Soil Water Tension

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Soil Water Tension
Department of Agricultural and Biological
Engineering
University of Illinois at Urbana-Champaign
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h
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-h
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Soil Water Characteristic (Release Curve)
Matric Potential
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Tensiometer for Measuring Soil Water Potential
Water Reservoir
Variable Tube Length (12 in- 48 in) Based on Root
Zone Depth
Vacuum Gauge
Porous Ceramic Tip
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Soil Water Potential

• Measure of the energy status of the soil water
• Important because it reflects how hard plants
  must work to extract water
• Units of measure are normally bars or atmospheres
• Soil water potentials are negative pressures
  (tension or suction)
• Water flows from a higher (less negative)
  potential to a lower (more negative) potential

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Soil Water Potential

• Components
• ?t total soil water potential
• ?g gravitational potential (force of gravity
  pulling on the water)
• ?m matric potential (force placed on the water
  by the soil matrix soil water tension)
• ?o osmotic potential (due to the difference in
  salt concentration across a semi-permeable
  membrane, such as a plant root)
• Matric potential, ?m, normally has the greatest
  effect on release of water from soil to plants

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Hydroscopic Water
Gravitational Water
Capillary Water
Water held in large pores Available for crop use
Water adheres to soil particles
Water drains through soil profile
Wilting Point 15 bars
Field Capacity 1/3 bar
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Water Movement in Plants

• Illustration of the energy differentials which
  drive the water movement from the soil, into the
  roots, up the stalk, into the leaves and out into
  the atmosphere.
• The water moves from a less negative soil
  moisture tension to a more negative tension in
  the atmosphere.

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Yw -1.3 MPa
Leaf air spaces
Yw -1.0 MPa
Yw -0.8 MPa
Xylem
Yw -0.75 MPa
Yw -0.15 MPa
Ys -0.025 MPa
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Effect of Soil Texture
Fine Textured Soil
Water Content
Coarse Textured Soil
Matric Potential
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Matric Potential and Soil Texture
The tension or suction created by small capillary
tubes (small soil pores) is greater that that
created by large tubes (large soil pores). At
any given matric potential coarse soils hold less
water than fine-textured soils.
Height of capillary rise inversely related to
tube diameter
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Effect of Soil Structure
Aggregated Soil
Water Content
Compacted Soil
Matric Potential
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Van Genuchten Representation
a empirical parameter h soil water
pressure head n empirical parameter that
affects the shape of the curve m empirical
parameter that affects the shape of the curve
(m
1 - 1/n) qr residual water content (an
extrapolated parameter) qs saturated water
content q volumetric water content at head,
h.
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Brooks and Corey Representation
q qr (qs - qr)
(hd/h)l
l empirical parameter hd soil water
pressure head qr residual water content (an
extrapolated parameter) qs saturated water
content q volumetric water content at head,
h.
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A moist sand sample has a volume of 456 cm3 in
the natural state and a weight of 843 g. The dry
weight is 763 g and the specific gravity of the
soil particles is 2.65. Determine the porosity,
volumetric moisture content, and degree of
saturation (water filled porosity/total porosity)
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A soil sample with a volume V100 cm3 was placed
in a pressure plate apparatus and brought to
apparent saturation. The following outflow
volumes were measured at equilibrium for the
given pressure settings. After
equilibrium was obtained at the 500 cm pressure,
the sample was found to have a dry bulk density
of 1.64 g/cm3, and a gravimetric water content
(mass of water/mass of solids) of 0.031. Evaluate
and plot the soil water retention curve from
these data
Pressure (cm) 0 10 20 30 40 50 60 70 80 100 150 200 500
Outflow Volume (cm3) 0 0.3 1.4 3.1 3.6 3.4 3.0 2.2 1.5 1.4 1.6 1.5 2.1
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Determine the volume (depth) of water that is
released from Tama Silt Loam when the water table
falls from the soil surface to 1 m below the soil
surface.
Depth (cm) Van Genuchten Parameters for Tama Van Genuchten Parameters for Tama Van Genuchten Parameters for Tama Van Genuchten Parameters for Tama
Depth (cm) qr qs a (1/cm) n
0 - 18 0.052 0.39 0.0063 1.34
18 - 48 0.057 0.44 0.0100 1.30
48 - 80 0.091 0.47 0.0055 1.46
80 - 100 0.088 0.59 0.0057 1.53
100 - 150 0.081 0.59 0.0052 1.56
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Drummer Silty Clay Loam
This poorly drained "prairie soil" formed in 40
to 60 inches of loess over Wisconsinan drift. It
occupies nearly 1.6 million acres. It has a high
management level productivity index (PI) of 150.