Time Domain Reflectometry Soil Moisture Measurement for

Hydrological Modelling Sustainable Urban Water Workshop

June 4, 2015

Xinbao Yu, Ph.D., P.E., Assistant Professor University of Texas at Arlington Arlington, TX, xinbao@uta.edu

TDR History

• Electrical Engineering (cable tester) Detect cable breaks • Agriculture and Hydrology Volumetric moisture content Topp- father of TDR • Geotechnical Engineering

Purdue University Research Group Water content and density, ASTM standard

• Various Area of Civil Engineering

Current TDR Research at UTA

Thermo-TDR • Soil moisture, dry density,

thermal properties

Suction-TDR • Soil moisture and suction

TDR Basics

A TDR System

RG58

TDR Instruments

• Different TDR devices

(a) 1502C (Tektronix Inc., Beaverton, OR); (b) TRIME-FM (IMKO, Ettlingen, Germany); (c) TRASE System I (Soil Moisture Equipment Corp., Goleta, CA); (d) TDR100 (Campbell Scientific Inc., Logan, UT)

TDR Instruments (cont.)

• TDR probes

Different type of TDR probes Design of your own probes for your research needs

A Field Monitoring System

• Data flow

What TDR measures?

-1.25

-0.75

-0.25

0.25

0.75

1.25

0 1 2 3 4 5 6 7 8

Scaled Distance (m)

Rel

ativ

e V

olta

ge (V

)

−= 11

f

sb V

VC

EC2

=

p

aa L

LK

Apparent Length, La

-1.25

-0.75

-0.25

0.25

0.75

1.25

0 1 2 3 4 5 6 7 8

Scaled Distance (m)

Rel

ativ

e V

olta

ge (V

)

−= 11

f

sb V

VC

EC2

=

p

aa L

LK

-1.25

-0.75

-0.25

0.25

0.75

1.25

0 1 2 3 4 5 6 7 8

Scaled Distance (m)

Rel

ativ

e V

olta

ge (V

)

-1.25

-0.75

-0.25

0.25

0.75

1.25

0 1 2 3 4 5 6 7 8

Scaled Distance (m)

Rel

ativ

e V

olta

ge (V

)

−= 11

f

sb V

VC

EC2

=

p

aa L

LK

Apparent Length, La

Vf

Vs/2

o

Kεε

=Note: dielectric constant

•(Apparent) Dielectric constant: Ka •Electrical conductivity: ECb

permittivity

Data Acquisition

•PMTDR

Signal Analyses

•Detection of reflections

Topp et al. (1982)

Baker and Allmaras (1990)

APPLICATION EXAMPLES

•Measurement of Moisture Content and Density •Infiltration Monitoring

Moisture Content and Density

• Earthwork control • Pavement

Current Methods

• Water Content – Oven Dry (ASTM D 2216) – Microwave (ASTM D 4643) – Nuclear Method (D 3017) – Calcium Carbide Gas Pressure (ASTM D 4944)

• Density

– Nuclear Method (ASTM D 2922) – Sand Cone (ASTM D 1556) – Rubber Balloon (ASTM D 2167) – Drive Cylinder ( ASTM D2937)

Nuclear Method – the State of Art

Different Modes of Nuclear Gauge (from Hausmaan, 1990)

(a) Backscatter (b) Direct Transmission

Advantages: Easy to use and accurate for most situations Disadvantages: Regulation, nuclear radiation; biased by hydrogen and certain types of ions; different sampling zone

-1.25

-0.75

-0.25

0.25

0.75

1.25

0 1 2 3 4 5 6 7 8Scaled Distance (m)

w=2.1% r d=1648 kg/m3

w=5.2% r d=1655 kg/m3

w=9.3% r d=1658 kg/m3

w=14.0% r d=1653 kg/m3

(a)

Rel

ativ

e V

olta

ge (V

)

Incr

easi

ng E

Cb Increasing Ka

Information from TDR Signal

Constant dry density; changing water content

-1.25

-0.75

-0.25

0.25

0.75

1.25

0 1 2 3 4 5 6 7 8

Scaled Distance (m)

w=10.7% r d=1256 kg/m3

w=10.7% r d=1490 kg/m3

w=10.6% r d=1583 kg/m3

w=10.4% r d=1871 kg/m3

(b)

Rel

ativ

e V

olta

ge (V

)Information in TDR Signal

Incr

easi

ng E

Cb Increasing Ka

Constant water content ; changing dry density

Soil Dielectric Constant, Ka

• Soil dielectric constant, Ka • Predominantly decided by water content • Topp’s equation relates Ka to volumetric water content (widely

applied in the current practice)

Soil Solids

Air

Water

222436 103.51092.2105.5103.4 −−−− ×−×+×−×= aaa KKKθ

( )ββ

βββ

θairasa

airasaa

KKnKKnK

,,

,,1−

−−−=

Utilizing Electrical Conductivity

Analogy Ka=81

Ka=1

Ka=3-5 ECw

ECa=0

ECs

bwaKd

wa +=ρρ

Dielectric Constant

dwcECd

wb +=ρρ

Electrical Conductivity

Soil Solids

Air Water Soil

Phases

0

0.05

0.1

0.15

0.2

0.25

0.3

0 2 4 6 8 10 12 14 16

Location No.

Wat

er C

onte

nt

TDROven DryNuclear

Glacial Till, INDOT Division of Research Testing Pavement

One-Step TDR Method versus Nuclear Method

Water Content Comparisons

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

Oven Dry Gravimetric Water Content

1:1 Line1:1 Line +1%1:1 Line -1%One-Step Method

Gra

vim

etric

Wat

er C

onte

nt b

y TD

R

Density Comparisons

800

1000

1200

1400

1600

1800

2000

2200

800 1000 1200 1400 1600 1800 2000 2200

Dry Density by Total Density and Oven Dry Water Content (kg/m3)

One-Step Method1:1 Line1:1 +3% Line1:1 -3% Line

Dry

Den

sity

by

One

-Ste

p M

etho

d (k

g/m

3 )

Rainfall Infiltration Monitoring

Flexible Flat Strip TDR Sensor

23

Coiled TDR strip Sensor Installation TDR strip Sensor in the Field

Cross Section View Sampling Area

Sensor Calibration

Photo of calibration experiment for the TDR strip sensor

Seepage Monitoring

Photo of the sand tank seepage monitoring Measured TDR signals during the seepage experiment

Infiltration Monitoring

Test set up for monitoring infiltration process

Measured TDR signals during the precipitation test

Conclusions

• TDR can accurately measure soil moisture content and dry density.

• TDR can monitor rainfall infiltration. • TDR probe sensor and flat strip sensor

will be used for soil moisture and infiltration monitoring.

27

Thank you for listening!