Measurement of Soil Water Potential Components 8 Hillel, pp. 161 - 168 Measurement of Soil Water Potential Components

Measurement of Matric Potential – Tensiometer A tensiometer consists of a porous cup, usually made of ceramic having very fine pores, that is connected to a vacuum gauge (or other measuring device) through a water filled tube. After installation of the tensiometer in the field the tube is filled with de-aired water and sealed airtight. In this initial stage water inside the tube is under atmospheric pressure. If the potential in the surrounding soil is lower than atmospheric pressure water will flow from the tensiometer through the porous cup into the soil till equilibrium is reached. Mass flow from tensiometer to the soil lowers the pressure in the tensiometer creating suction sensed by the gauge or transducer. When the soil is wetted flow may occur in the reverse direction towards a new equilibrium.

Measurement of Matric Potential – Tensiometer The gauge or transducer reading has to be corrected to account for the positive head that is exerted by the water column inside the tensiometer on the point of interest in depth of the ceramic cup Measurement Range ygauge = - 1.2 m 0 to – 100 kPa (-1bar; -10 m head) zgauge=0.2 m Tensiometer Equation ym = ygauge + (zgauge – zcup) zcup=0.5 m ym = ygauge + (zgauge – zcup) Tensiometer Equation ym = -1.2 + (0.2 – (-0.5)) ym = -0.5 m

Measurement of Matric Potential – Tensiometer Sketch showing tensiometers with vacuum gauges and electronic pressure transducers.

Tensiometer & Potential Diagrams - Example The cups of tensiometers 1 and 2 are at a depth of 0.6 and 0.8 m below the soil surface. The gauges are 0.2 m above the soil surface. The gauge in tensiometer 1 indicates gauge = -0.9 m. Draw a potential diagram with the soil surface as reference level. Assume static equilibrium conditions. Estimate the gage reading in tensiometer 2. First we set our reference level at the soil surface and calculate the matric potential in 0.6 m depth using the tensiometer equation:

Tensiometer & Potential Diagrams - Example With known matric potential and assuming zero solute potential we now can calculate the hydraulic potential in 0.6 m depth. Note that under equilibrium conditions the hydraulic potential is uniform throughout the soil profile. With known hydraulic potential we now can calculate the matric potential throughout the profile (tabulated values are in m head) The reading in tensiometer 2 is calculated as:

Tensiometer & Potential Diagrams - Example As the final step we can draw the potential diagram for equilibrium conditions:

Measurement of Matric Potential – Heat Dissipation The rate of heat dissipation in a porous medium is dependent on the medium‘s specific heat capacity, the thermal conductivity, and the density. The thermal conductivity and heat capacity of a porous matrix is affected by its water content (matric potential). The measurement is based on application of a heat pulse through a heating element and analysis of the temperature response measured with a thermocouple. The measured magnitude of temperature change during a given heating period is linearly related to the natural logarithm of the matric potential. measured from calibration The linearity coefficient a has to be determined through calibration of the heat dissipation sensor.

Measurement of Matric Potential – Heat Dissipation A typical line source heat dissipation sensor consists of a fine wire heating element that is axially centered in a cylindrical ceramic matrix having a diameter of about 1,5 cm and a length of 3,2 cm. The thermocouple is located adjacent to the heating element at mid-length. Both the thermocouple and the heating element are placed in the shaft portion of a hypodermic needle. -10 to – 1000 kPa (-0,1 to -10 bar; -1 to – 100 m head) Measurement Range

Measurement of Matric Potential- The Kelvin Equation In cases where the solute potential is negligible, matric potential can be inferred from measurements of vapor pressure over the wet soil sample. The potential energy of soil water is in thermodynamic equilibrium with the potential energy of water vapor in the ambient air. A psychrometer measures the relative humidity in the ambient air (vapor pressure in the soil air relative to the saturation vapor pressure of air at the same temperature) that is related to potential energy yv of water vapor: Mw Molecular weight of water (0,018 kg/mol) R Ideal gas constant (8.31 J K-1 mol-1) T Absolute temperature (K) rw Density of water (1000 kg/m3 at 20°C)

Measurement of Matric Potential- Psychrometer Rearranging and log-transformation yields: yv = ym (no salts) Matric Potential yv = yw = ym + ys (salts present) Water Potential This equation can be further simplified for RH close to 1, a value often encountered in agricultural soils (entire range of plant available water is between RH = 0.98 and RH = 1.0). Note:

Measurement of Matric Potential- Psychrometer Measurement Principle: A psychrometer infers relative humidity from the difference in temperature between a dry non evaporating surface (called dry bulb temperature), and the temperature of an evaporating surface (called wet bulb temperature) The wet bulb temperature is usually below the dry bulb temperature because of the latent heat loss that is associated with the evaporation process. The rate of evaporation from a wet surface depends on the relative humidity or vapor pressure of the ambient air. Low humidity = high evaporation rate High humidity = lower evaporation rate The higher the evaporation rate the larger is the temperature depression of the wet bulb below the dry bulb temperature.

