Essentially, there are only two primary measurement methods for water potential—tensiometers and vapor pressure methods. Tensiometers work in the wet range—special tensiometers that retard the boiling point of water have a range from 0 to about -0.2 MPa. Vapor pressure methods work in the dry range—from about -0.1 MPa to -300 MPa (0.1 MPa is 99.93% RH; -300 MPa is 11%).
Historically, these ranges did not overlap, but recent advances in tensiometer and temperature-sensing technology have changed that. Now, a skilled user with excellent methods and the best equipment can measure the full water potential range in the lab.
There are reasons to look at secondary measurement methods, though. Vapor pressure methods are not useful in situ, and the accuracy of the tensiometer must be paid for with constant, careful maintenance (although a self-filling version of the tensiometer is available).
Additionally, there are traditional methods like gypsum blocks, pressure plates, and filter paper that should be understood. This section briefly covers the strengths and limitations of each method.
The pressure plate was introduced in the 1930s by L.A. Richards. It doesn’t actually measure the water potential of a sample. Instead, it brings the sample to a specific water potential by applying pressure to the sample and allowing the excess water to flow out through a porous ceramic plate. When the sample comes to equilibrium, its water potential will be equivalent to the pressure applied.
Pressure plates are typically used to make soil moisture characteristic curves. Once the soil samples reach a specific water potential under pressure, the researcher can remove the sample from the plate and dry it to measure its water content. A soil moisture characteristic can be produced by making these measurements at different pressures in the pressure plate apparatus.
The accuracy of pressure plates is important, because they are often used to calibrate other secondary measurement methods.
In order to make an accurate moisture release curve with a pressure plate, you have to ensure that the sample has fully come to equilibrium at the designated pressure. Several reviewers, including Gee et. al (2002), Cresswell et. al (2008), and Bittelli and Flury (2009) have noted problems with this assumption.
Errors, particularly at low water potentials, may be caused by clogged pores in the ceramic of the pressure plate, flow restriction within the sample, loss of hydraulic contact between the plate and the soil due to soil shrinkage, and re-uptake of water when the pressure on the plate is released. At low water potentials, low hydraulic conductivities can cause equilibrium to take weeks or even months. Gee et. al (2002) measured the water potentials of samples equilibrated for 9 days on 15 bar pressure plates and found them to be at -0.5 MPa instead of the expected -1.5 MPa. Especially when constructing a moisture release curve to estimate hydraulic conductivity and determine plant available water, pressure plate measurements at potentials less than -0.1 MPa (-1 bar) can cause significant error (Bittelli and Flury, 2009).
Additionally, Baker and Frydman (2009) establish theoretically that the soil matrix would drain differently under a positive pressure than it does under suction. They postulate that equilibrium water contents achieved using suction will be significantly different than those that occur under natural conditions. Anecdotal evidence seems to support this idea, though further testing is needed. Ultimately, pressure plates may have enough accuracy in the wet range (0 to -0.5 MPa) for some applications, but other methods can provide better accuracy, which may be especially important when using the data for modeling or calibration.
The WP4C Dew Point Hygrometer is one of the few commercially available instruments that currently uses this technique. Like traditional thermocouple psychrometers, the dew point hygrometer equilibrates a sample in a sealed chamber.
A small mirror in the chamber is chilled until dew just starts to form on it. At the dew point, the WP4C measures both mirror and sample temperatures with 0.001◦C accuracy to determine the relative humidity of the vapor above the sample.
The most current version of this dew point hygrometer has an accuracy of ±1% from -5 to -300 MPa and is also relatively easy to use. Many sample types can be analyzed in five to ten minutes, although wet samples take longer.
At high water potentials, the temperature differences between saturated vapor pressure and the vapor pressure inside the sample chamber become vanishingly small.
Limitations to the resolution of the temperature measurement mean that vapor pressure methods will probably never supplant tensiometers.
The dew point hygrometer has a range of -0.1 to -300 MPa, though readings can be made beyond -0.1 MPa using special techniques. Tensiometers remain the best option for readings in the 0 to -0.1 MPa range.
The HYPROP is a unique lab instrument that uses the Wind/Schindler evaporation method to make moisture release curves on soils with water potentials in the tensiometer range.
Hyprop uses two precision mini-tensiometers to measure water potential at different levels within a saturated 250 cm3 soil sample while the sample rests on a laboratory balance. Over time, the sample dries, and the instrument measures the changing water potential and the changing sample weight simultaneously. It calculates the moisture content from the weight measurements and plots changes in water potential correlated to changes in moisture content.
Results are verified, and values for dry range and saturation are calculated according to a selected model (i.e., van Genuchten/Mualem, bimodal van Genuchten/Mualem, or Brooks and Corey).
Hyprop has high accuracy and produces a complete moisture release curve in the wet range. The curve takes three to five days to complete, but the instrument operates unattended.
Hyprop’s range is limited by the range of tensiometers, although the mini-tensiometers have been used to measure beyond -250 kPa (-0.25 MPa) because of their boiling retardation feature.
Below -250 kPa the tensiometers cavitate. Power users have the option of adding a final point to the curve at the air-entry point for the ceramic tensiometer cup (-880 kPa; -0.88 MPa).
