Soil Water Measurement

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Soil Water Measurement
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Soil Water Measurement

• Soil water affects plant growth through its
  controlling effect on plant water status.
• Two ways to assess soil water availability for
  plant growth
• by measuring the soil water content and
• by measuring how strongly that water is retained
  in the soil (soil water potential).

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Soil Water Content

• saturated soil All soil voids (pore space) are
  filled with water.
• Field Capacity (FC) All readily drainable water
  (by gravity) are vacated macro-pores,
  approximately 0.33 bar (330 cm or pF 2.5).
• Permanent Wilting Point (PWP) The soil moisture
  content at which the leaves of sunflower plants
  wilt permanently and do not recover if water is
  applied, approximately 15 bars (15,000 cm, pF
  4.2). Water is left only in micro-pores.

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Available Water Capacity (AWC)

• Volume of water that is kept in the soil between
  FC and PWP.
• This water is potentially available to the
  plant and the value is generally used for
  determining frequency of irrigation and the depth
  of water that should be applied.
• AWC (mm m-1)(FC by volume-PWP by volume)x(10)

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Readily available water capacity (RAWC)

• Not all the water held between FC and PWP is
  available at the same rate to the plants.
• RAWC, kept at the lower tension (lower pF
  values), is considered a better indicator of soil
  moisture stress and should be used for irrigation
  scheduling.
• Rule of thumb 50 to 75 of AWC is considered as
  RAWC, varying based on crop physiology, rooting
  depth and volume, and moisture extraction pattern
  of each crop.

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Measurement of FC and PWP

• FC and PWP can be measured in the laboratory,
  using appropriately sized pressure plates and
  corresponding pressure membranes.
• PWP measurement Use of pressure plate (at -15
  bars matric potential or pF4.2) is an accepted
  method.
• Many question the validity of laboratory
  measurement of FC, and prefer field measurement.

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AWC calculation

• Soil moisture is determined on a weight basis.
• Using Db values, MC on a weight basis is
  converted to MC on a volume basis
• MC ( by volume v/v) MC ( by weight w/w)x(Db)
• or,
• MC ( v/v) (water weight/dry soil weight) x
  (weight of dry soil/total soil volume)
• Where,
• Db Bulk density, and MC Moisture content

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Soil moisture characteristics curve

• As water content in soil decreases, the matric
  potential decreases (becomes larger negative
  number).
• The functional relationship between matric
  potential (the potential resulting from
  attractive forces between the soil matrix and the
  water) in the soil and changes in soil water
  content is named the soil moisture
  characteristics (retention) curve.

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Moisture retention curve determination

• Moisture content at saturation (water-content at
  pF 0) is an indication of soils total
  pore-volume percentage.
• Retention curve is produced for different soils
  by determining water content at different
  tensions between saturation and PWP.
• Normal tensions applied (vacuum) are 0.05, 0.2,
  0.33 (FC), 1.0, 3.0, 15 bars (PWP) that are
  equivalent to 1.7, 2.0, 2.5, 3.0, 3.5, and 4.2 pF
  values, respectively.
• Moisture content of oven dry soil can be used as
  the equivalent tension of 9,800 bars (pF value of
  7.0).

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Laboratory Procedures for pF Curves

• Saturate the soil cores until a film of water is
  formed on soil surface, letting water to be
  adsorbed from the bottom
• After weighing, place pre-saturated soils on top
  of the ceramic plate
• Make sure that there is a good contact between
  the soil cores and the ceramic plate

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Laboratory Procedures for pF Curves (cont.)

• The outlet tube of the ceramic plate should then
  connected to the outflow tube of the pressure
  chamber
• The chamber should be pressurized to intended
  positive pressure
• The system should stay pressurized until
  equilibrium is reached with the applied water
  pressure. The equilibrium is reached when outflow
  of water has ceased which may even take three to
  four days

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Laboratory Procedures for pF Curves (cont.)