Measurement of Matric Potential- Psychrometer A thermocouple psychrometer consists of a fine wire chromel constantan or other standard bimetallic thermocouple. A thermocouple is a double junction of two dissimilar metals. When the two junctions are subjected to different temperatures they generate a voltage difference explained as the SEEBECK effect. Conversely when an electric current is applied through the junctions it creates a temperature difference between the junctions by heating one and cooling the other dependent on the currents direction. For typical soil use one junction of the thermocouple is suspended in a thin wall ceramic or stainless steel cup that is buried in the soil while the other one is embedded in an insulated plug to measure the ambient temperature at the same location. Field Psychrometer

Measurement of Matric Potential- Psychrometer In psychrometric mode the suspended thermocouple is cooled below the dew point so that a droplet of water forms on the junction. (this is called Peltier cooling). Then the cooling stops and water evaporates from the junction drawing heat and depressing the temperature below that of the ambient air. The difference in temperature between the wet bulb (suspended junction) and the dry bulb (insolated junction) is measured and used to infer relative vapor pressure using the psychrometer equation. measured S....Slope of the saturation water vapor curve y....Psychrometer constant (about 0,067 kPa K-1 at 20°C)

MATRIC POTENTIAL – PSYCHROMETER The slope of the water vapor curve is temperature dependent and can be approximated according to Brutsaert [1982]: Where tR=1-373.15/T The saturation vapor pressure e0 is also temperature dependent and can be approximated by integrating the previous equation: The relation between water vapor pressure e, relative humidity RH, and temperature T is uniquely defined. That means knowledge of any two of them leads automatically to the third one:

MATRIC POTENTIAL – PSYCHROMETER A good Psychrometer can measure temperature depressions in the order of 0.000085 oC per kPa. Any errors in measuring wet bulb depression introduces large errors into psychrometric determinations of water potential. THERMODYNAMIC EQUILIBRIUM BETWEEN SAMPLE AND AMBIENT AIR IS REQUIRED TO ACHIEVE ACCURATE MEASUREMENTS!

MATRIC POTENTIAL – PSYCHROMETER Laboratory Psychrometer -800 to – 10000 kPa (-80 to – 1000 m head) Measurement Range

MATRIC POTENTIAL – PSYCHROMETER Laboratory Psychrometer

MATRIC POTENTIAL – PSYCHROMETER A second procedure inferring water potential using thermocouple psychrometers is called DEWPOINT METHOD In this method one surface is brought to dew point temperature and kept at exactly this temperature using a monitoring system and electronic circuitry. State of the art equipment (e.g., WP4 Potentiameter) uses chilled mirror dewpoint technique combined with a photoelectric detection system to keep the surface of a mirror at dewpoint temperature. Ambient temperature at the sample surface is measured with an infrared thermometer. WP4 Potentiameter

MEASUREMENT OF PRESSURE POTENTIAL The pressure potential may be measured with a Piezometer. A Piezometer is a tube with perforated bottom that is driven or flushed to depths below the water table. Water enters the tube and rises to a height equal to that of the unconfined water table. The elevation of the free water table is measured relative to the soil surface using a steel tape with bell sounder, or other electro-optic devices that indicate water table depth. The value of pressure potential expressed as energy per weight is simply the vertical distance from a point of interest to the surface of the free water table.

MEASUREMENT OF SOLUTE POTENTIAL A useful approximation that is used to estimate ys from the electrical conductivity of the soil solution at saturation (ECs) is: . Standard measurement of soil EC is usually based on extracting solution from saturated soil samples and measure the solution EC using a electrical conductivity meter.

MEASUREMENT OF SOLUTE POTENTIAL Electrical conductivity meters are based on Ohm‘s law that is obeyed by salt solutions. The solution EC is determined from known voltage and electrode geometry and measurement of the electric current. E = IxR E...electromotive force (volts) I....current flow (amperes) R...resistance (ohms)

Measurement of solute potential - TDR Rhoades and Oster (1986) [SSSA Methods of Soil Analysis Book] discuss several in-situ sensors for soil bulk EC measurement (the electrical conductivity through bulk soil rather than soil solution only) The proportional reduction in signal voltage serves as a basis for simultaneous measurement of bulk soil electrical conductivity EC and bulk dielectric constant eb The energy of the electromagnetic wave transversing the waveguide is attenuated proportional to the electrical conductivity along the travel path. Electrical Conductivity TDR waveforms show proportional reduction in signal voltage with soil bulk EC, hence enable simultaneous measurement of bulk soil EC and dielectric constant eb

MEASUREMENT OF SOLUTE POTENTIAL

Solute Potential - Example Irrigation water having EC of 10 dS/m was added to a cropped field which then drained to field capacity (v = 0.25), followed by a few weeks of plant roots extracting soil water to wilting point (v = 0.1). Find s at: saturation, field capacity and wilting point. 1.) For saturation we can apply Eq.29 (classnotes, page 44) 2.) Because soil water changed from saturation to field capacity by liquid flow there was no change in ys. 3.) From field capacity to wilting point however, water uptake was accomplished through selectively permeable membranes (plant roots) which absorb water while leaving most salts behind. The increased salt concentration affects the soil's solute potential at wilting point and is estimated by: