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Methods of Media Characterization

A challenging area of rapid advancement

A challenging area of rapid advancement

Williams, 2002 http://www.its.uidaho.edu/AgE558

Modified after Selker, 2000 http://bioe.orst.edu/vzp/

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TopicsMeasurement of pressure potentialThe tensiometerThe psychrometer

Measurement of Water ContentTDR (dielectric)Neutron probe (thermalization)Gamma probe (radiation attenuation)Gypsum block (energy of heating)

Measurement of PermeabilityTension infiltrometerWell permeameter

Measurement of pressure potentialThe tensiometerThe psychrometer

Measurement of Water ContentTDR (dielectric)Neutron probe (thermalization)Gamma probe (radiation attenuation)Gypsum block (energy of heating)

Measurement of PermeabilityTension infiltrometerWell permeameter

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Physical Indicators of Moisture

All methods measure some physical quantity What can be measured?weight of soilpressure of water in soilhumidity of air in soilscattering of radiation that enters soildielectric of soilresistance to electricity of soiltexture of soiltemperature/heat capacity of soil

Each method takes advantage of one indicator

All methods measure some physical quantity What can be measured?weight of soilpressure of water in soilhumidity of air in soilscattering of radiation that enters soildielectric of soilresistance to electricity of soiltexture of soiltemperature/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

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: directGravimetric

Dig some soil; Weigh it wet; Dry it; Weigh it dry

VolumetricTake 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 consumingper sample - free

GravimetricDig some soil; Weigh it wet; Dry it; Weigh it dry

VolumetricTake 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 consumingper sample - free

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Methods: Indirect via pressure

Tensiometers

Psychrometers

Indirect2: Surrogate mediaGypsum blocks (includes WaterMark etc.)

Tensiometers

Psychrometers

Indirect2: Surrogate mediaGypsum 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

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 barEmployed since 1940’sNeed replicates to be

reliable (>4)

Can be made in many shapes, sizes.

Require maintenance to keep device full of water

Useful to -0.8 barEmployed since 1940’sNeed replicates to be

reliable (>4)Cup

Gauge

Reservoir

Body

Removable

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Pressure measurement: The tensiometer

Can be made in many shapes, sizes.

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 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 (won’t measure <-0.8bar)

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 (won’t measure <-0.8bar)

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Pressure potential: The psychrometer

Porous Ceramic Cup

Thermocouple

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

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

Pressure

TemperatureGas constant

Relative humidity

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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

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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Heater

Thermocouple

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

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

Indirect pressure: Gypsum block, Watermark et al.

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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)

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)

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

Idea of indirect pressure measurements:Measure water content of surrogate media, infer pressure,

then infer water content in soil

SoilSurrogate Media

Pre

ssur

e

Pre

ssur

e

Water content Water content

We measure water content in the surrogate media

We want a value for watercontent in our soil

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Indirect Pressure: Gypsum block, Watermark et al.

Thumbnail:Generally a low cost optionCalibration typically problematic in time

and between unitsPoor in swelling soilsPoor in highly variable soilsSometimes adequate for yes/no

decisionsSelker had very poor luck with these in

Willamette valley (no correlation!)

Thumbnail:Generally a low cost optionCalibration typically problematic in time

and between unitsPoor in swelling soilsPoor in highly variable soilsSometimes adequate for yes/no

decisionsSelker had very poor luck with these in

Willamette valley (no correlation!)

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Dielectric

• A dielectric is a substance that doesn’t 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)

• A dielectric is a substance that doesn’t 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)

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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)

• 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)

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$70

$500

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

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

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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

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-nuclearuniversal calibrationmeasures volumetric water content directlywide variety of configurations possible

Long probes (up to 10 feet on market)Short probes (less than an inch)Automated with many measuring pointsElectronics coming down in price (soon <$500)Potentially accurate (+/- 2% or better)

Lots of excitement surrounding TDR now. Why?Non-nuclearuniversal calibrationmeasures volumetric water content directlywide variety of configurations possible

Long probes (up to 10 feet on market)Short probes (less than an inch)Automated with many measuring pointsElectronics coming down in price (soon <$500)Potentially accurate (+/- 2% or better)

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Indirect Electrical

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

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

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Indirect radiation: interactions between soil & radiation

When passing through, radiation can either:be adsorbed by the stuffchange 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

When passing through, radiation can either:be adsorbed by the stuffchange 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 neutronsWhen 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 methodQuite accurate (simply

wait for lots of counts)Lots of soil constituents

can effect calibration

Send out high energy neutronsWhen 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 methodQuite accurate (simply

wait for lots of counts)Lots of soil constituents

can effect calibration

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Indirect radiation: Neutron probe (thermalization)

Pro’sPotentially Accurate

Widely availableInexpensive per

locationFlexible (e.g., can go

very deep)

Pro’sPotentially Accurate

Widely availableInexpensive per

locationFlexible (e.g., can go

very deep)

ConsNeeds soil specific

calibration (lots of work)

