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Lecture20-23(2008)

# Lecture20-23(2008) - SOIL WATER POTENTIAL Read pages 105 to...

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SOIL WATER POTENTIAL Read : pages 105 to 125 Introduction to Environmental Soil Ph ysics The two forms of energy influencing water flow in soil are: -- 1. potential and 2. kinetic --(energy—the abilit y to do work ). Kinetic energy (KE--the energy which matter has by virtue of its motion; thus, KE = 1/2 mv 2 , KE v 2 ). Soil water velocities are very slow, so KE is generally considered to be negligible. Thus, the potential energy is the primary energy source determining the state and movement of water in a porous system (soil). The potential energy is determined by the water position and/or internal condition. There is a spontaneous and universal tendency for all matter to achieve a state of low potential energy. Accordingly, soil water flows (from high to low) in the direction of decreasing potential energy.

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The absolute amount of energy that a body of water may contain cannot be determined. (Absolute energy includes electronic and nuclear bonding energies as well as kinetic energy and energy derived from cohesive and adhesive forces and potential energy resulting from elevation). The Ener gy Status of Soil Water Potential Ener gy, PE Potential ener gy (PE) is the energy which a body has by virtue of its position in a force field. Forces Associated with the Potential Ener gy of Water in Soils In soils, the water is acted upon by many force fields, thus the total potential, M t , is a summation of these force fields :-- M t = M g + M p + M o the subscripts are defined as following: t, total; g, gravity; p, pressure; and o, osmotic in other literature and textbooks this will be expressed as : .. R t = R g + R p + R o .
The gravitational potential , M g , arising from the earth’s gravitational field, i.e. each earthly body is attracted toward the earth's center by a gravitational force equal to the weight of that body.

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Lecture20-23(2008) - SOIL WATER POTENTIAL Read pages 105 to...

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