Soil-Water-Content-Measurement-Methods-and-Field-Applications.pptx

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Water ContentMeasurement Methodsand Field Applications

Doug Cobos Ph.D.

Decagon Devices and Washington

State University



Bacground

About the presenter Ph.D. in Soil Physics! "##$! University o%

Minnesota

Director o% &esearch and Development!Decagon Devices! 'nc.

Ad(unct Faculty in )nvironmentalBiophysics! Washington State University

*ead )ngineer on +)CP instrument %or,ASA "##- Phoeni Mars *ander

"



/utline

Direct vs. 'ndirect measurements Water content0 1ravimetric vs. 2olumetric

Water content measurement techni3ues

,eutron probe Dual4needle heat pulse

1ravimetric sampling

Dielectric sensors +ime Domain

Fre3uency Domain

Sensor installation methods

Field applications5eamples



Measurement +echni3ues

Direct measurements )valuate property directly

*ength 6ith calipers

Mass on a balance

'ndirect measurements Measure another property and

relate it to the property o%interest through a calibration )pansion o% li3uid in a tube to

determine temperature



De7nition0 2olumetric Water Content

Separate into constituent parts

θ is volumetric 6atercontent 82WC9!

26 is the volume o%

6ater

2 + is total sample

volume

#.:# m$

#.$: m$

#.;: m$

Air

Water

Soil

#.$: m$5m$

or$:< 2WC

θ =V wV T



De7nition0 1ravimetric 6ater content

1ravimetric6ater content8w)

m = mass

6 = 6ater d = dry solids

w=

mw

md



2olumetric vs. 1ravimetric WaterContent

Two important notes:1. In situ 7eld measurement methods can only

measure volumetric 6ater content

". >ou must tae soil cores o% no6n volume inthe 7eld to measure 2WC %rom gravimetric

method

2olumetricWaterContent

82WC9 Water

volume perunit totalvolume

1ravimetricWaterContent

81WC9 Water6eight perunit dry soil6eight

Soil bul Density! ρ b

ρ b=

md

V T



Sample soil8seal in bag9

Weigh onprecision

balance 86et6eight9

Dry ? ;#:o C%or "@ h

Weigh onprecision

balance 8Dry6t9

Direct Water Content0 1ravimetric 8w9 +echni3ue

1enerate volumetric 6ater content Same as gravimetric ecept soil is sampled 6ith no6n volume

Calibration instructions0

666.decagon.com5appnotes5Calibrating)C"/SoilMoistureProbes.pd%



Direct Water Content Measurements

Advantages Simple

Direct measurement

Can be inepensive

Disadvantages Destructive

does not account %or temporal variability +ime consuming

&e3uires precision balance oven



'nstruments %or Measuring in situ WaterContent 8indirect9

,eutron thermaliation ,eutron probes

Dual needle heat pulse probe

Dielectric measurement Capacitance5Fre3uency Domain

&eectometery 8FD&9

+ime Domain &eectometry 8+D&9



,eutron +hermaliationProbe0 o6 +hey Wor

&adioactive source igh4energy epithermal neutrons

&eleases neutrons into soil 'nteract 6ith atoms in the soil

slo6ing them do6n /ther common atoms

Absorb little energy %rom neutrons

*o64energy detector Slo6ed neutrons collected

Ethermal neutrons

+hermal neutrons directly related to atoms! 6atercontent



,eutron +hermaliation Probe0'nstallation and Calibration

'nstallation Auger installation

hole Manual auger or

1iddings probe 'nstall access tube

Calibrate sensor 1ravimetric

method 6ith coreso% no6n volume

Singlerepresentative site

Data courtesy o% Scott Stanislav! *eo &ivera andCristine Morgan! +eas AM University



eu ron ro eMeasurements

Measurements Uncap hole *o6er probe to speci7c depth

+ae reading at each depth ;@ s to " min per reading *onger read times give better

accuracy



eu ron erma a onProbe

Advantages *arge measurement

volume;# to "# cm radius!

depending on 6atercontent

1ets a6ay %rom issues6ith spatial variability

Single instrument can

measure multiple sites 'nsensitive to salinity!

