Potentials are defined in terms of work
Definition and Application of Water Potential Concepts
Potentials are defined in terms of work. The total soil-water potential is defined by the International Soil Science Society and the Soil Science Soc. of America as 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 from a pool of pure water at a specified elevation at atmospheric pressure to the soil water at the point of under consideration. Gravimetric potential and matric potential are defined in similar ways as the work required to move a small amount of water from a source pool top a destination pool. The exact nature of these pools and their locations are summarized nicely in the a table within the glossary of terms provided by the SSSA.
Each of the definitions of potential refers to work per unit quantity of water. Work per unit volume of water has units of pressure. Work per unit weight of water has units of length and is known as head. Work per unit mass has units of energy per unit mass. Different terms are sometimes used to describe these potentials. In this document, total potential and total head will be used interchangeably, gravitational potential and gravitational head will be used interchangeably, and matric potential and pressure head will be used interchangeably.
The figure below contains a diagram of these potentials on a unit weight basis for a saturated soil.
The gravitational heads are positive at both ends of the soil system since work would need to be done to move water from a pool at the reference level to a pool at the elevation of the bottom or the top of the soil. The pressure heads are greater than zero at both ends since work must be done to move water from a pool at the elevation of the point of interest into the soil at that elevation. The total head is the sum of the gravitational and pressure heads. In this example, water is moving downward even though the pressure head is greater at the bottom than at the top.
The figure below contains a diagram of the gravitational, pressure, and total heads for a horizontal flow system. Note that the gravitational heads are the same at both ends of the soil.
Piezometers can be used to measure hydraulic heads in saturated soils where pressure heads are positive. A piezometer is essentially an open pipe connected to the soil at the point of interest. The height of water in the pipe above some reference level indicates the total hydraulic head in the soil.
In each of the examples above, the pressure heads are positive. In unsaturated soils, pressure heads are less than zero. A tensiometer is used to measure hydraulic heads in unsaturated soils. It differs from a piezometer in that it contains a porous ceramic cup that is placed in contact with the soil. The cup permits water to pass through it until water in the tube or manometer is in equilibrium with the soil water. The figure below is a diagram of a soil containing tensiometers at two points. The pressure head at point B is less than zero. As a result the total head at B is less than the gravitational head at B. Water in this system will flow downward since the total head at A is greater than that at B.
