Methods of Media Characterization

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Methods of Media Characterization

• A challenging area of rapid advancement

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Topics

• Measurement of pressure potential
• The tensiometer
• The psychrometer
• Measurement of Water Content
• TDR (dielectric)
• Neutron probe (thermalization)
• Gamma probe (radiation attenuation)
• Gypsum block (energy of heating)
• Measurement of Permeability
• Tension infiltrometer
• Well permeameter

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Physical Indicators of Moisture

• All methods measure some physical quantity What
  can be measured?
• weight of soil
• pressure of water in soil
• humidity of air in soil
• scattering of radiation that enters soil
• dielectric of soil
• resistance to electricity of soil
• texture of soil
• temperature/heat capacity of soil
• Each method takes advantage of one indicator

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Methods Direct versus indirect

• Direct methods measures the amount of water that
  is in a soil
• Indirect methods estimates water content by a
  calibrated relationship with some other
  measurable quantity (e.g. pressure)
• We will see that the vast majority of tools
  available are indirect
• The key to assessing indirect methods is the
  quality/stability/consistency of calibration

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Methods direct

• Gravemetric
• Dig some soil Weigh it wet Dry it Weigh it dry
• Volumetric
• Take a soil core (undisturbed) Weigh wet, dry
• Pros Cons
• - Accurate (/- 1) - Cant repeat in spot
• - Cheap - Slow - 2 days
• equipment - free - Time consuming
• per sample - free

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Methods Indirect via pressure

• Tensiometers
• Psychrometers
• Indirect2 Surrogate media
• Gypsum blocks (includes WaterMark? etc.)

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Communicating with soil Porous solids

• The tensiometer employs a rigid porous cup to
  allow measurement of the pressure in the soil
  water.
• Water can move freely across the cup, so pressure
  inside is that of soil

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Pressure measurement The tensiometer

• Can be made in many shapes, sizes.
• Require maintenance to keep device full of water
• Useful to -0.8 bar
• Employed since 1940s
• Need replicates to be reliable (gt4)

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Pressure measurement The tensiometer

• Can be made in many shapes, sizes.

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Pressure measurement The tensiometer

• Thumbnail Watch out for
• Swelling soils
• tensiometer will loose contact, and not function
• Inept users!
• Poor for sites with low skill operators of units
• Easy to get garbage data if not careful
• Fine texture soils (wont measure lt-0.8bar)
• Most useful in situations where you need to know
  pressure (engineered waste etc.)

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Pressure potential The psychrometer

• A device which allows determination of the
  relative humidity of the subsurface through
  measurement of the temperature of the dew point

Pressure
Relative humidity
Gas constant
Temperature
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Pressure potential The psychrometer

• Thumbnail most likely not your 1st choice...
• Great for sites where the typical conditions are
  very dry. In fact, drier than most plants
  prefer.
• Low accuracy in wet range (0 to -1 bar)
• Need soil characteristic curves to translate
  pressures to moisture contents - problem in
  variable soils
• Great for many arid zone research projects

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Indirect pressure Gypsum block, Watermark et al.

• Using a media of known moisture content/pressure
  relationship
• Energy of heating a strong function of ?
• Resistance embedded plates also f(?).
• Measure energy of heating, or resistance infer
  pressure
• Problems
• The properties of the media change with time
  (e.g., gypsum dissolves clay deposition)!
• Making reproducible media very difficult (need
  calibration per unit)
• Hysteresis makes the measurement inaccurate (more
  on this later)

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Example Watermark
260 for meter 27 for probes
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Indirect Pressure Gypsum block, Watermark et al.

• Idea of indirect pressure measurements
• Measure water content of surrogate media, infer
  pressure, then infer water content in soil

Surrogate Media
Soil
Pressure
Pressure
Water content
Water content
We want a value for water content in our soil
We measure water content in the surrogate media
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Indirect Pressure Gypsum block, Watermark et al.