temperature

Disadvantages,o continuous record

&e3uires radiationcerti7cation to use

)pensive

eavy



Dual ,eedle eat Pulse 8D,P9 +echni3ue

+heory Changes in heat capacity o% soil is linear %unction o% 6ater

content

Create calibration that relates 2WC to heat capacity

Measurement Use dual needle probe

/ne needle contains a heater! the other a temperaturemeasuring device

eat one needle and record temperature over time on

the other Use maimum temperature rise 8delta +9 to calculate

heat capacity and convert to 2WC



Dual ,eedle eat Pulse +echni3ue

'nstallation Push sensor into soil

Make sure needs do not bend during insertion

Connect to datalogger 6ith precision

temperature and data analysis5manipulationcapabilities

D l , dl t P l



Dual ,eedle eat Pulse +echni3ue

Advantages

Smallmeasurementvolume

Most location4speci7c methodavailable

Can measure

6ater contentaround gro6ingseed

Disadvantages

&e3uires datalogger6ith precisetemperature

measurement andanalysis

Can be susceptible totemperaturegradients in soil

timedepth

'ntegrates small soilvolume

Fragile >oung et at. 8"##-9 Correcting Dual4Probe eat4Pulse &eadings %orChanges in Ambient +emperature! 2adose Gone Hournal I0""4$#



Dielectric +heory0 o6 it 6ors

'n a heterogeneousmedium0 2olume %raction o% any

constituent is related to thetotal dielectric permittivity

Changing any constituentvolume changes the totaldielectric

Because o% its high dielectric

permittivity! changes in6ater volume have the mostsigni7cant eJect on the totaldielectric

Material DielectricPermittivity

Air ;

Soil Minerals $ 4 I

/rganic Matter " 4 :

'ce :

Water -#



Dielectric Miing Model0 F>'

+he total dielectric o% soil is made upo% the dielectric o% each individualconstituent

+he volume %ractions! V x , are 6eighting%actors that add to unity

Where ε is dielectric permittivity! b is a constant around#.:! and subscripts t, m, a, om, i, and w represent total!mineral soil! air! organic matter! ice! and 6ater.

i

b

iom

b

om

b

wa

b

am

b

m

bV V V V

t

ε ε θ ε ε ε ε ++++=

2 l t i W t C t t d



2olumetric Water Content andDielectric Permittivity

&earranging the e3uation sho6s 6atercontent! θ ! is directly related to the totaldielectric by

+ae home points 'deally! 6ater content is a simple 7rst4order %unction

o% dielectric permittivity +here%ore! instruments that measure dielectric

permittivity o% media can be calibrated to read 6atercontent

5.0

5.05.05.05.05.0

5.0

)(1

w

iiomomaamm

w

V V V V

t

ε

ε ε ε ε ε

ε

θ +++

−=

+i D i



+ime Domain&eectometry

+i D i



+ime Domain&eectometry

Measures apparent length 8*a9 o% probe %roman )M 6ave propagated along metallic rods

*a is related to ε and there%ore θ



+ime Domain &eectometery

AdvantagesCalibration is

relatively insensitiveto tetural diJerence

/utput 6ave provideselectrical conductivityin%ormation

1ood accuracy

'nsensitive to salinitychanges 6hen )C islo6 to moderate.

Disadvantages)pensive

Does not 6or athigh )C 8trace 6illatten9

&e3uires 6ave%orm

analysis 8comes 6ithmost pacages9

Sensitive to gaps insoil contact



+ime Domain +ransmission Sensors

2ariation o% time domainreectometry *oo at transmission o% 6ave around

loop instead o% reection Utilie %ast response circuitry to

digitie 6ave%orm using onboard

sensor /utput dielectric permittivity



+ime Domain +ransmission

Advantages

*o6er sensitivity totemperature

variation *ittle salinity aJect

at lo6 to mediumelectricalconductivity 8)C9levels

*o6er cost

Disadvantages

Small volume o%

inuence *imited 7eld in the

soil

Cannot be installed

into undisturbed soil *ose signal at high

soil )Cs

Di l i '



