MS 04 Soil Matric Potential

ETSEA

Jorge Lampurlanés ([email protected])

Department of Agricultural and Forestry Engineering

ETSEA

Soil Matric Potential

� Components of soil water potential

� Measurement of matric potential

� Tensiometer.

� Electrical Resistance sensor.

� Head Dissipation sensor.

� Thermocoupule Psychrometer.

2Soil Matric Potential Department of Agricultural and Forestry Engineering

ETSEA

Components of Soil Water Potential

� Water in soil


ETSEA


� 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 +++=


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.

Soil Matric Potential Department of Agricultural and Forestry Engineering 5

ETSEA

� 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−=



ETSEA

� Conversion units of the soil water potential


Components of Soil Water PotentialETSEA

� Relationship matric potential-volumetric water content



ETSEA

� Relationship matric potential-volumetric water content


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.


ETSEA

� 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).


Tensiometer

ETSEA

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.


ETSEA

� 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.


Tensiometer

ETSEATensiometer

� Commercial sensors:

� Irrometer (Irrometer)

� Quickdraw (Soilmoisture)


ETSEATensiometer


� T4, T5, T8, TS1 (Decagon)


ETSEA

Tensiometer




ETSEA


� 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.


ETSEA


� 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.


ETSEA


� 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.


ETSEA


� WARNINGS

� Very sensitive for θ between 0-0,35 (dry soil).

� Not sensitive for θ > 0.9 (near saturation).


ETSEA


� 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.


ETSEAElectrical Resistance sensor


� 223-L, 0,1 to 1000 kPa (Campbell)

� Watermark, 0- 200 kPa (Irrometer)


ETSEA

Tensiometer




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.


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).



� 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.




• 229-L 0,1-10 bar (Campbell Sci)


ETSEA

Tensiometer




ETSEA


� 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


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.


ETSEA



ETSEAThermocouple Psychrometer


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.


ETSEA

Tensiometer 0 → -85 kPa

Gypsum blocks -100 → -1500 kPa

Thermocouple Psycrometer -100 → -1500 kPa

Head dissipation (irrigation)

ETSEA

MS 04 Soil Matric Potential

Documents

Transcript of MS 04 Soil Matric Potential