In this 20-minute webinar, Dr. Colin Campbell demystifies the differences between soil water content measurement methods. He explores the scientific measurement theory and the pros and cons of each method. He also explains which technology might apply to different types of field research, and why modern sensing is about more than just the sensor. Learn: • Measurement theory behind the gravimetric method, capacitance, time-domain reflectometry (TDR), time-domain transmission (TDT), frequency-domain reflectometry (FDR), resistance sensors, and more • Which technology applies to different field situations • What factors matter when choosing a sensor type • Why some methods are not research-grade • How modern sensing is about more than just the sensor • How to determine a good price-to-performance ratio for your unique application

2. SOIL MOISTURE 102
IN SITU MEASUREMENT
TECHNIQUES
Dr. Colin Campbell
Senior Research Scientist
METER Group, Inc. USA

3. INTRODUCTION
ABOUT ME
● Background in Soil and Environmental Physics
● 19 years in instrumentation development at METER Group
● Teach Environmental Biophysics at Washington State University
● 1000s of hours installing sensors and monitoring, interpreting,and
publishing soil moisture data

4. ELECTRONIC WATER CONTENT
FIRST MEASUREMENTS IN THE FIELD
Photo courtesy of CSIRO

5. GOOGLE SEARCH
SOIL MOISTURE SENSOR

6. SOIL MOISTURE
AMBIGUOUS PHRASE
Water Content: Amount Water (Matric) Potential: Energy
• q is volumetric water
content (VWC)
• Vw is the volume of
water
• VT is total sample
volume
q =
Vw
VT
• Sum of matric forces (adhesive
and cohesive, Ym) and others
(osmotic, gravitational, and
pressure)
Soil
Minerals
~55%
Pore Water
~42%
Air ~3%
Soil
Minerals
~55%
Pore Water
~10%
Air
~35%
Soil
Minerals
~55%
Pore Water
~1%
Air
~44%

7. CONSIDER THE APPLICATION
Field, catchment, beyond
• Satellite
• Cosmic Ray
Site scale (sub-field, treatment, plot)
• Resistance
• Dielectric permittivity
• Thermal conductivity
• Thermalized Neutron
0.1 – 0.7 m
10s meters to km

8. IN SITU SENSORS
RESISTANCE

9. IN SITU SENSORS
RESISTANCE
Overview:
• Incredibly cheap
• Reacts to changes in water content
• Simple to integrate into project
Uses:
• Home/science fair projects
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07
VolumetricWaterContent(m3/m3)
Electrical Conductivity (Bulk dS/m)
0.12 dS/m Saturation Extract
0.17 dS/m Saturation Extract
0.23 dS/m Saturation Extract
0.46 dS/m Saturation Extract

10. IN SITU SENSORS
DIELECTRIC PERMITTIVITY

11. IN SITU SENSORS
DIELECTRIC PERMITTIVITY
Dielectric constant: Ability to store charge
1 5 20 40 80
Soil Minerals
Organic
Matter
Air
Ice
Water

12. IN SITU SENSORS
DIELECTRIC PERMITTIVITY
1 5 20 40
Soil Minerals
~55%
Pore Water
~1%
Air ~44%
Soil
Minerals
~55%
Pore Water
~10%
Air ~35%
Soil
Minerals
~55%
Pore Water
~42%
Air ~3%
Water
~100%
80

13. IN SITU SENSORS
DIELECTRIC PERMITTIVITY
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0 5 10 15 20 25 30 35
VolumetricWaterContent(m3/m3)
Dielectric
1.5 g/m3 Bulk Density
1.0 g/m3 Bulk Density

14. High Performance
IN SITU SENSORS
DIELECTRIC PERMITTIVITY
Sensor Measurement Frequency
kHz MHz GHz
Salt Sensitivity Cost

15. TECHNOLOGY
DIELECTRIC MEASUREMENT
Capacitance
• Sensor uses soil as capacitor element with charge storage calibrated to water content
Time Domain Reflectometry
• Travel time of reflected wave of electrical energy along a transmission line determined charge
storage of soil and calibrated to water content
Frequency Domain Reflectometry
• Sensor uses soil as a capacitor where maximum resonance (resonant frequency) is calibrated to
charge storage and water content

16. SYSTEM VIEW
INSTALLATION

17. SYSTEM VIEW
INSTALLATION

18. SYSTEM VIEW
INSTALLATION

19. SYSTEM VIEW
DEPLOYMENT &
VISUALIZATION

20. IN SITU SENSORS
FIELD PERFORMANCE

21. IN SITU SENSORS
FIELD PERFORMANCE

22. IN SITU SENSORS
FIELD PERFORMANCE

23. IN SITU SENSORS
FIELD PERFORMANCE

24. IN SITU SENSORS
FIELD PERFORMANCE

25. SUMMARY
Overwhelming number of soil moisture measurement options
Generally only two options for field measurements
1. Resistance-based sensors
• Measure resistance or electrical conductivity of soil
• Shows a relationship to water content but swamped by electrical conductivity
• Cheap, wildly inaccurate, unreliable
2. Dielectric-based sensors
• Measure charge storing capacity of soil (pore water)
• Use high-frequency signal to polarize water
• Higher frequencies perform much better
• Higher frequencies increase sensor cost
System around sensors essential to successful outcomes
Installation, system deployment, and data visualization are critical
considerations