PRESENTED BY

Bhushan N. Patil

M.E. Scholar (DE-E&TC)

A REVIEW OF VARIOUS SOIL MOISTURE

MEASUREMENT TECHNIQUES

S.S.B.T’s COET, Bambhori, Jalgaon

GUIDED BY

Prof. P. H. Zope

Prof. K. S. Patil

CONTENT

1. INTRODUCTION

2. SOIL MOISTURE MEASUREMENT TECHNIQUES

i. Gravimetric Technique

ii. Neutron Scattering

iii. Time Domain Reflectometer (TDR)

iv. Frequency Domain Reflectometer (FDR)

v. Amplitude Domain Reflectometer (ADR)

vi. Phase Transmission

vii. Time Domain Transmission (TDT)

viii. Tensio-meter Method

3. ADVANTAGES & DISADVANTAGES

4. OPERATING RANGE & ACCURACY

5. MEASUREMENT VOLUME & COST

6. CONCLUSION

REFERENCES

1. INTRODUCTION

NEED OF SOIL MOISTURE MEASUREMENT

In agriculture & Plant science field to determine best time to Sow & plow the field.

Various physical & chemical properties of soil changes with amount of moisture

present in soil.

To measure changes in infiltration, irrigation.

To study ground water recharge & Evapo-transpiration.

It is also important in the fields like Hydrology, Forestry, Agrology.

To study & determine the parameters like soil profile, surface tension related with

civil & soil engineering.

2. SOIL MOISTURE MEASUREMENT TECHNIQUES

I. GRAVIMETRIC TECHNIQUE

It is the most oldest, satisfactory & widely used soil moisture measurement

technique. Used for calibration of other techniques.

Principle- Based on calculations of the weighing of the soil sample before & after

the drying

Methodology- Measurement is carried out in following steps :

1) Collecting soil sample- Using Sampling Augers, Sampling Tubes, Core Barrels

or Open-drive Sampler.

2) Weighing original soil sample

3) Drying of soil- Oven drying or Radiation drying

4) Weighing dried sample of soil

5) Calculating amount of moisture content

% Moisture Measurement = * 100

II. NEUTRON SCATTERING

Principle- High energy is lost in Neutron collision with atoms having low atomic

weight like hydrogen atoms. Water content these atoms on very large scale.

Methodology-

1) Uses a long & narrow cylindrical probe containing source & detector for

neutrons.

2) Probe is then inserted in soil & source is then activated

3) The effect of collisions changes a fast neutron to a slow neutron.

4) The number of slow neutrons detected by a counter tube electronically

5) Measuring Gauge is calibrated in terms of moisture content

.

Fig-1: Schematic of Neutron Scattering [18]

III. TIME DOMAIN REFLECTOMETER (TDR)

Principle- Dielectric constant of soil is the function of content of moisture present

in soil. Traveling time of a EM wave changes as velocity of traveling wave is

affected by the dielectric constant of soil.

Ka = (c/v)2 = [(c × t)/(2 × L)]2

Methodology-

1) An open ended sensor transmission waveguide is designed & inserted in soil.

2) A pulse of EM is transmitted through it. A part of signal reflected back which

then observed using Sampling Oscilloscope.

3) Using Time delay between transmitted & reflected signal soil moisture

measurement is carried out.

Fig-2: Operation of TDR measurement [1] Fig-3: TDR system [10]

IV. FREQUENCY DOMAIN REFLECTOMETER (FDR)

Principle- The electrical capacitance of a capacitor that uses the soil as a dielectric

depends on the soil water content. When connecting this capacitor (made of

metal plates or rods imbedded in the soil) together with an oscillator to form an

electronic circuit, changes in the circuit operating frequency detects changes in

soil moisture.

Methodology-

1) Probes are designed using two or more electrodes (i.e., plates, rods, or metal

rings around a cylinder) inserted into the soil.

2) An electrical field is given then the soil around the electrodes serves as the

dielectric of the capacitor that completing the oscillating circuit.

3) By varying the frequency soil moisture is then measured

Fig-4: Capacitance probes and FDR [10]

V. AMPLITUDE DOMAIN REFLECTOMETRY (ADR)

Principle- When an EM wave traveling along a TL reaches a section, part of the

energy reflected back due to impedance mismatch. This reflected wave

interacts with the original incident wave producing a voltage standing wave.

Impedance of soil depends on the content of moisture that changes wave

amplitude along the length of the TL.

Methodology-

1) Sinusoidal EM wave is generated & applied to a coaxial TL that extends into

the soil through an array of parallel metal rods.