Working with radiationExpensive to buyExpensive to disposeSlow to usecan’t be automated

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Indirect radiation: Gamma probe

Radiation attenuationSource & 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

Radiation attenuationSource & 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

Source Detector

Sample

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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

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

Source Detector

Sample

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Indirect via feel:getting to know your soil

A reasonable soil water status may be obtained by checking the feel of the soilDoes 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

A reasonable soil water status may be obtained by checking the feel of the soilDoes 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

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Directions in the future

Much lower cost TDR

Much more flexible systemsradio telemetry for cheapauto-logging systemscomputer based tracking

Much more call for precise and frequent water monitoring

Much lower cost TDR

Much more flexible systemsradio telemetry for cheapauto-logging systemscomputer based tracking

Much more call for precise and frequent water monitoring

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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.

• Measure flux (q) because you need to know it per se,

• ……or to infer K

• See Hubbell presentation on student project page.

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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

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

Infiltration under the inner ring is approximately one dimensional

Constant head must be maintained, to be equal in inner and outer rings

Install deep enough to avoid leakage

Typical Double Ring Infilrometer Set-up

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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

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 0<<1 pore size distribution parameter. wide pore

size distributions = ;1 other soils = 2/3The Brutsaert cumulative infiltration is

from which you can determine Ksat and S.

The Brutsaert Model

S = sorptivity 0<<1 pore size distribution parameter. wide pore

size distributions = ;1 other soils = 2/3The Brutsaert cumulative infiltration is

from which you can determine Ksat and S.

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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

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

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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

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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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

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

Septum Filling Port

Dir

ect

Re

ad

ing

Sca

le

Marriotte Bottle

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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 K=Ksexp(-t)

r is the disk radius. Using either multiple tensions or multiple radii, you can solve for Ks and

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

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Typical Tension infiltrometer Data

BOREAS 1994 Tension Infiltration TestNSA-YJP 8 cm Disk DOY 251 (c)

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

0 400 800 1200 1600 2000

Time (s)

De

pth

Infi

ltra

ted

(c

m)

15 cm

6 cm

3 cm

Interpretation requires fitting a straight line to the “steady-state” data.

Note: noise increases as flow decreases

Interpretation requires fitting a straight line to the “steady-state” data.

Note: noise increases as flow decreases

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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

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

Shut off valve

Graduated Cylinder

Bubbler

Device Outlet

Support plate

H

a

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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 infiltration 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

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/a<2 by

For H/a>2, error can be reduced by using Reynolds and Elricks result

Where * is tabulated

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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

Basically reproduces Darcy’s experiment

Important to measure head loss in the media

Typically use “Tempe Cells” for holding cores, which are widely available

Constant Head in flow

Packed Column

h2

h1

Constant Head out flow

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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

Better for low permeability samples.

Need to account for head loss through instrument

Measure time rate of falling head and fit to analytical solution

kr x

R tLn

h

h

2

21

2

radius r

Core radius R

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Interpreting Infiltration Experiments, cont.

The Green and Ampt Model (constant head)

L = depth of wetting frontn = porosityd = depth of ponding

hf = water entry pressureThe 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.

The Green and Ampt Model (constant head)

L = depth of wetting frontn = porosityd = depth of ponding

hf = water entry pressureThe 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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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 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]

Water Flood Valve

Air Purge Valve

Vacuum Gauge

O-Ring Seal

2r

L

2R Approx

10 cm

GH

Volumetricly Graduated Reservoir

SteelWool

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The Device

Key Parts:

Reservoir

Pressure Gauge

Infiltration Ring

Key Parts:

Reservoir

Pressure Gauge

Infiltration RingWater Flood Valve

Air Purge Valve

Vacuum Gauge

O-Ring Seal

2r

L

2R Approx

10 cm

GH

Volumetricly Graduated Reservoir

SteelWool

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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

ha – air entry pressure

hw – water entry pressure

Typically assume that

ha = 2 hw

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Procedure1. Pound Ring in with slide hammer about 10

cm

2. Purge air and allow infiltration until wetting front is at 10 cm

3. Measure dH/dt to obtain infiltration rate

4. Close water supply valve

5. Record pressure on vacuum gauge: record minimum value

1. Pound Ring in with slide hammer about 10 cm

2. Purge air and allow infiltration until wetting front is at 10 cm

3. Measure dH/dt to obtain infiltration rate

4. Close water supply valve

5. Record pressure on vacuum gauge: record minimum value

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Water Entry PressureThe 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

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

1. All parameters are “operational” rather than fundamental

2. Conductivity is less than K found in labs due to trapped air

3. 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-

147Aldabagh and Beer, 1971 TASAE 14:29-31

1. All parameters are “operational” rather than fundamental

2. Conductivity is less than K found in labs due to trapped air

3. 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-

147Aldabagh and Beer, 1971 TASAE 14:29-31

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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

Recall standard falling head result from lab methods:

Remember that Kfs is about 0.5 Ks

2

12

2

h

hLntR

xrK fs