Theory: Water Potential; Water Potential; Measuring Water Potential - Decagon Devices WP4C Operator's Manual

Dewpoint potentiameter

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PotentiaMeter

9. Theory: Water Potential

Water potential is defined as the potential energy per unit

volume of water in a sample. The total water potential of a

sample is the sum of four component potentials: gravita-

tional, matric, osmotic, and pressure. Gravitational poten-

tial depends on the position of the water in a gravitational

field. Matric potential depends on the adsorptive forces

binding water to a matrix. Osmotic potential depends on

the concentration of dissolved substance in the water.

Pressure potential depends on the hydrostatic or pneu-

matic pressure on the water.

The WP4C measures the sum of the osmotic and matric

potentials in a sample. Often one or the other of these

potentials will be the dominant factor in determining the

total potential. For example, solutions like the KCl calibra-

tion standard have only an osmotic component. Soils bind

water mainly through matric forces, and therefore have

mainly a matric component (though salt-affected soils can

have a significant osmotic component).

The water potential of a solid or liquid sample can be

found by relating the sample water potential reading to the

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