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Methods of Media Characterization. A challenging area of rapid advancement. Topics. Measurement of pressure potential The tensiometer The psychrometer Measurement of Water Content TDR (dielectric) Neutron probe (thermalization)

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methods of media characterization

Methods of Media Characterization

A challenging area of rapid advancement

  • 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




physical indicators of moisture
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
methods direct versus indirect
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
methods direct
Methods: direct
  • Gravemetric
      • Dig some soil; Weigh it wet; Dry it; Weigh it dry
  • Volumetric
      • Take a soil core (“undisturbed”); Weigh wet, dry

Pro’s Con’s

- Accurate (+/- 1%) - Can’t repeat in spot

- Cheap - Slow - 2 days

equipment - free - Time consuming

per sample - free


methods indirect via pressure
Methods: Indirect via pressure
  • Tensiometers
  • Psychrometers
  • Indirect2: Surrogate media

Gypsum blocks (includes WaterMark etc.)

communicating with soil porous solids
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
pressure measurement the tensiometer






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 1940’s
  • Need replicates to be reliable (>4)
pressure measurement the tensiometer1
Pressure measurement: The tensiometer
  • Can be made in many shapes, sizes.
pressure measurement the tensiometer2
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 (won’t measure <-0.8bar)
  • Most useful in situations where you need to know pressure (engineered waste etc.)
pressure potential the psychrometer
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 pg_hydro.html

Relative humidity





pressure potential the psychrometer1
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

indirect pressure gypsum block watermark et al


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)
example watermark
Example: Watermark

$260 for meter

$27 for probes

indirect pressure gypsum block watermark et al1
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






Water content

Water content

We want a value for water

content in our soil

We measure water content

in the surrogate media

indirect pressure gypsum block watermark et al2
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!)
indirect electrical the nature of soil dielectric
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 (>200 mHz)
indirect electrical capacitance dielectric low frequency
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


  • soils have very different dielectrics at low frequency



high frequency capacitance dielectric
High Frequency Capacitance (Dielectric)
  • 80 mHz
  • $250 meter
  • $250 sensor
  • $20 access tube
  • Calibration fairlystable
indirect electrical tdr dielectric
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
indirect electrical tdr dielectric1
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 <$500)
      • Potentially accurate (+/- 2% or better)
indirect radiation interactions between soil radiation
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

indirect radiation neutron probe thermalization
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(access tube)
  • Quite accurate (simply wait for lots of counts)
  • Lots of soil constituentscan effect calibration


Thermalized Neutrons

High Energy Neutrons

indirect radiation neutron probe thermalization1
Indirect radiation:Neutron probe (thermalization)
  • Pro’s
    • Potentially Accurate
    • Widely available
    • Inexpensive per location
    • Flexible (e.g., can go very deep)
  • Cons
    • Needs soil specific calibration (lots of work)
    • Working with radiation
    • Expensive to buy
    • Expensive to dispose
    • Slow to use
    • can’t be automated
indirect radiation gamma probe
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

gamma attenuation
Gamma Attenuation
  • Attenuation follows Beer’s law: each frequency attenuated at different rate; each material having a different attenuation rate.
    • I= incident radiation
    • I= transmitted radiation
    • xi=thickness of medium i
    • ai=attenuation coefficient for material i at frequency 
indirect via feel getting to know your soil
Indirect via feel:getting to know your soil
  • Soil water status obtained checking the feel of the soil
    • Does It make a ribbon?
    • Does it stick to your hand?
    • Does it crumble?
  • Although crude, the information immediate; gets farmer in field thinking about water and her soil
  • Possibly the most effective water monitoring strategy


directions in the future
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


permeability double ring infiltrometer
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
available in a wide range of sizes
Available in a Wide Range of Sizes!

Photo: Paul Measles

interpreting infiltration experiments
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
the brutsaert model
The Brutsaert Model
  • The Brutsaert Model
  • S = sorptivity
  • 0<<1 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.
interpreting infiltration experiments cont
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
interpreting infiltration experiments cont1
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.
permeability tension infiltrometer
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 1980’s, becoming the industry standard
interpreting tension infiltrometer data
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 K=Ksexp(-t)
  • r is the disk radius. Using either multiple tensions or multiple radii, you can solve for Ks and 
Interpretation requires fitting a straight line to the “steady-state” data.
  • Note: noise increases as flow decreases
permeability well permeameter
Permeability: Well permeameter
  • Establishes a fixed height of ponding
  • Measure rate of infiltration
  • Can estimate K(h) relationship via time rate of infiltration
making sense of well permeameter data
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/a<2 by
  • For H/a>2, error can be reduced by using Reynolds and Elricks result
  • Where * is tabulated
k s lab methods constant head
Ks - Lab methods: constant head
  • Basically reproduces Darcy’s experiment
  • Important to measure head loss in the media
  • Typically use “Tempe Cells” for holding cores, which are widely available
k s lab methods falling head
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


radius R

measuring green and ampt parameters
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]
the device
The Device
  • Key Parts:
  • Reservoir
  • Pressure Gauge
  • Infiltration Ring
identify the air and water entry pressures
Identify the Air and Water Entry Pressures
  • ha – air entry pressure
  • hw – water entry pressure
  • Typically assume that
  • ha = 2 hw
  • 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
employ falling head method for k s
Employ falling head method for Ks
  • Recall standard falling head result from lab methods:
  • Remember that Kfs is about 0.5 Ks
water entry pressure
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
limitations on bouwer method
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 56:139-147
  • Aldabagh and Beer, 1971 TASAE 14:29-31