Methods of Media Characterization

1
Methods of Media Characterization

• A challenging area of rapid advancement

Williams, 2002
http//www.its.uidaho.edu/AgE558 Modified after
Selker, 2000
http//bioe.orst.edu/vzp/
2
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

3
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

4
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

5
Methods direct

• Gravimetric
• 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

6
Methods Indirect via pressure

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

7
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

8
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)

9
Pressure measurement The tensiometer

• Can be made in many shapes, sizes.

10
Pressure measurement The tensiometer

• Thumbnail Watch out for
• Swelling soils
• tensiometer will loose contact during drying, and
  not function
• Inept users!
• Poor for sites with low skill operators of units
• Easy to get garbage data if not careful
• Fine-textured soils (wont measure lt-0.8bar)

11
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
12
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

13
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

14
Gypsum block, continued

• Problems
• The properties of the media change with time
  (e.g., gypsum dissolves clay deposition on
  surface changes gypsum moisture curve)
• Making reproducible media very difficult (need
  calibration per each unit)
• Hysteresis makes the measurement inaccurate (more
  on this later)

15
Example Watermark
260 for meter 27 for probes
16
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
17
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
• Selker had very poor luck with these in
  Willamette valley (no correlation!)

18
Dielectric

• A dielectric is a substance that doesnt conduct
  electricity (an insulator)
• Word dielectric used when considering the effect
  of AC fields on the substance usually a
  non-metal.
• Commonly considered synonymous with insulator
  used when material is used to withstand a high
  electric field (e.g. in a capacitor)

19
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)

20
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
21
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

22
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)

23
Indirect Electrical

• Other Surface and Subsurface Geophysical Methods
• DC Resistivity
• Electromagnetic Induction (Emag)
• Ground-penetrating radar (GPR)

24
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

25
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

26
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)

27
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

28
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 ?

29
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 soil scientist into the field and
  thinking about water and soil
• Possibly the most effective water monitoring
  strategy

30
Directions in the future

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

31
Ways to measure Flux

• Measure flux (q) because you need to know it per
  se,
• or to infer K
• See Hubbell presentation on student project page.

32
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

33
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

34
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.

35
New term Sorptivity

• 1957, Sorptivity introduced by Philip
• measure of the capacity of a medium to adsorb or
  desorb a liquid.
• Where I is the cumulative infiltration at time t,
  and S is the sorptivity

36
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

37
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

38
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 ?

39
Typical Tension infiltrometer Data
40

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

41
Permeability Well permeameter

• Establish a fixed height of ponding
• Variation on this design BAT
• Measure rate of infiltration
• Can estimate K(h) relationship via time rate of
  infiltration

42
Making sense of Well Permeameter data

• Interpretation of well permeameter data typically
  employs the result of Glover (as found in Zanger,
  1953) for steady infiltration 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

43
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

44
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
45
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.

46
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

47
The Device

• Key Parts
• Reservoir
• Pressure Gauge
• Infiltration Ring

48
Identify the Air and Water Entry Pressures

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

49
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

50
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

51
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

52
Employ falling head method for Ks

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