2) It’s outer cover forms an electrical shield across the central rod. Rod assembly

acts as an additional section of the TL having impedance depending on the

dielectric constant of the soil between the rods.

3) Depending on the amplitude change in reflected signal moisture content is

determined

Fig-5: ADR sensor probe [10]

VI. PHASE TRANSMISSION

Principle- In travelling sinusoidal wave phase is shifted relatively after traveling

fixed distance with respect to initial phase at the origin.

This phase shift depends on i) length of travel along the TL

ii) frequency and

iii) the velocity of propagation.

As velocity of propagation is related to soil moisture content, so it can be

determined by this phase shift at fixed frequency and length of travel.

Methodology-

1) Two open concentric metal rings are used to design a waveguide probe

2) Phase is then measured at the beginning and ending of the waveguides.

3) At the end soil moisture content can be devaluated easily depending on the

phase shift . [10]

Fig-6: Phase transmission probe and sensor [10]

VII. TIME DOMAIN TRANSMISSION (TDT)

Principle- Measurement of the propagation time over a known distance of the

wave can be used to calculate the dielectric characteristics. To determine water

content in a porous dielectric medium, an EM pulse is guided through the

medium by a transmission line.

Methodology-

1) It is similar to TDR, but requires an electrical connection at the beginning as

well as ending of the TL.

2) The probe has a waveguide design as bent rods of metals which are inserted

into the electronic block on both side.

3) Another way is the sensor consists of a long band (~3 ft), with an electronic

block at both ends.

4) Soil moisture is then easily calculated depending on the travel time of EM wave.

Fig-7: Time domain transmission probe [17]

VIII. TENSIOMETER METHOD

Principle- Based on the property of the absorption i.e. Suction force of water for

soil.

water comes into equilibrium with the soil solution through a permeable &

saturated porous material if a sealed water-filled tube is placed with the soil.

Methodology-

1) The permeable ceramic cup tip is placed with the soil in the root zone of plant.

2) Water moves from the tube into the soil as the soil is normally not saturated,

So that a partial vacuum(pressure) is made and evaluated by the gauge.

3) Pressure varies with amount of water potential thus gauge indicate the

moisture content in the soil.

Fig-8: Tensiometer example [10]

3. ADVANTAGES & DISADVANTAGES

Parameter (I)

GM

(II)

NM

(III)

TDR

(IV)

FDR

(V)

ADR

(VI)

PT

(VII)

TDT

(VIII)

Tensi.

1) Requirement of Specific

Calibration

N N Y Y Y Y Y N

2) Affected by Soil Salinity & air

gaps

N Y N/Y Y Y Y Y N

3) Measurement at different depths Y Y N Y N N N N

4) Connection with Data Logger N N Y Y Y Y Y N

5) Time efficient N N Y Y Y Y Y N

6) Permanent Installation N Y Y Y Y Y Y N

7) Safety Y N Y Y Y Y Y Y

8) Automation N N Y Y Y Y Y N

4. OPRATING RANGE AND ACCURACY

Technique Operating Range

(ft3 per ft3)

Accuracy

(ft3 per ft3)

Neutron Moderation 0 to 0.6 ± 0.005

TDR 0.05 to saturation ± 0.01

FDR 0 to saturation ± 0.01

ADR 0 to saturation ± 0.01 to 0.05

PT 0.05 to 0.5 ± 0.01

TDT 0 to 0.7 ± 0.05

Tensiometer 0-0.80 bar ±0.01 bar

5. MEASUREMENT VOLUME AND COST

Technique Measurement volume Cost

Neutron Moderation Sphere(radius 6-16

inches)

$10,000-15,000

TDR About 1.2 inches radius

around waveguide

$400-23,000

FDR Sphere(radius 1.6 inches) $100-3,500

ADR Cylinder (radius 1.2

inches)

$500-700

PT Cylinder $200-400

TDT Cylinder (radius 2 inches) $400-1,300

Tensiometer Sphere (Greater than 4

inch radius)

$75-250

6. CONCLUSION

It is concluded that each technology has some limitations, so it is said that the

ideal method for measurement of soil moisture under field conditions has yet to

be perfected.

Individual requirements should be indentified sufficiently before adopting a soil

moisture monitoring methodology.

TDR method can be most superior for soil moisture measurement if it’s cost is

reduce and if limitation over high salinity operation is removed.

FUTURE SCOPE :

Limitation over high salinity operation is removed by use of Polyolefin coated

TDR probes.

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