Dielectric 'nstruments0Capacitor5FD& Sensor Basics

Sensor probes %orm a large capacitor Steel needles or copper traces in circuit

board are capacitor plates

Surrounding medium is dielectricmaterial

)lectromagnetic 8)M9 7eld is producedbet6een the positive and negative plates



+ypical Capacitor

Capacitor

Electromagnetic

Field

Dielectric

Material

Positive Plate egative Plate



)ample0 o6 Capacitance Sensors Function

)MField

Sensor 8Side 2ie69

# cm

; cm

" cm



1etting to Water Content

Charging o% capacitordirectly related todielectric

Sensor circuitry converts

capacitor charge to anoutput o% voltage orcurrent

Sensor output is calibrated

to 6ater content using thedirect volumetric 6atercontent method discussedearlier



Capacitance5FD&

Advantages *o6er cost

&e3uire simple readoutdevice

)asy to install5use Best resolution to

changes in 6ater contento% any method &esolve changes o%

#.####; m$ m4$

*o6 Po6er

Disadvantages Some probes are

sensitive to soil tetureand temperatureuctuationsDepends on probe

measurement %re3uency

Some re3uire do6n4holeinstallation

Sensitive to air gaps insoil contact



Sensor 'nstallation

+hree types o% instruments Access tube

Permanent installation

EPush4in and &ead

Access +ube Auger hole to installation depth

'nsert access tube sleeve into hole Air gaps MUS+ be minimied during installation o%

sleeve

'nstall dielectric probe in sleeve and seal /& lo6erdielectric probe into sleeve at depths o% interest



Permanent 'nstallation

Many techni3ues %orsensors installation

;. +rench 6all

". : cm diameter auger

hole0 bottom$. ;# cm diameter

auger hole0 side 6all

@. @:o angled : cmauger hole0 bottom

Sensor insertion.Sensor 6idth must be

vertical not horiontal

@

$";

'nstall video0 666.decagon.com5videos



Sensor 'nstallation

EPush4in and &ead Sensors Purpose

Spot measurements o% 2WC

Many measurements over large area ,o need %or data on changes in 2WC over

time

+echni3ue

Push probe into soil )nsure ade3uate soil to probe contact

+ae reading %rom on4board display

Kuestion0 What +echni3ue



Kuestion0 What +echni3ueis Best %or My &esearchL

Ans6er0 't depends on 6hat you 6ant. )very techni3ue has advantages and disadvantages All techni3ues 6ill give you some in%ormation about

6ater content

So 6hat are the important considerationsL

)perimental needs Current inventory o% e3uipment Budget &e3uired accuracy5precision Manpo6er available to 6or Certi7cation

+echni3ues to Field



+echni3ues to FieldMeasurement

Case ;0 'rrigation scheduling5monitoring Details

"# sites! measurements %rom .": m to " m Spread over 7eld system Continuous data collection is desirable Money available %or instrumentation )ventually moving to controlling irrigation 6ater

Choice Capacitance sensors

1ood accuracy 'nepensive )asy to deploy into undisturbed soil &adio telemetry available to simpli%y data collection

+echni3ues to Field




Case "0 Plot monitoring Details

"# measurement locations! @ m spacing

2WC measurements at several depths in each location

Measurements re3uired at least daily *abor available to collect data

*imited budget

Decision ,eutron probe

Accurate Cost is price o% instrument

Measures at multiple depths in access tube

&eliable

+echni3ues to Field




Case $0 1eostatistical survey o%catchment 6ater content Details

Point measurement o% 6ater content at

statistically signi7cant intervals across acatchment *o6 budget *abor available to tae measurements Spatial variability ey to analysis

Decision Single EPush4in and &ead capacitance

instrument *o6 cost! easy to use ,o installation necessary

Standard calibration available



Conclusion

Many choices %or 7eld 6ater contentmeasurement

Several things must be considered to getthe right system

Many resources available to mae decisions Manu%acturerNs 6ebsites

*istservs51oogle 1roup

http055666.so6acs.com AgSciences 1oogle 1roup0 send email to

agscience?googlegroups.com

Application scientists

http://www.sowacs.com/

mailto:[email protected]

mailto:[email protected]

http://www.sowacs.com/



+echni3ue is BestL



+echni3ue is BestLComparison Chart

,eutron Probe +D& +D+ Capacitance

Data *oggingL ,one Specialiedreader

Digitalcommunication

Standard datalogger

Calibration &e3uired %orbest accuracy

&e3uired %orbest accuracy



Accuracy 54 #.#$ m$

m4$

'ncreases 6ithcalib.

54 #.#$ m$

m4$


54 #.#$ m$

m4$


54 #.#$ m$

m4$


+emperatureSensitivity

'nsensitive Soildependent!

can be

signi7cant

Soildependant!

can be

signi7cant

Soil5sensordependent!

can be

signi7cantSalinity

Sensitivity'nsensitive *o6 levels0

lo6R ighlevels0 Fails

*o6 levels0lo6R igh

levels0 Fails

*o6 levels0lo6R igh

levels0 lo6 tohigh! probe

speci7c

Soil-Water-Content-Measurement-Methods-and-Field-Applications.pptx

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