Lecture 17 Soil Water (4)

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Lecture 17 Soil Water (4)
Soil Water Measurement

• Soil moisture
• Soil water potential
• Infiltration rate

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Soil Moisture Measurement

• Gravimetric method
• Coring soil samples of known volume and
  determining
• their weight loss when dried in an oven at 105?C.
  
• Standard method
• Widely used
• Laborious and time consuming
• Prone to errors in sampling and repeated weighing
• Cant distinguish between structural and
  non-structural water
• Clays may not be totally dried while organic
  matter may oxidize

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Soil Moisture Measurement

• 2. Neutron Probe
• Measuring soil water content from the energy
  reduction of neutrons released in the soil. A
  radioactive source of fast (high energy) neutrons
  is released at a given depth of the soil and the
  number of neutrons which are slowed or
  thermalized by the collisions with hydrogen
  nuclei, mainly in soil water, is measured by a
  detector. (Figure 5.28 Jones)
• Measuring soil water content in a radius of about
  10 cm for wet soil and 25 cm for dry soil
• Not useful for top layer of soil (0-5 cm)
• Needs to be calibrated against gravimetric method
• Fast and little labour
• Accurate for measuring changes in water content
  rather than the absolute value
• Precautions for radioactivity

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Soil Moisture Measurement

• 3. Time-domain reflectometry (TDR)
• Determine soil water content from measuring the
  dielectric constant of the soil between two
  lines. The higher the soil water content, the
  larger the dielectric constant. The dielectric
  constant is measured between two transmission
  lines (rods) inserted in the soil either
  horizontally or vertically.
• Good for taking the average soil water content at
  a depth when rods are installed horizontally
• Good for taking the average soil water content of
  a layer of soil when rods are installed
  vertically
• Accuracy is comparable to the gravimetric method
• Allows for non-manual, continuous monitoring
• Non-radioactive

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Soil Moisture Measurement

• 4. Capacitance probe
• Determine soil water content from measuring the
  dielectric constant of the soil at a point. A
  probe is inserted to the soil and the tip is in
  contact of soil where the dielectric constant is
  measured
• Point measurement
• Particularly useful for top layers of soil
• Sensitive to local inhomogeneities and air gaps
  between probe and soil.
• Needs calibration using gravmetric method
• Non-radioavtive

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Soil Moisture Measurement

• 5. Synthetic aperture radar (SAR)
• Inferring soil water from active microwave
  (RADAR, RAdio Detection And Ranging) interaction
  with the soil surface. Wet soil surfaces have
  higher dielectric constants and therefore higher
  reflectivity for radar
• Areal average
• Depth depends on the radar wavelength
• Useful for detecting soil moisture change and
  spatial distribution rather than the absolute
  value
• SAR is also strongly affected by soil surface
  roughness

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Soil Water Potential Measurement
Water potential is related to soil moisture, but
is also unique measure of the effects of soil
water on water movement and extraction of water
by plants.
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Soil Water Potential Measurement

• Tensiometers
• Measures soil water suction. It comprises of a
  porous pot filled with deionized water and
  inserted to the soil. The pressure within the pot
  equalizes with the tension or pressure in the
  surrounding soil is measured using either of
  pressure transducer or a manometer.

• Oldest and widely used
• Often used for unsaturated soil
• Can measure total water potential below water
  table,
• i.e., piezometer
• Lowest suction to measure is 80kPa, below which
  air
• may enter into the porous pot, introducing
  errors.
• Doesnt work well in clays

http//www.soilmeasurement.com/tensimeters.html
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Tensiometers

Figure 5.27, Jones
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Soil Water Potential Measurement

• 2. Resistance block
• Deriving soil water potential from the
  electrical resistance of a block of porous
  material buried in the soil and in equilibrium in
  water suction with the soil.
• Can work at very low water potentials (as low as
  1500 kPa)
• Useful for clays
• Sensitive to temperature and salinity
• Gradual change in resistance over time
• Compatibility of physical characteristics between
  the soil and the burried block

Gypsum Block
http//www.sowacs.com/sensors/gypsum.html
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Infiltration measurements

• Infiltrometers
• Measures the rate of water entering the soil
  surface, i.e., mm/h
• Permeameters
• Measures infiltration at a given depth
• 3. Throughflow pits
• Measures infiltration at different depths