soil water potential measurement doug cobos, ph.d. decagon devices and washington state university
TRANSCRIPT
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Soil Water Potential Measurement
Doug Cobos, Ph.D.Decagon Devices and Washington
State University
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Two Variables are Needed to Describe the State of Water
Water content andQuantityExtent
Related Measuresheat content andcharge and
Water potential
QualityIntensity
temperaturevoltage
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Extensive vs. Intensive
Heat Content
Temperature
Energy flow?
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Water Potential PredictsDirection and rate of water flow in Soil,
Plant, Atmosphere ContinuumSoil “Field Capacity”Soil “Permanent Wilting Point”Seed dormancy and germinationLimits of microbial growth in soil and
food
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Water Potential
Energy required, per quantity of water, to transport, an infinitesimal quantity of water from the sample to a reference pool of pure, free water
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Water Potential: important pointsEnergy per unit mass, volume, or
weight of waterWe use units of pressure (Mpa, kPa, m
H2O, bars)
Differential property A reference must be specified (pure, free
water is the reference; its water potential is zero)
The water potential in soil is almost always less than zero
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Water potential is influenced by:Binding of water to a surface Position of water in a gravitational
fieldSolutes in the waterPressure on the water (hydrostatic or
pneumatic)
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Total water potential = sum of components
T = m + g + o + p
T – Total water potential
m – matric potential - adsorption to surfaces
g – gravitational potential - position
o – osmotic potential - solutes
p – pressure potential - hydrostatic or pneumatic
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Matric potential (Ψm)
adsorptive forces
From Jensen and Salisbury, 1984
Hydrogen bonding of water to surfaces Always negative Most important component in
soil Highly dependent on surface
area of soil
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Soil Water Retention Curves
Matric potential (kPa)
Vol
umet
ric w
ater
con
tent
(m
3/m
3)
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Gravitational potential (Ψg)
10 m
Reference Height
Ψg = g * h * ρwater = 9.81 m s-2 * 10 m * 1 Mg m-3 = + 98.1 kPa
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Gravitational potential (Ψg)
1 m
Reference Height (soil surface)
Ψg = g*h = 9.81 m s-2 *1 m = - 9.81 kPa
Ψg = g * h * ρwater = 9.81 m s-2 * 1 m * 1 Mg m-3 = - 9.81 kPa
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Osmotic potential (Ψo) - solutes
Arises from dilution effects of solutes dissolved in water Always negative Only affects system if semi-permeable
barrier present that lets water pass but blocks saltsPlant rootsPlant and animal cellsAir-water interface
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Osmotic potential (Ψo) - solutes
C = concentration of solute (mol/kg)φ = osmotic coefficient - 0.9 to 1 for most solutesν = number of ions per mol (NaCl = 2, CaCl2 = 3, sucrose = 1)R = gas constantT = Kelvin temperature
Ψ0 = CφvRT
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Pressure potential (Ψp)
Hydrostatic or pneumatic pressure (or vacuum) Positive pressure
Surface waterGroundwaterLeaf cells (turgor pressure)Blood pressure in animals
NegativePlant xylem
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Water potential ranges and units
Condition Water Potential (MPa)
Water Potential (m H2O)
Relative Humidity (hr)
Freezing Point (oC)
Osmolality (mol/kg)
Pure, free water 0 0 1.00 0 0
Field Capacity -0.033 -3.4 0.9998 -0.025 0.013
-0.1 -10.2 0.9992 -0.076 0.041
-1 -102 0.993 -0.764 0.411
Permanent wilting point
-1.5 -15.3 0.989 -1.146 0.617
-10 -1020 0.929 -7.635 4.105
Air dry -100 -10204 0.478 -76.35 41.049
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Water potentials in Soil-Plant-Atmosphere Continuum
Atmosphere -100
-3.0
-2.5
-1.7-1.5
Soil
Root
Xylem
Leaf
Permanent wilt(MPa)
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Measuring Soil Water Potential Solid equilibration methods
Electrical resistance Capacitance Thermal conductivity
Liquid equilibration methods Tensiometer Pressure chamber
Vapor equilibration methods Thermocouple psychrometer Dew point potentiameter
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Electrical Resistance Methods for Measuring Water Potential
Standard matrix equilibrates with soil
Electrical resistance proportional to water content of matrix
Inexpensive, but poor stability, accuracy and response
Sensitive to salts in soil Sand
Gypsum capsule
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Capacitance Methods for Measuring Water Potential
Standard matrix equilibrates with soil
Water content of matrix is measured by capacitance
Stable (not subject to salts and dissolution
Fair accuracy from -0.01 to -0.5 MPa (better with calibration)
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Heat Dissipation Sensor
Robust (ceramic with embedded heater and temperature sensor)
Large measurement range (wet and dry end)
Stable (not subject to salts and dissolution
Requires complex temperature correction
Requires individual calibration
Ceramic
Heater and thermocouple
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Liquid Equilibration: Tensiometer
Equilibrates water under tension with soil water through a porous cup
Measures tension of water Highest accuracy of any sensor
in wet range Limited to potentials from 0 to -
0.09 MPa Significant maintenance
requirements
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Liquid Equilibration: Pressure chamber
Moist soil placed on saturated porous plate
Plate and soil sealed in chamber and pressure applied, outflow at atmospheric pressure
Ψsoil ≈ negative of pressure applied
Common method for moisture characteristic curves
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Liquid Equilibration: Pressure chamber
Equilibrium time Hours at wet end Months or more at dry end
(maybe never)
Recent work shows that samples at -1.5 Mpa only reached -0.55 Mpa Hydraulic contact between plate
and soil sample Low Kunsat at low water potential
Gee et. al, 2002. The influence of hydraulic disequilibrium on pressure plate data. Vadose Zone Journal. 1: 172-178.