• Thumbnail
• Generally a low cost option
• Calibration typically problematic in time and
  between units
• Poor in swelling soils
• Poor in highly variable soils
• Sometimes adequate for yes/no decisions
• We have had very poor luck with these in
  Willamette valley (no correlation!)

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Indirect electrical the nature of soil
dielectric

• Soils generally have a dielectric of about 2
  to 4 at high frequency.
• Water has a dielectric of about 80.
• If we can figure a way to measure the soil
  dielectric, it shows water content.
• WATCH OUT the soil dielectric is a function of
  the frequency of the measurement! For it to be
  low, need to use high frequency method (gt200 mHz)

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Indirect electrical Capacitance (dielectric,
low frequency)

• Stick an unprotected capacitor into the soil
  and measure the capacitance.
• Higher if there is lots of dielectric (i.e.,
  water)
• Need to Calibrate capacitance vs volumetric water
  content per soil
• PROBLEM
• soils have very different dielectrics at low
  frequency

70
500
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Indirect electrical TDR (dielectric)

• Observe the time of travel of a signal down a
  pair of wires in the soil.
• Signal slower if there is lots of dielectric
  (i.e., water)
• Calibrate time of travel vs volumetric water
  content
• Since high frequency, can use universal
  calibration

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Indirect electrical TDR (dielectric)

• Lots of excitement surrounding TDR now. Why?
• Non-nuclear
• universal calibration
• measures volumetric water content directly
• wide variety of configurations possible
• Long probes (up to 10 feet on market)
• Short probes (less than an inch)
• Automated with many measuring points
• Electronics coming down in price (soon lt500)
• Potentially accurate (/- 2 or better)

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Indirect radiation interactions between soil
radiation

• When passing through, radiation can either
• be adsorbed by the stuff
• change color (loose energy)
• pass through unobstructed
• Which of these options occurs is a function of
  the energy of the radiation
• Each of these features is used in soil water
  measurement

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Indirect radiation Neutron probe
(thermalization)

• Send out high energy neutrons
• When they hit things that have same mass as a
  neutron (hydrogen best), they are slowed.
• Return of slow neutrons calibrated to water
  content (lots of hydrogen)
• Single hole method
• Quite accurate (simply wait for lots of counts)
• Lots of soil constituentscan effect calibration

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Indirect radiation Neutron probe
(thermalization)

• Cons
• Needs soil specific calibration (lots of work)
• Working with radiation
• Expensive to buy
• Expensive to dispose
• Slow to use
• cant be automated

• Pros
• Potentially Accurate
• Widely available
• Inexpensive per location
• Flexible (e.g., can go very deep)

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Indirect radiation Gamma probe

• Radiation attenuation
• Source detector separated by soil.
• Water content determines adsorption of beam
  energy.
• Must calibrate for each soil.
• Same used in neutron and x-ray attenuation.
• Can use various frequencies to determine fluid
  content of various fluids (e.g., Oils)
• Not used in commercial agriculture

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Gamma Attenuation

• Attenuation follows Beers law each frequency
  attenuated at different rate each material
  having a different attenuation rate.
• I?? incident radiation
• I? transmitted radiation
• xithickness of medium i
• ai?attenuation coefficient for material i at
  frequency ?

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Indirect via feelgetting to know your soil

• A reasonable soil water status may be obtained by
  checking the feel of the soil
• Does It make a ribbon?
• Does it stick to your hand?
• Does it crumble?
• Although crude, the information is immediate, and
  gets the farmer into the field and thinking about
  water and her soil
• Possibly the most effective water monitoring
  strategy

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Directions in the future

• Much lower cost TDR
• Much more flexible systems
• radio telemetry for cheap
• auto-logging systems
• computer based tracking
• Much less water to work with
• Much more call for precise and frequent water
  monitoring

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Permeability Double ring infiltrometer

• Establishes 1-d flow by having concentric sources
  of water
• measure rate of infiltration in central ring
• Easy, but requires lots of water, and very
  susceptible to cracks, worm holes, etc.
• Interogates large area

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Interpreting Infiltration Experiments

• Horton Equation
• Rate of infiltration, i, is given by
• i if (io - if) exp(-?t)
• where if is the infiltration rate after long
  time, io is the initial infiltration rate and ?
  is and empirical soil parameter. Integrating
  this with time yields the cumulative
  infiltration

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The Brutsaert Model

• The Brutsaert Model
• S sorptivity
• 0lt?lt1 pore size distribution parameter. wide
  pore size distributions ? 1 other soils ?
  2/3
• The Brutsaert cumulative infiltration is
• from which you can determine Ksat and S.