• After reaching the equilibrium, the pressure
  should be released and the core samples should be
  weighed
• This procedure should be repeated for all
  intended matric potentials, until all
  measurements are completed
• After all measurements are completed, soil cores
  should be dried in forced air oven at 105oC.

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Laboratory Procedures for pF Curves (cont.)

• The volumetric water content for each matric
  potential will be calculated using
• Volumetric water content ()Vol. of water
  (cm3)/Core volume (cm3)
• The volume of water at each matric potential (pF
  value) is then determined from
• Vol. of water(Mass of equilibrated soilMass of
  oven dried core)/DbH2O
• Where DbH2O 1
• The soil moisture characteristic curve is then
  produced by plotting the soil water matric
  potential (bar or pF value) against soil
  volumetric water content ().

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Soil water characteristics (retention) curves
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Field measurement

• It is best to directly measure the degree of
  wetness (soil moisture content) or the matrix
  potential, rather than using calibration curves
  for estimating soil water content for irrigation
  scheduling, because of the effect of hysteresis
  caused by wetting and drying of soil samples.

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Non-destructive water content measurementNeutron
Probe

• Neutron probe uses the property of scattering and
  slowing down neutrons (H ions).
• Alpha particles emitted by the decay of the
  americium (241) collide with the light beryllium
  nuclei, producing fast neutron.
• Fast neutrons, encountering hydrogen in the soil,
  lose their energy and are slowed down or
  thermalized.
• The detection of slow neutrons returning to the
  probe allows estimation of the amount of H ions
  present.
• Since most of the H ions in the soil is
  associated with soil water, it provide water
  content estimate.

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Non-destructive water content measurementTime
Domain Reflectometry (TDR)

• TDR measures the spread of an electromagnetic
  wave through the soil.
• The characteristics of this propagation depends
  on soil water content.
• A good agreement exist between the TDR and
  neutron probe measurements.
• The cost of neutron probe and TDR are prohibitive.

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Non-destructive water potential
measurementGypsum block/Granular Matrix Sensors

• Exhibit a wide range relationship between their
  electric conductivity and soil water potential.
• Somewhat unreliable in some soils caused by loss
  of contact with the soil due to dissolving of
  gypsum, inconsistence pore size distribution and
  soil salinity effects.
• GMS works based on Gypsum block technology, but
  reduces the general inherent problems of gypsum
  blocks, using a granular matrix mostly supported
  in a metal or plastic screen.

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Non-destructive water potential
measurementTensiometers

• Another type of instrument that measures the
  energy status (or potential) of soil water.
• Tensiometers are extensively used for irrigation
  scheduling because they provide direct
  measurements of soil moisture status and are easy
  to manage.
• Tensiometers are available at BoWRD.

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Non-destructive water potential
measurementTensiometers (Components)

• A porous ceramic cup and a rigid body tube that
  is connected to a manometer or a vacuum gauge
  with all components filled with water, having an
  air-tied seal.
• A Bourdon tube vacuum gauge is commonly used for
  water potential measurements.

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Non-destructive water potential
measurementTensiometers (Operation Principles)

• Tensiometers are placed with ceramic cup firmly
  in contact with soil in plant root zone.
• Since ceramic cup is porous, water moves through
  it to equilibrate with soil water, causing a
  hydraulic contact between water in the cup and
  soil water.
• Water moving out of the cup develop a suction or
  negative pressure (partial vacuum) that causes a
  reading on the vacuum gauge.
• Gauge reading, an indication of the attractive
  forces between water and soil particles, is a
  measure of the energy that would need to be
  exerted by the plant to extract water from the
  soil.

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Non-destructive water potential
measurementTensiometers (Operation Principles)

• Tensiometer is able to follow changes in the
  matric potential as a result of soil drying out
  due to drainage, evaporation or plant uptake of
  water (transpiration).
• When moisture is replenished by rain or
  irrigation, the matric potential will drop.
• Tensiometer continuously records fluctuations in
  soil water potential under field conditions.

