ms 04 soil matric potential
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Jorge Lampurlanés ([email protected])
Department of Agricultural and Forestry Engineering
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Soil Matric Potential
� Components of soil water potential
� Measurement of matric potential
� Tensiometer.
� Electrical Resistance sensor.
� Head Dissipation sensor.
� Thermocoupule Psychrometer.
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Components of Soil Water Potential
� Water in soil
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Components of Soil Water Potential
� Soil Water Potential
� Potential energy of water in the soil with respect to a defined reference state (pure water under atmospheric pressure at a known vertical position and temperature)
� Hydraulic Potential: Ψh = Ψp + Ψg
� Water Potential: Ψw = Ψm + Ψo
omgp ψψψψψ +++=
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Ψp pressure potential (under the water table, saturated soil).Ψg gravitational potential (position).Ψm matric potential (above the water table, unsaturated soil).Ψo osmotic potential (solutes).
ETSEAComponents of Soil Water PotentialDriving force for the movement of water in soil and plants.
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� Units� Energy/mass (Soil Water Potential, Ψ)
� Energy/volume (pressure equivalent, p)
� Energy/weight (head equivalent, h)
Conversion
ρ, density of water (1000 kg/m3)
g, acceleration due to gravity (9,81 m/s2)
Logarithmic scale
kg
J
M
E⇒
[ ]PPam
N
m
mN
m
J
V
E⇒==⋅=⇒
233
[ ]LmN
mN
N
J
F
E⇒=⋅=⇒
)()()( 1 mg
PakgJψψρψ =⋅=⋅ −
))((log10 cmpF ψ−=
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Components of Soil Water Potential
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� Conversion units of the soil water potential
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Components of Soil Water PotentialETSEA
� Relationship matric potential-volumetric water content
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Components of Soil Water Potential
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� Relationship matric potential-volumetric water content
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Components of Soil Water PotentialETSEA
Tensiometer
Electrical Resistance sensor
Head Dissipation sensor
Thermocouple Psychrometer
ETSEATensiometer
� PRINCIPLE� A tensiometer is a porous cup
connected to a manometer by an airtight tube with all parts of the system water-filled.The porous cup is water permeable but air impermeable.
� When the cup is in contact with the soil water moves into or out of the cup unitil the (negative) perssure inside the cup equals the matric potential of the soil water outside.
� Measuring the pressure inside the tensiometer we can calculate the matric potential of the soil.
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� MATERIALS & METHODS
� Fill the tensiometer with de-aired water to avoid bubble formation (vacuum).
� Place the tensiometer to the required depht.
� Hole diameter = Tensiometer diameter (to prevent water chaneling).
� Before to install apply presure to detect leaks.
� Ensure good cup-soil contact (pure fine earth or a small amount of water down the hole to make a slurry).
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Tensiometer
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Tensiometer
� MATERIALS & METHODS
� Peassurement of the pressure inside the tensiometer:� Bourdon vacuum gauges (low precission ± 2 kPa).
� Mercury Manometers (± 0.25 kPa, banned).
� Pressure transducers (± 0.1 kPa, high sensitive, fast response)
� Vented to the atmosphere (differential).
� Temperature compensated.
� Portable (pucture tensiometer) or fixed.
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� WARNINGS� Lower limit around -85 kPa (pores diameter of the cup).
� Periodic purging (read, refill, restore vacuum).
� Response time (from matric potential change to reading change)� Conductance of the porous cup.
� Sensor or “gauge” sensitivity.
� Unsaturated hydraulic conductivity of the soil.
� Keep all water-filled parts out ofdirect sunlight to avoid volumechanges that produce a temporaryperturbation of the reading.
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Tensiometer
ETSEATensiometer
� Commercial sensors:
� Irrometer (Irrometer)
� Quickdraw (Soilmoisture)
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ETSEATensiometer
� Commercial sensors:
� T4, T5, T8, TS1 (Decagon)
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Tensiometer
Electrical Resistance sensor
Head Dissipation sensor
Thermocouple Psychrometer
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Electrical Resistance sensor
� PRINCIPLE
The electrical resistance of two electrodes embedded in a porous media is a function of its water content, hence of its matric potential.
In the soil, the matric potential of the sensor equals the matric potential of the soil.
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Electrical Resistance sensor
� MATERIALS & METHODS� The Electrical Resistance sensor includes 2 metallic
electrodes (parallel or concentric grids) embedded in a block of porous material (gypsum, fiberglass or nylon).
