [ML–1T–2], or pressure (P); and Energy per unit weight, [L], or head (H); where M, L, and T represent mass, length, and time dimensions, respectively, in a chosen unit system. Because the change in energy drives water movement, a reference state is necessary. Thus, potential is the work per unit quantity (mass, volume, or weight) required to transport an infinitesimal parcel of water reversibly and isothermally to a point of interest from a point representing a reference state for a pool of water defined by the following criteria: (1) pure water, (2) free water (not bound, i.e., adsorbed to solid phase), (3) air phase at atmospheric pressure, and (4) located at an established vertical datum (arbitrary reference elevation) (Hillel, 1998). Total potential is used to define the subsurface flow of water under saturated and unsaturated conditions. In the vadose zone, total potential is commonly called soil water potential and obtained by summing the gravitational pressure (Φg), pressure (Φp) and osmotic potentials (Φo) generated by the respective force fields acting on water. Pressure potential (Φp)results from forces acting at the air–water and water–solid interfaces and is equal to the sum of pneumatic (Φa) and matric potentials (Φm) respectively. Osmotic potential results from solutes in the soil water solution, which lower the potential energy of the soil water by reducing its vapor pressure, and, in the presence of a semipermeable membrane, produce an osmotic pressure gradient. Osmotic potential typically can be neglected for dilute soil solutions. Where the air phase pressure is equal to its reference pressure (atmospheric pressure), as often is the case for a well-aerated subsurface, the pneumatic potential is zero. Therefore, for a dilute soil solution in a well-aerated porous medium, soil water potential (total potential) is equal to the sum of gravitational and matric potentials, making Φm measurements a prerequisite for predicting water movement in the subsurface. Matric potential represents the specific potential energy attributable to capillary and adsorptive forces, i.e., the total effect resulting from the affinity of water to the whole soil matrix (pores and particle surfaces). The values of Φp and Φm (where Φa= 0) are negative above a free water surface and thus are negative at elevations above the water table. Sometimes, the terms suction or tension are used to treat these potentials as positive values above the water table.