Decagon Devices MPS-2, Руководство оператора

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3 THEORY MPS-2 & MPS-6
3 Theory
3.1 Water Potential Measurement
There are two basic parameters that describe the state of water in
soil: one is soil water content, or the amount of water per unit of soil,
and the other is soil water potential, or the energy state of water in
the soil. Although water content is useful when trying to describe
the water balance of a soil, i.e. how much water is moving in, out,
or being stored, water potential is often preferred over water content
because it determines how water moves in a soil or from the soil to
the plant. In addition, you can use water potential to determine
plant availability of water, schedule irrigation, or determine the me-
chanical stress state of soil.
All soil water potential measurement techniques measure the poten-
tial energy of water in equilibrium with water in the soil. The Second
Law of Thermodynamics states that connected systems with differing
energy levels move toward an equilibrium energy level. When an ob-
ject comes into hydraulic contact with the soil, the water potential of
the object comes into equilibrium with the soil water potential. For
example, tensiometers make use of this principal to measure the po-
tential energy of a liquid water reservoir in equilibrium with the soil
water (liquid equilibration), and psychrometers/dew point hygrome-
ters measure the potential energy of water vapor in equilibrium using
soil water (vapor equilibration).
Another category of water potential sensors use a solid matrix equi-
libration technique to measure the water potential of the soil. This
technique introduces a known material with a static matrix of pores
into the soil and allows it to come into hydraulic equilibrium ac-
cording to the Second Law of Thermodynamics. Because the two
are in equilibrium, measuring the water potential of the solid matrix
gives the water potential of the soil. Historically, instruments have
measured the thermal conductivity or electrical conductivity of the
solid matrix to determine its water potential with varying degrees of
success.
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