Plant and Soil
Water Potential
INTERNATIONAL
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Research Background
• Hawkesbury Forest Experiment
• University of Western Sydney,
Richmond, NSW
• Whole Tree Chambers
• CO2 x Drought Experiment
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Research Background
• Ambient CO2: 380ppm
• Elevated CO2: Am + 300ppm
• Irrigated: 10mm every 3 days
• Drought: no water
• Control treatment
• My Job: The Drought
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Quantifying Drought
• Rainfall
• Soil Water Content
– Volumetric
– Gravimetric
• Soil Water Availability
– Dependent on Soil Texture
– Soil Water Potential
Image: Craig Barton
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Definition
• Water Potential:
– Free energy associated with water
– Free energy is the potential to do work
– Water moves from higher to lower potentials
– Water moves from mountains to ocean
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Definition
• Water Potential Units of Measurement:
J/kg
MPa
Bars
RH
-1
-0.001
-0.01
0.999993
-10
-0.01
-0.1
0.999926
-33
-0.033
-0.33
0.999756
-100
-0.1
-1
0.999261
-1000
-1
-10
0.992638
-1500
-1.5
-15
0.988977
-10000
-10
-100
0.928772
Air Dry
-100000
-100
-1000
0.477632
Oven Dry
-1000000
-1000
-10000
0.000618
High Potential
Field Capacity
Wilting Point
Low Potential
Source: Decagon Inc.
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Soil-Plant-Atmosphere
Continuum
Low Potential
Atmosphere = -100 Bar
Leaf air = -7.0 Bar
Leaf cell = -1.0 Bar
Trunk = -0.8 Bar
Soil = -0.33 Bar
High Potential
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Quantifying Drought - Atmosphere
• Weather Station:
– Vapour Pressure Deficit
• Armidale in Summer approx. 3 kPa
• Sydney in Summer approx. 6 kPa
• Alice Springs in Summer approx. 10 kPa
– Potential Evaportranspiration
• Penman-Montieth Equation
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Quantifying Drought - Soils
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Source: Decagon Inc.
Quantifying Drought - Soils
•
Thermocouple inside ceramic
•
Heat pulse sent through sensor
•
Temperature rise measured
•
Dry soil = large increase
•
Wet soil = small increase
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Quantifying Drought - Soils
• Thermal Matric Potential
Sensor:
– Advantages:
• Highly accurate
• Large measurement range
• Logging capability
– Disadvantages:
• Calibration required
• Careful installation
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Quantifying Drought - Soils
• Moisture Release Curves:
– Quantifying soil moisture content
Against soil water potential for
Varying Soil Textures
– Standard practice to measure
Between Field Capacity and
Graph Source:
Remko Duursma, UWS
Plant Wilting Point: -0.33 to -15 Bar
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Graph Source:
Remko Duursma, UWS
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
• Moisture Release Curves:
– Advantages:
• Relatively cheap
• Established methodology
– Disadvantages:
• Time consuming (months)
• Soils highly variable
• Many curves needed
• Reliance on extrapolation
Image: Craig Barton
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Quantifying Drought - Soils
• Moisture Release Curves:
– Advantages:
• Cheap (per sample)
• Fast response time (one day)
• Many curves can be constructed
• Larger portion of curve (-1000MPa)
– Disadvantages:
• Poor resolution at wet end of curve
• (Pressure plate can resolve wet end)
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Soil-Plant-Atmosphere
Continuum
Low Potential
Atmosphere = -100 Bar
Leaf air = -7.0 Bar
?
Leaf cell = -1.0 Bar
Trunk = -0.8 Bar
Soil = -0.33 Bar
High Potential
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Quantifying Drought - Plants
• Leaf Water Potential with WP4-T:
– Advantages:
• True leaf water potential
– Disadvantages:
• Tricky methodology
• Slow response time (30 minutes)
• Rarely done
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Quantifying Drought - Plants
• Leaf Water Potential with Pressure Bomb:
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Quantifying Drought - Plants
• Leaf Water Potential with Pressure Bomb:
– Advantages:
• Proven, accurate technique
• Widely used, easy to do
• Data is potentially powerful
– Disadvantages:
• Spot measurement, destructive
• Slow sampling (for large sample size)
• Xylem water potential
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Predawn Leaf Water Potential
Ambient CO2 Results Only:
Pre-Dawn Leaf Water Potential
(MPa)
0
-0.2
-0.4
-0.6
-0.8
Irrigated
Drought
Graph Source:
David Ellsworth, UWS
-1
June
October
Nov
Dec
Jan
late Jan
Month
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Midday Leaf Water Potential
Graph Source:
David Ellsworth, UWS
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Unexpected Results
Ambient
Drought
Elevated
Drought
Graph Source:
Remko Duursma, UWS
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Combining Instruments
Leaf Water Potential + Neutron Probe Soil Moisture
Graph Source:
Remko Duursma, UWS
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Combining Instruments
Leaf Water Potential + Neutron Probe Soil Moisture
+ Soil Water Potential =
COMPLETE STORY
Elevated
Drought
Ambient
Drought
Graph Source:
Remko Duursma, UWS
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Diurnal Leaf Water Potential
Time of Day
Hydraulic Architecture?