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Interpreting Infiltration Experiments, cont.

• The two term Philip model suggests fitting the
  rate of infiltration to
• i 0.5 S t-1/2 A
• and the cumulative infiltration as
• I S t1/2 At

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Interpreting Infiltration Experiments, cont.

• The Green and Ampt Model (constant head)
• L depth of wetting front
• n porosity
• d depth of ponding
• hf water entry pressure
• The cumulative infiltration is simply I nL.
• To use this equation you must find the values of
  Ksat and hf which give the best fit to the data.

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Permeability Tension infiltrometer

• Applies water at set tension via Marriotte bottle
• Using at sequence of pressures can get K(h) curve
• Read flux using pressure sensors
• Introduced in 1980s, becoming the industry
  standard

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Interpreting Tension Infiltrometer Data

• The data from the tension infiltrometer is
  typically interpreted using the results for
  steady infiltration from a disk source develped
  by Wooding in 1968 for a Gardner conductivity
  function KKsexp(-?t)
• r is the disk radius. Using either multiple
  tensions or multiple radii, you can solve for Ks
  and ?

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Typical Tension infiltrometer Data
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• Interpretation requires fitting a straight line
  to the steady-state data.
• Note noise increases as flow decreases

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Permeability Well permeameter

• Establishes a fixed height of ponding
• Measure rate of infiltration
• Can estimate K(h) relationship via time rate of
  infiltration

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Making sense of Well Permeameter data

• Interpretation of well permeameter data typically
  employs the result of Glover (as found in Zanger,
  1953) for steady infioltration from a source of
  radius a and ponding height H
• The geometric factor c is given, for H/alt2 by
• For H/agt2, error can be reduced by using Reynolds
  and Elricks result
• Where ? is tabulated

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Ks - Lab methods constant head

• Basically reproduces Darcys experiment
• Important to measure head loss in the media
• Typically use Tempe Cells for holding cores,
  which are widely available

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Ks - Lab methods falling head

• Better for low permeability samples.
• Need to account for head loss through instrument
• Measure time rate of falling head and fit to
  analytical solution

radius r
Core radius R
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Measuring Green and Ampt Parameters

• The Green and Ampt infiltration model requires a
  wetting front potential and saturated
  conductivity. The Bouwer infiltrometer provides
  these parameters
• WRR 4(2)729-738, 1966

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The Device

• Key Parts
• Reservoir
• Pressure Gauge
• Infiltration Ring

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Identify the Air and Water Entry Pressures

• ha air entry pressure
• hw water entry pressure
• Typically assume that
• ha 2 hw

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Procedure

• Pound Ring in with slide hammer about 10 cm
• Purge air and allow infiltration until wetting
  front is at 10 cm
• Measure dH/dt to obtain infiltration rate
• Close water supply valve
• Record pressure on vacuum gauge record minimum
  value

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Employ falling head method for Ks

• Recall standard falling head result from lab
  methods
• Remember that Kfs is about 0.5 Ks

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Water Entry Pressure

• The water entry pressure will be taken as half
  the value of the measured air entry pressure (the
  minimum pressure from the vacuum gauge on the
  infiltrometer)
• WATCH OUT correct observed pressure for
  water column height in unit

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Limitations on Bouwer Method

• All parameters are operational rather than
  fundamental
• Conductivity is less than K found in labs due to
  trapped air
• Rocks and cracks can render measured value of hw
  incorrect.
• For more details on method see
• Topp and Binns 1976 Can. J. Soil Sci 56139-147
• Aldabagh and Beer, 1971 TASAE 1429-31