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Non-destructive water potential
measurementTensiometers (Operation Principles)

• Accurate tensiometer response will occur only if
  air does not enter the water column.
• Air expands and contracts with changes in
  pressure and temperature, thus causing inaccurate
  tensiometer readings.
• Air leaks or dissolved air can enter through the
  ceramic cup during normal operation of the
  instrument.
• If a significant amount of air enters the
  instrument, it must be expelled and the
  tensiometer refilled with water before it can
  reliably operate again.

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Non-destructive water potential
measurementTensiometers (Operation Range)

• The useful range of a tensiometer is limited from
  0 (saturation) to as high as 0.85 bar (85 cm
  head).
• Above 0.85 bar the column of water in the tube
  will form water vapor bubbles (cavitate), causing
  instrument to stop functioning.
• In many agricultural soils, the tensiometer range
  accounts for 50 of the soil water that is taken
  up by the plants (almost RAWC)

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Non-destructive water potential
measurementTensiometers (Site selection)

• Tensiometers measure soil water tension in a
  small volume of soil immediately around the
  ceramic cup.
• Should be placed within the root active zone(s)
  of the crop for which irrigation is scheduled.
• Depending on crop type and its root distribution,
  one or more tensiometers of variable length may
  be required.

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Non-destructive water potential
measurementTensiometers (Placement in the field)

• Site(s) selected for installation must be
  representative of the surrounding field
  conditions.
• Tensiometers should be placed within the active
  root zone, in the plant canopy in positions,
  receiving typical amounts of rainfall and
  irrigation as the intended crop.
• shallow-rooted crops (vegetables) need only one
  tensiometer, centered in the crop root zone,
  10-15 cm below the surface.
• Deep rooted crops (tree crops, most row crops)
  two tensiometer should be used at each site.

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Non-destructive water potential
measurementTensiometers (Installation)

• Before field installation, each tensiometer
  should be tested to ensure it is working.
• Fill tensiometers with clean water (deionized
  water) and keep vertically for at least 30
  minutes to saturate the ceramic tip.
• After fully wetting the ceramic tip, it can be
  refilled and capped.

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Non-destructive water potential
measurementTensiometers (Installation)

• Tensiometer will not be serviceable immediately
  after filling because of air bubbles in the
  vacuum gauge.
• small vacuum hand pump should be used to remove
  all air bubbles from the tube and vacuum gauge
  and test for air leaks.
• After air bubbles are removed, tensiometers
  should be installed in previously cored holes to
  the appropriate depth in the field.

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Non-destructive water potential
measurementTensiometers (Installation)

• Soil around tensiometer should be tamped at the
  surface.
• After installation, several hours is required,
  before tensiometer can read the correct soil
  water potential value due to installation induced
  disturbance of the soil and the need for water to
  move through the ceramic cup before equilibrium
  is reached.

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Non-destructive water potential
measurementTensiometers (Installation)

• Tensiometers must be periodically serviced in the
  field.
• Under normal operation, air will be extracted
  from water under tension and becomes trapped
  within the tensiometer, reducing response time
  and its operability.
• Tensiometer tube should be inspected each time
  the tensiometer is read.
• If more than 0.5 cm of air is accumulated beneath
  the service cap, the trapped air should be
  removed and the tube refilled with deionized
  water.

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Tensiometers (Automation)

• Tensiometers can be instrumented to provide
  automatic control of irrigation systems.
• Vacuum gauge is equipped with a magnet and a
  magnetic pick-up switch so that, when a desired
  (and preset) water tension occurs, the switch
  closes, starting the irrigation pump.
• Pump operates for a preset period of time,
  lowering the tensiometer reading, after which the
  tensiometer is again monitored until the critical
  water tension again occurs.

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I hope we all enjoyed the five days of office
discussions. I certainly did and look forward to
the upcoming field work. Let us make our hands
dirty!