ms 03 soil water content

7/30/2019 MS 03 Soil Water Content

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ETSEA

Soil Water Content

Jorge Lampurlans ([email protected])Department of Agricultural and Forestry Engineering

ETSEA

Soil Water Content Measurement

Thermogravimetric Method Neutron ProbeTime Domain Reflectometry (TDR) Capacitance Method

Frequency Domain Reflectometry (FDR) Ground Penetrating Radar (GPR) From soil water potential

ETSEAThe Water in Soil

Soil Water Content:

Gravimetric basis: [g water/g water]

Volumetric basis: [m3 water/ m3 water]

Related by bulk density: [g soil/ m3 soil]

=

On the soil profile: W [m3 water/ m2 water]

W = i zi (zi, soil depth in mm)

3Soil Water Content Department of Agricultural and Forestry Engineering

ETSEA


The Water in Soil


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ETSEA

Lysmeter


The Water in SoilETSEA

Lysimeter


The Water in Soil

ETSEA

Lysimeter


The Water in SoilETSEA

Soil Water Balance

Equations

Complete: ET = Wi + [P + I +Re] - [Rl + D] - Wf Simplified: ET = W i + P - Wf

I+Re RlP

W i W f

D

ET


The Water in Soil

ET: EvatopranpirationW: Soil Water Content (inicial, final)

P: PrecipitationI: IrrigationR: Surface Runoff (entering, leaving)


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ETSEAThermogravimetric Method

PRINCIPLE

Soil water content definition: the water that may beevaporated from a soil by heating between 100 y 110 C(105 C) until there is no further weight loss.


ETSEAThermogravimetric Method MATERIAL & METHODS

Soil sampling (50 to 100 g of soil): auger (hand operated)


Edelman Riverside Stony soil


Soil sampling (50 to 100 g of soil): auger (soil column cylinder auger)



Soil sampling (50 to 100 g of soil): auger (soil column cylinder auger)



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ETSEAThermogravimetricThermogravimetric MethodMethod MATERIAL & METHODS

Soil sampling (50 to 100 g of soil): auger (hydraulic drive)

hermetic containers (plastic pots)



Soil water evaporation at 105 C until constant weight(weight change in 0,5 to 4 hours < 0,1% of the initialweight): 24 to 48 hours. Forced air oven Laboratory balance


ETSEA

WARNINGSAt 110 C not all water removed

T > 110 C organic mater oxidation

T > 80 C gypsium hydratation water lost

CALCULATIONS = (MwetMdry)/Mdry

STONY SOILS: = (


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ETSEANeutron Probe PRINCIPLE

H slows down fast neutrons (thermalization)

Most H in soil is in soil water

Changes in soil H Changes in water content

[Thermalized Neutrons] = f(t)


ETSEANeutron Probe PRINCIPLE

Calibration: = R/R s a + b

: Volumetric water contentR: Neutron count rate in soilRs: Count rate in a standard medium (Water)R/Rs: Count-rate ratioa y b: Calibration constants


ETSEANeutron Probe MATERIAL & METHODS

Access Tubes Aluminium, brass, stainless steel, plastic.

Transparency to neutrons (not PVC).

Mechanical strength.

Resistance to corrosion.

Stoppers to seal the top and end pieces to close the botton.

Access Tube Installation Minimize dirturbance to the soil, soil surface and vegetation to obtain

truly representative measurements of the area

Acces tube fit tightly into the soil to prevent voids and channeling ofwater dow besides it.


ETSEA

MATERIAL & METHODS Neutron probe

Prove (Sourve & Detector)

Prove cable (strong, waterproof,with stoppers)

Stable power supply (1-2 kV)

Counter unit (8 to 64 s)

Prove carrier (limits radiation)


Neutron Probe


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ETSEA

WARNINGS

Sphere of importance Decreases with soil water content

(0.15 m in wet soil0.5 in very dry soil)

Depth increments > 0.1 m

Smoot water content profile

Total profile water content: OK

Errors at the soil surface(< 0.15-0.30 m)


Neutron Probe ETSEAWARNINGS

Random Counting Errors Increase the counting time (64 s)

Weekly standard counts (1 hour, in water)

Need of site-specific calibration Presence of strong neutron absorbers: iron, chlorine

Bulk density increases count rate specially in wet conditions

Marked textural boundaries

Stones

Access tubes


Neutron Probe

ETSEA

FIELD CALIBRACINTake soil samples (water content determination)

during the excavation for the access tube installation. Record neutron counts at the required depths.

Determine volumetric water content of the samplesthermogravimetrically.

Repeat at different access tubes when the soil is atdifferent water contents (drying, irrigation).

Take undisturbed samples of known volume near theaccess tube at the same depths.


Neutron Probe ETSEA FIELD CALIBRATION


Neutron Probe


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ETSEA

Commercial neutron probes: Troxler 3% (Troxler) (Cambell Pacific)


Neutron Probe ETSEAMethods based on K PRINCIPLE

Include de moist soil as part of the dielectric of a capacitor and

measure its capacitance that gives the dielectric constant (K) orrelative permittivity ():

capacitance with soil

capacitance with air

K is frequency (F) dependent: F < 30 MHz: proportion and configuration of air filled pores

F between 30-3.000 MHz K = f(water, soil, air)

(Kwater 80, Ksoil 3-5, Kair= 1) F > 3000 MHz: water dipoles do not follow the electromagnetic fields

K = =


ETSEA

PRINCIPLE


Methods based on K ETSEA METHODS

TDR (Time Domain Reflectometry): measurement of

the travel time of and electromagnetic wide range highfrequency pulse (step voltage) through the soil.

Frequency Domain (FD)fixed low frequency (around 100 MHz) Capacitance: measurement of the capacitance of the soil.