� Ensure good contact between the sensor and the soil.
� Measure the resistance with alternate current.
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Electrical Resistance sensor
� WARNINGS� Long response time (48 h).
� Needs calibration (pressure membranes or pressure plates) and temperature correction.
� Hysteresis in the calibration curve due to the hysteresis in the water retention characteristic of the porous media.
� Gypsum sensor insensible to the electrical conductivity of the soil solution. Fiberglass and nylon are sensible and are not suitable for saline soils.
� Gypsum sensor have limited lifetime, slowly dissolves and changes its calibration.
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Electrical Resistance sensor
� WARNINGS
� Very sensitive for θ between 0-0,35 (dry soil).
� Not sensitive for θ > 0.9 (near saturation).
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Electrical Resistance sensor
� ADVANTAGES� Gypsum sensors:
� Can cover a range of potentials from -30, -80 kPa to -1,5 MPa.
� Useful complement to the use of tensiometers.
� Fiberglass and nylon:� Extended lifetime.
� Range of potentials: near saturation to -100 kPa.
� Cheap => attractive proposition in nonsaline soils, large numbers of sensors and/or data logging.
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ETSEAElectrical Resistance sensor
� Commercial sensors:
� 223-L, 0,1 to 1000 kPa (Campbell)
� Watermark, 0- 200 kPa (Irrometer)
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Tensiometer
Electrical Resistance sensor
Head Dissipation sensor
Thermocouple Psychrometer
ETSEAHeat Dissipation sensor� PRINCIPLE
The thermal diffusivity of a porous material is related to its water content, hence to its matric potential.
+ water content ⇒ + thermal diffusivity ⇒ + head dissipation
� MATERIAL & METHODS
� Heat dissipation is measured as the difference between the temperature at the centre of the sensor before and after a short (150 s) heat pulse has been applied.
� The temperature difference is related with the matric potential.
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ETSEAHeat Dissipation sensor
� MATERIAL & METHODS
� Sensor
� Temperature sensor (germanium junction diode).
� Heating coil (wrapped around the temperature sensor).
� Both encased in a cylinder of gypsum or a ceramic material, large enough to contain the heat pulse (50 mm long, 20 mm diameter).
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ETSEAHeat Dissipation sensor
� ADVANTAGES� Unaffected by the thermal diffusivity of the soil.� Unaffected by the salinity of the soil solution.
� WARNINGS� Calibration in pressure plates.� Hysteresis of the calibration curve (porous material).� Accuracy: ±20 kPa from 0 to -300 kPa.
±100 kPa from -300 to -600 kPa.� Cannot be interrogated too frequently (time to head
pulse dissipation).� Not for scientific applications. Well suited for irrigation
scheduling.
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ETSEAHeat Dissipation sensor
� Commercial sensors:
• 229-L 0,1-10 bar (Campbell Sci)
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Tensiometer
Electrical Resistance sensor
Head Dissipation sensor
Thermocouple Psychrometer
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Thermocouple Psychrometer
� PRINCIPLE
The relative humidity (p/po) of the air in equilibrium with the liquid phase in the soil is related to its water potential.
The relative can be determined by the temperature difference between the dry and the wet bulb (psychrometer)
=
0
lnp
p
M
TRw
ρψ
Tp
s
M
TRw ∆
+=0
γρψ
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ETSEAThermocouple Psychrometer
� MATERIAL & METHOS � Insert the thermocouple psychrometer into the soil,
horizontally with 50-100 mm of the lead wire, at the desired depth.
� Psychrometers:� Richards: evaporation of a water drop.
� Spanner: Peltier effect.
� Dew point hygrometer: maintains the temperature at dew point temperature.
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Thermocouple Psychrometer
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ETSEAThermocouple Psychrometer
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ETSEAThermocouple Psychrometers
� WARNINGS
� Very sensitive to temperature variations. For high potential (> -100 kPa) very small ∆T.
� Calibration with salt solutions of different concentration at different temperatures. Temporal shift.
� Require temperature correction (especially at the surface).
� Install horizontally to minimize the influence of soil temperature gradients.
� Accuracy ±50 kPa.
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Tensiometer 0 → -85 kPa
Gypsum blocks -100 → -1500 kPa
Thermocouple Psycrometer -100 → -1500 kPa
Head dissipation (irrigation)
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