There are two basic methods of characterizing or measuring the water in soil. The first is to measure the amount of water in the soil. This approach is the basis for the soil condition water content. An alternative to measuring the amount of water in soil is to measure the energy state of the water. This approach leads to the soil condition water potential.
Differences and changes in potential energy can be measured in terms of work. In an "ideal" system (e.g., no loss of energy to entropy production), the change in potential energy of an object as it moves from one location or state to another is numerically equal to the work required in the move or change of state. The difference in potential energy between two objects is numerically equal to the work required to move the objects to the same state or location. Work, and changes in potential energy can be either positive (+) or negative (-). An object loses potential energy when it does positve work. For example, a rock (spontaneously) rolling down a hill loses potential energy and does positive work. As the rock is moved (by exteranl forces) back up the hill, it gaines potential energy and does negative work. In the latter case, the system (i.e., the source of the external forces) loses potential energy and does postive work.
For example, common experience tells us that water will flow from a small creek at a higher elevation to a reservoir at a lower elevation. This will happen regardless of whether the creek and the reservoir are located in a valley (lower absolute concentration of potential energy) or at the top of a mountain (higher absolute concentration of potential energy), as long as the relative elevation (relative concentration of potential energy) between the two bodies of water remains the same. Water from the small creek also moves into the large reservoir in spite of the fact that the reservoir as a whole may contain more total potential energy than the creek.
The soil water potential parameter must describe the relative conentration of potential energy in soil water at different locations in the soil and plant sytem. A parameter describing the concentration of potential energy (potential energy per unit quantity of water) is called a potential. Therefore, soil water potential must be a potential. The major constraint on defining the soil water potential parameter is that the absolute potential energy of water cannot be measured.
Choose one building (either randomly or for some reason of convenience) and call this building the reference building. Define a parameter called DH (Delta Height). Assign each building in the city a value for DH which is equal to the difference in height between the building and the reference building. A building shorter than the reference building is assigned a negative DH while a building taller than the reference building is assigned a positive DH. As a consequence, the reference building has a DH equal to zero. The difference in height between any two buildings is equal to the difference between the DH of each building. Creating the building paramater DH allows the measurement of the relative height between buildings without ever measuring the absolute height of any building.
The strategy for defining the soil water potential parameter is the same as the DH paramter. A reference body of water is defined and the soil water potential of any other body of water is defined as the difference in potential energy concentration (potential energy per unit quantity of water) between the soil water and the reference body of water. The reference body of water is usually called the reference state, and as in the building height analogy, the water potential of water in the reference state is zero.
Definition (1) includes the conditions of reversible and isothermal water movement which is required to relate the change in potential energy to work done. Definition (1) also includes an explicit definition of the sign of soil water potential. If the potential energy per unit quantity of soil water is less than water in the reference state then the soil water will be moving "uphill" to the reference state, gaining potential energy, and therefore doing negative work. In this case the soil water will have a negative value of soil water potential.
Definition (2) explicitly describes water potential as a difference in potential energy per unit quantity of water (P) which may be more direct than the reference to work in definition (1). However, there is no explicit indication of the sign of soil water potential contained in definition (2). The implication is that the "difference" in potential energy is defined as the P(soil water) - P(ref state). This will produce the same sign convention as definition (1).
A rigorous definition of soil water potential must include the following three characteristics:
| Factor affecting Potential Energy | Component name | Reference State | Sign |
|---|---|---|---|
| Adsorption of Water to Soil | Matric Potential | Free Water | neg "-" |
| Dissolved Solutes | Osmotic or Solute Potential | Pure Water | neg "-" |
| Elevation in Gravitational Field | Gravitational Potential | Reference Elevation | pos "+" (above ref. elev.) or neg "-" (below ref. elev.) |
| Applied Pressure | Pressure Potential | Atmospheric Pressure | pos "+" (applied pressure) or neg "-" (applied suction) |
Hydraulic Potential
Water Potential
Total Water Potential
Units
of Soil Water Potential
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tljones@taipan.nmsu.edu
Last modified 08/09/95