FDR (Frequency Domain Reflectometry): Measurement ofthe complex impedance.

WARNINGS Measurement of free water only


Methods based on K


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ETSEATime Domain Reflectometry (TDR) PRINCIPLE

The propagation velocity of an electromagnetic wave down atransmission line in a nonmagnetic medium (soil) is related

with the dielectric constant (K) or permittivity () of themedium

A high-frequency electromagnetic pulse is fed into the soilbetween two metal rods. Part of the pulse is reflected back upthrough the soil from the bottom of the rods, and the time

interval between the incident and reflected pulses is measured.

If Kv t

K

cv =


t

Lv

2=

(c = 3108 m/s)

ETSEATime Domain Reflectometry (TDR) PRINCIPLE


2

2

=

L

tcK

2

tcLAB =

2

=

L

LK AB

K

cv =

t

Lv

2=

ETSEA

MATERIAL & METHODS Different proves (transmission lines)


Time Domain Reflectometry (TDR) ETSEA MATERIALS & METHODS

Transmission line or wave guide Coaxial Cable 50 (no more than 2 m)

2 to 3 rigid parallel metallic rodsinserted into the soil Rod diameter: 3 - 5 mm.

Rod spacing 1.5 - 10 cm(Rod spacing / Rod diameter < 10).

Rod length: 5 - 50 cm (attenuation).

Vertically or horizontally inserted.

Avoid rod to soil air gaps(rods diameters as large as possible).

Parallelism is important forconductivity measurements.


Time Domain Reflectometry (TDR)


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ETSEA

MATERIAL & METHOS Cable tester (Tectronyc 1502C)

Wave Form Analysis: beginning and end of thetransmission line (manual or automatic)


Time Domain Reflectometry (TDR) ETSEA

WARNINGS Signal attenuation in saline soils by ionic conduction

Insulation of the central rod.

Bulk Electrical Conductivity measurement:



app

f

f

u

s

RZ

KmS

+

=

1

1)/(

0

0

V

VVff

= (final reflection coefficient)

Zu = 50 for Tektronix 1502B cable tester.Rapp measured by sorting the sensor rods.Ks sensor constant obtained by calibration

with saline solutions.(Temperature correction)

ETSEA

WARNINGSThe method gives and average value of water content

over the length of the transmission line.

Volume of influence: cylinder of = rod distance


Time Domain Reflectometry (TDR) ETSEA CALIBRATION

Universal calibration or Topp equation

Works better for sandy soils. Good for relative values. Low bulk densities,

specific mineralogical properties,clays and soil structure modifies K().=> Soil-specific calibration.

Field calibration Only a dry and wet point and adjust

the Topp equation.The complete relationship.

43210)043.05,5292530(

++= KKK




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ETSEA

Commercial equipment TRASE (Soilmoisture)


Time Domain Reflectometry (TDR) ETSEA Commercial equipment

Hydrosense 3% (Campbell)

TDR100 (Campbell) TRIME 3% (SDEC)



ETSEACapacitance Method

PRINCIPLE Measurement of the soil capacitance K

With a bridge at a frequency of 100-150 MHz

With a LC oscillator circuit, the frequency of oscillation is a directmeasure of the capacitance.

MATERIALS & METHODS Access Tube

Plastic (metal are not suitable, it acts as a barrier to the electric field)

The gap between access tube and soil < 0.5 mm

SAKC 108542,8 12=


LCF

2

1=

ETSEA

MATERIALS & METHODS Prove

Parallel electrodes

Cylindrical


Capacitance Method


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ETSEA

WARNINGS Distance between electrodes. Compromise between:

Radius of influence (3-4 cm)

Depth resolution (2 cm)

Measurement dominated by the soil around theelectrodes: small-scales lateral heterogeneity, air gapsand channelling of water may interfere.

Requires soil-specific calibration.


Capacitance Method ETSEAWARNINGS

No linear relationship between K and


Capacitance Method

ETSEAFrequency Domain Reflectometry (FDR) PRINCIPLE

A low frequency (100 MHz) sinusoidal signal is propagatedalong the transmission line to the soil probe and reflects back

(change in K) causing a voltage standing wave to be set up onthe transmission line. The amplitude difference between the beginning and at the end

of the transmission line is empirically related to the complexrelative dielectric constant:

K = K - j K K is a function of . K is related to the bulk electrical conductivity.


ETSEA

ADVANTAGES The sensor is factory calibrated with a reference

impedance. This improves accuracy by eliminating

influences of cable lengths and quality, connectors andswitches. Cheaper sensors. They measure also temperature to make corrections. K more sensitive to changes than K potentially more

accurate than TDR. DISADVANTAGES

Due to the low frequency the relationship between K a is more influenced by the soil calibration


Frequency Domain Reflectometry (FDR)


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ETSEA

SOIL-SPECIFIC CALIBRACIN


Frequency Domain Reflectometry (FDR) ETSEA

Commercial proves: Diviner2000 (Sentek)

Enviroscan (Sentek)



ETSEA

Comercial probes: Thetaprove (Delta-T)

ML2x 1%

SM200 3%

Profile probe (Delta-T)


Frequency Domain Reflectometry (FDR) ETSEA

Commercial probes: ECH2O 3% (Decagon)




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ETSEAGround Penetrating Radar (GPR)

PRINCIPLE

Uses short pulses of high frequency electromagnetic waves (50-1500 MHz)

ADVANTAGES Noninvasive

Fast

Suitable forlarge areas


ETSEAFromFrom SoilSoil WaterWater PotentialPotential

PRINCIPLE

Estimate soil water content () measuring soil water potential(h).

DRAWBACKS: We need to know the (h)

Hysteresis of the (h) curve

We need to measure soil waterpotential


ms 03 soil water content

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