Methods of media characterization
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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 l.jpg

Methods of Media Characterization

A challenging area of rapid advancement

Topics l.jpg

  • 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 l.jpg
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

Methods direct l.jpg
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



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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

    Pressure measurement the tensiometer l.jpg






    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 tensiometer9 l.jpg
    Pressure measurement: The tensiometer

    • Can be made in many shapes, sizes.

    Pressure measurement the tensiometer10 l.jpg
    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 l.jpg
    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 psychrometer12 l.jpg
    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 l.jpg


    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 l.jpg
    Example: Watermark

    $260 for meter

    $27 for probes

    Indirect pressure gypsum block watermark et al15 l.jpg
    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 al16 l.jpg
    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 (>200 mHz)

    Indirect electrical capacitance dielectric low frequency l.jpg
    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 l.jpg
    High Frequency Capacitance (Dielectric) frequency)

    • 80 mHz

    • $250 meter

    • $250 sensor

    • $20 access tube

    • Calibration fairlystable

    Indirect electrical tdr dielectric l.jpg
    Indirect electrical: TDR (dielectric) frequency)

    • 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 dielectric22 l.jpg
    Indirect electrical: TDR (dielectric) frequency)

    • 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 l.jpg
    Indirect radiation: frequency)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 l.jpg
    Indirect radiation: frequency)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 thermalization25 l.jpg
    Indirect radiation: frequency)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 l.jpg
    Indirect radiation: frequency) 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 l.jpg
    Gamma Attenuation frequency)

    • 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 l.jpg
    Indirect via feel: frequency)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 l.jpg
    Directions in the future frequency)

    • 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 l.jpg
    Permeability: Double ring infiltrometer frequency)

    • 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 l.jpg
    Available in a Wide Range of Sizes! frequency)

    Photo: Paul Measles

    Interpreting infiltration experiments l.jpg
    Interpreting Infiltration Experiments frequency)

    • 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 l.jpg
    The Brutsaert Model frequency)

    • 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 l.jpg
    Interpreting Infiltration Experiments, cont. frequency)

    • 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 cont35 l.jpg
    Interpreting Infiltration Experiments, cont. frequency)

    • 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 l.jpg
    Permeability: Tension infiltrometer frequency)

    • 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 l.jpg
    Interpreting Tension Infiltrometer Data frequency)

    • 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 

    Slide39 l.jpg

    Permeability well permeameter l.jpg
    Permeability: Well permeameter “steady-state” data.

    • 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 l.jpg
    Making sense of Well Permeameter data “steady-state” 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 l.jpg
    K “steady-state” data.s - 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 l.jpg
    K “steady-state” data.s - 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 l.jpg
    Measuring Green and Ampt Parameters “steady-state” data.

    • 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 l.jpg
    The Device “steady-state” data.

    • Key Parts:

    • Reservoir

    • Pressure Gauge

    • Infiltration Ring

    Identify the air and water entry pressures l.jpg
    Identify the Air and Water Entry Pressures “steady-state” data.

    • ha – air entry pressure

    • hw – water entry pressure

    • Typically assume that

    • ha = 2 hw

    Procedure l.jpg
    Procedure “steady-state” data.

    • 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 K “steady-state” data.s

    • Recall standard falling head result from lab methods:

    • Remember that Kfs is about 0.5 Ks

    Water entry pressure l.jpg
    Water Entry Pressure “steady-state” data.

    • 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 l.jpg
    Limitations on Bouwer Method “steady-state” data.

    • 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