ATS2545 Lecture Notes - Lecture 6: Permanent Wilting Point, Field Capacity, Hydrogen Bond
25 May 2018
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Lecture 6 – Water in Soil: Infiltration
Water Molecule
• Polar
o Electrical forces not balanced within molecule
o Positive end (hydrogen)
o Negative end (oxygen)
o Allows water to stick together
o Strong bonds – hydrogen bonding (electrical attraction)
Silica Unit
• Oxygen most abundant element
• Electrical charges
o Si: 4+
o O: 2-
• Overall charge:
o 4-
• Characteristics
o Tetrahedron bond arrangement
o Tetrahedra link into chains and other forms by sharing oxygen atoms
o E.g. clay
• Other small ions (Aluminium: +3) substitute for central silicon
o If charge less than +4 → overall negative charge remains
• Excess negative charge around edges of clay crystal → leaving overall
structure with ability to attract positive ions (cations) to balance excess
negative charge
• Exchange complex
o Electrical field of ions since negative charged clay fragments attract
water as well as cations
o Results in more water to be drawn to outer negative ‘ends’ of those
closest to clay
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Soil Water
• Wilting point
o When plants start to wilt and suffer from water shortage
• Field capacity
o When soil reaches a lower water content than saturation
o E.g. a very wet soil will release some weekly held water to the pull of
gravity so that water will drain downwards towards by seepage
• As a plant extracts water from the soil, or as the soil dries, it is the outermost,
loosely held water molecules that are most readily extracted and used first
o As the layer of attached water becomes thinner, the water molecules
become harder to detach
Soil Suction
• The force with which a soil retains water, related to thickness of attached layer
of water molecules
• Polar water molecule interacting with unbalanced molecules in clay due to
presence of aluminium silica
• Surface tension
o Force that arise at air
o Arises because water molecules are drawn to each other more strongly
than to gas molecules
o Tiny pockets of water have higher surface tension
o Unbalanced lateral and downward forces arise at water surface
o Directions are equal as water becomes deeper
Flow of water in an Unsaturated Soil
• Within water films, water molecules do move around by random molecular
and thermal motion
• Molecules that most likely to drift are those least strongly held
• If they move to a location where the film is thinner, they will become more
strongly attached and will have a lower probability of moving randomly back
to their original site
• There will be a net flow of water molecules from sites where they are weakly
held to sites where they will be more firmly held
o From wet soil → dry soil
• Electrical diffusion
• Infiltration is driven by molecular forces which arise from unbalanced
electrical charges in the soil
• Water drawn in when it rains, stuck in soil by electrical forces
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find more resources at oneclass.com
Document Summary
Water molecule: polar, electrical forces not balanced within molecule, positive end (hydrogen, negative end (oxygen, allows water to stick together, strong bonds hydrogen bonding (electrical attraction) Flow of water in an unsaturated soil: within water films, water molecules do move around by random molecular and thermal motion, molecules that most likely to drift are those least strongly held. Infiltration is driven by molecular forces which arise from unbalanced electrical charges in the soil: water drawn in when it rains, stuck in soil by electrical forces. Infiltration: the entry of water into the surface of a soil, followed by re-distribution of water below the surface through pore spaces, expressed in mm/h. Lecture 7 water in the soil #2: groundwater flow. Degree of saturation: saturated, if all intergranular void spaces are filled by water, in soils, only during and shortly after rain, unsaturated, if some air remains within the voids, in soils, mostly after rain.
