Hydrology BC ENV 3025
Water in the unsaturated zone
Definitions
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experiment: wetting soil in a beaker, what
makes the grains stick to each other?
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the zone between the ground surface and the water table where pore spaces
of the rock or soil may be partly filled with air and partly with water
is referred to as the unsaturated zone or vadose zone and
water in this zone is referred to as soil moisture (Fig)
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the capillary fringe is a saturated zone above the water table where
water is affected by capillary forces
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distribution of moisture in the vadose zone (Fig
8.1)
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grain sizes in soils (and thus pore sizes) vary over several orders of
magnitude (Fig)
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look at sand gauges
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definitions of porosity, field capacity, specific retention,
specific
yield (Fig)
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porosity, field capacity, and wilting points for typical soils (Fig)
Movemement of water in the unsaturated zone
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the potential in the unsaturated and saturated zone are similar: h = z
+ p/(rg) = z + y
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this pressure is negative in unsaturated soils and is often called
pressure,
suction or tension head (y),
y
is given as "cm water column"
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Water pressure within capillary tubes is less than atmospheric pressure
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y can be measured with
a tensiometer (Fig)
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y as a function of moisture
content for a fine sand (Fig
8.4)(Fig)
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soil water status as a function of pressure (tension) (Fig)
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driving forces for flow of water in the unsaturated zone are: gravity and
adhesion/cohesion
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Darcy's law describes the flow in porous media:
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q = -K * dh/dz = -K * d(z+y)/dz = -K * d(z +
p/(rg))/dz
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K: hydraulic conductivity, function of saturation (Fig
8.5)
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pressure head and elevation as a function of depth in saturated and unsaturated
zone under no flow conditions (=> y = -z) (Fig
8.8)
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wetting fronts
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A Capillary Barrier
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Explore 8
Resources
Dingman, S.L. (1994) Physical Hydrology. Prentice Hall, Englewood Cliffs,
575pp.