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Water Potential and Relative Humidity
Relative humidity (hr) and water potential (Ψ) related by the Kelvin equation:
rw
hM
RTln
R is universal gas constantMw is molecular mass of waterT is temperature
Condition Water Potential (MPa)
Relative Humidity (hr)
Pure, free water 0 1.000
Field Capacity -0.033 0.9998
Permanent wilting point
-1.5 0.989
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Vapor Equilibrium Methods
Thermocouple psychrometerMeasure wet bulb temperature depression
of head space in equilibrium with sample
Dew point hygrometerMeasure dew point depression of head
space in equilibrium with sample
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Thermocouple Psychrometer
Chromel-constantanthermocouple
sample
Thermocoupleoutput
Measures wet bulb temperature depression
Water potential proportional to cooling of wet junction
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Sample Chamber Psychrometer
Measures water potential of soils and plants
Requires 0.001C temperature resolution
0 to – 6 MPa (1.0 to 0.96 RH) range
0.1 MPa accuracy (problems in wet soil)
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In Situ Soil Water Potential
Soil Psychrometer
Readout
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Chilled Mirror Dew Point
Infrared SensorMirror
Optical Sensor
Fan
Sample
Cool mirror until dew forms
Detect dew optically
Measure mirror temperature
Measure sample temperature with IR thermometer
Water potential is approximately linearly related to Ts - Td
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WP4 Dew Point Potentiameter
Range is 0 to -300 MPa
Accuracy is 0.1 MpaExcellent in dry soilProblems in wet soil
Read time is 5 minutes or less
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Some applications of soil water potential Soil Moisture Characteristic
Plant Available Water Surface Area Soil Swelling
Hydropedology
Water flow and contaminant transport
Irrigation management
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Soil Moisture Characteristic Relates water content to water
potential in a soil Different for each soil Used to determine - plant available water - surface area - soil swelling
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Plant Available Water
Two measurement methods needed for full range Hyprop, tensiometer, pressure
plate in wet end Dew point hygrometer or
thermocouple psychrometer in dry end
Field capacity (-0.033 Mpa) Upper end of plant available water
Permanent wilting point (-1.5 Mpa) Lower end of plant available water Plants begin water stress much lower
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Surface Area from aMoisture Characteristic
y = 1231.3x2 + 406.15x
R2 = 0.9961
0
50
100
150
200
250
0 0.05 0.1 0.15 0.2 0.25 0.3
Slope of Semilog plot
EG
ME
Su
rfac
e A
rea
(m2/
g)
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pF Plot to get Soil Swelling
y = -17.02x + 7.0381
R2 = 0.9889y = -29.803x + 7.0452
R2 = 0.9874
y = -97.468x + 6.8504
R2 = 0.968833.5
44.5
55.5
66.5
77.5
0 0.05 0.1 0.15 0.2
Water Content (g/g)
Su
ctio
n (
pF
)
L-soil
Palouse
Palouse B
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Expansive Soil Classification from McKeen(1992)
Class Slope Expansion
I > -6 special case
II -6 to -10 high
III -10 to -13 medium
IV -13 to -20 low
V < -20 non-expansive
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Hydropedology Requirements:
Year around monitoring; wet and dry
Potentials from saturation to air dry
Possible solutions: Soil psychrometers (problems with
temperature sensitivity) Capacitance matric potential sensor
(limited to -0.5 MPa on dry end) Heat dissipation sensors (wide
range, need individual calibration)
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Water Flow and Contaminant Transport
Requirements: Accurate potentials and gradients
during recharge (wet conditions) Continuous monitoring
Possible solutions: Capacitance matric potential
sensor Pressure transducer tensiometer
(limited to -0.09 MPa on dry end)
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Irrigation Management Requirements:
Continuous during growing season
Range 0 to -0.1 Mpa
Possible solutions: Tensiometer (soil may get too
dry) Electrical resistance (poor
accuracy) Heat dissipation or capacitance
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Measuring water content to get water potential
Requires moisture characteristic curve for converting field measurements from q to y
Conventional wisdom: time consuming Most moisture release curve have been done on
pressure plates Long equilibrium times, labor intensive
New techniques Fast (<24 hours) Automated
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Bridging the gap
10%
20%
30%
40%
50%
60%
70%
Vo
lum
etr
ic W
ate
r Co
nte
nt
Volumetric water content at various depths over over the growing season of wheat grown in a Palouse Silt Loam (Location: Cook's Farm, Palouse, WA)
30 cm
60 cm
90 cm
120 cm
150 cm
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SummaryKnowledge of water potential is
important for Predicting direction of water flowEstimating plant available waterAssessing water status of living organisms
(plants and microbes)
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Summary Water potential is measured by equilibrating
a solid, liquid, or gas phase with soil water
Solid phase sensors Heat dissipation Capacitance Granular matrix
Liquid equilibrium Tensiometers Pressure plates
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Summary
Vapor equilibration Thermocouple psychrometers Dew point potentiameters
No ideal water potential measurement solution exists Maintenance and stability Accuracy and calibration Ease of use Range of operation