Leaf Water Potential (MPa)
8p
m
7p
m
5p
m
3p
m
1p
m
11
am
9a
m
7a
m
What regulates Plant Water
Status?
Stomatal Closure?
5a
m
0
-0.5
-1
-1.5
-2
Solutions for soil, plant & meteorology
Ambient
Graph Source:
Michael Forster, UWS; Derek Eamus, UTS
Elevated
www.ictinternational.com.au
International
-2.5
Combining Instruments
Leaf Water Potential + Sap Flow (Tree Water Use)
Graph Source:
Michael Forster, UWS; Derek Eamus, UTS
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Combining Instruments
Time of Day
Leaf Water Potential (MPa)
8p
m
7p
m
5p
m
3p
m
1p
m
11
am
9a
m
7a
m
5a
m
0
-0.5
Graph Source:
Michael Forster, UWS; Derek Eamus, UTS
-1
-1.5
-2
Ambient
Elevated
-2.5
Slope of lines is similar
No difference in soil to leaf
hydraulic pathway
Plant water status under
control of stomata
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Combining Instruments
Leaf Water Potential + Stomatal Conductance (Porometer)
Graph Source:
David Ellsworth, UWS;
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Pressure Bomb Data
Time of Day
0
Leaf Water Potential (MPa)
m
5a
m
7a
m
9a
am
1
1
m
1p
m
3p
m
5p
m
7p
m
8p
-0.5
-1
-1.5
-2
Ambient
Elevated
-2.5
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Stem Psychrometer Data
Solutions for soil, plant & meteorology
Graph Source:
Alec Downey, ICT International
International
www.ictinternational.com.au
PSY-1 New Instrument
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Psychrometer/Hygrometer
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Psychrometer/Hygrometer
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Definition
• Water Potential Units of Measurement:
J/kg
MPa
Bars
RH
-1
-0.001
-0.01
0.999993
-10
-0.01
-0.1
0.999926
-33
-0.033
-0.33
0.999756
-100
-0.1
-1
0.999261
-1000
-1
-10
0.992638
-1500
-1.5
-15
0.988977
-10000
-10
-100
0.928772
Air Dry
-100000
-100
-1000
0.477632
Oven Dry
-1000000
-1000
-10000
0.000618
High Potential
Field Capacity
Wilting Point
Low Potential
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
A New Generation
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
A New Generation - Psychrometer
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
The Ideal System
Atmosphere
Weather Station
WP4-C
Porometer
Leaves
Pressure Bomb
Psychrometer
Sap Flow
Thermal Matric Potential Sensor
WP4-C
Soil Moisture Sensors
Stem / Trunk
Roots
Soil
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Continuous Logging Options
Atmosphere
Weather Station
Leaves
Psychrometer
Sap Flow
Thermal Matric Potential Sensor
Soil Moisture Sensors
Stem / Trunk
Roots
Soil
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
Acknowledgements
Thank you to David Ellsworth, David Tissue, Craig Barton,
Derek Eamus and Remko Duursma for kind permission to use data.
The Hawkesbury Forest Experiment involved researchers from:
University of Western Sydney, University of Technology, Sydney,
University of New South Wales, Macquarie University,
Industry & Investment, NSW, and Swedish University of Agricultural Sciences
All researchers involved with the HFE acknowledge the provision
of funding by the Australian Government's Department of Climate Change
and the Department of Agriculture, Fisheries and Forestry
for the Hawkesbury Forest Experiment.
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International
ICT International Pty Ltd
Solutions for soil, plant and environmental research
International
www.ictinternational.com.au
sales@international.com.au
Phone: 61 2 6772 6770
Fax:
61 2 6772 7616
PO Box 503, Armidale, NSW, Australia, 2350
Solutions for soil, plant & meteorology
www.ictinternational.com.au
International