Water potential, by definition, is a measure of the difference in potential energy between the water in a sample and the water in a reference pool of pure, free water. The tensiometer is an actualization of this definition.
The tensiometer tube contains a pool of (theoretically) pure, free water. This reservoir is connected (through a permeable membrane) to a soil sample. Thanks to the second law of thermodynamics, water moves from the reservoir to the soil until its energy is equal on both sides of the membrane. That creates a vacuum in the tube. The tensiometer uses a negative pressure gauge (a vacuometer) to measures the strength of that vacuum and describes water potential in terms of pressure.
Tensiometers are probably the oldest type of water potential instrument (the initial concept dates at least to Livingston in 1908), but they can still be quite useful. In fact, in the wet range, a high-quality tensiometer used skillfully, can have excellent accuracy.
The tensiometer’s range is limited by the ability of water inside the tube to withstand a vacuum. Although water is essentially incompressible, discontinuities in the water surface such as edges or grit provide nucleation points where water’s strong bonds are disrupted and cavitation (low-pressure boiling) occurs. Most tensiometers cavitate around -80 kPa, right in the middle of the plant-available range.
However, METER Group Ag, in Germany, builds tensiometers that are modern classics thanks to precision German engineering, meticulous construction, and fanatical attention to detail. These tensiometers have terrific accuracy and a range that (with a careful operator) can extend to -250 kPa.
Water content tends to be easier to measure than water potential, and since the two values are related, it’s possible to use a water content measurement to find water potential.
A graph showing how water potential changes as water is adsorbed into and desorbed from a specific soil matrix is called a moisture characteristic or a moisture release curve.
Every matrix that can hold water has a unique moisture characteristic, as unique and distinctive as a fingerprint. In soils, even small differences in composition and texture have a significant effect on the moisture characteristic.
Some researchers develop a moisture characteristic for a specific soil type and use that characteristic to determine water potential from water content readings. Matric potential sensors take a simpler approach by taking advantage of the second law of thermodynamics.
Matric potential sensors use a porous material with known moisture characteristic. Because all energy systems tend toward equilibrium, the porous material will come to water potential equilibrium with the soil around it.
Using the moisture characteristic for the porous material, you can then measure the water content of the porous material and determine the water potential of both the porous material and the surrounding soil. Matric potential sensors use a variety of porous materials and several different methods for determining water content.
At its best, matric potential sensors have good, but not excellent, accuracy. At its worst, the method can only tell you whether the soil is getting wetter or drier. A sensor’s accuracy depends on the quality of the moisture characteristic developed for the porous material and the uniformity of the material used. For good accuracy, the specific material used should be calibrated using a primary measurement method. The sensitivity of this method depends on how fast water content changes as water potential changes. Precision is determined by the quality of the moisture content measurement.
Accuracy can also be affected by temperature sensitivity. This method relies on isothermal conditions, which can be difficult to achieve. Differences in temperature between the sensor and the soil can cause significant errors.
All matric potential sensors are limited by hydraulic conductivity: as the soil gets drier, the porous material takes longer to equilibrate. The change in water content also becomes small and difficult to measure. On the wet end, the sensor’s range is limited by the air-entry potential of the porous material being used.
The filter paper method was developed in the 1930s by soil scientists as an alternative to the methods then available. A specific type of filter paper (Whitman No. 42 Ashless) is used as the porous medium. Samples are equilibrated with the filter paper medium. Samples are equilibrated with the filter paper in a sealed chamber at constant temperature. Gravimetric water content of the filter paper is determined using a drying oven, and the water potential is inferred from the predetermined moisture characteristic curve of the filter paper. Deka et al. (1995) found that at least 6 days were required for full equilibration.
The range of filter paper is commonly accepted to be down to -100 MPa if allowed to equilibrate fully. However, as illustrated, errors from temperature gradients become exceptionally large at water potentials near zero.
This method is inexpensive and simple, but it is not accurate. It requires isothermal conditions, which can be difficult to achieve. Small temperature variations can cause significant errors.
Gypsum blocks are often used as simple indicators of irrigation events. Gypsum blocks measure the electrical resistance of a block of gypsum as it responds to changes in the surrounding soil. The electrical resistance is proportional to water potential.
Gypsum blocks are incredibly cheap and fairly easy to use.
The readings are temperature-dependent and have very low accuracy. Also, gypsum dissolves over time, especially in saline soils, and loses its calibration properties. Gypsum blocks tell you wet or dry but not much more.
Like gypsum blocks, granular matric sensors measure electrical resistance in a porous medium. Instead of gypsum, they use granular quartz surrounded by a synthetic membrane and a protective stainless steel mesh.
Compared with gypsum blocks, granular matric sensors last longer and work in wetter soil conditions. Performance can be improved by measuring and compensating for temperature variations.
Measurements are temperature-dependent and have low accuracy. Also, even with good soil-to-sensor contact, granular matric sensors have rewetting problems after they have been equilibrated to very dry conditions because water has a reduced ability to enter the coarse medium of the granular matrix from a fine soil. Range is limited on the wet end by the air entry potential of the matrix. Granular matric sensors can only start measuring water content/potential when the largest pores in the matrix start to drain. Additionally, these sensors use a gypsum pellet, which dissolves over time, giving poor long-term stability.
Ceramic-based sensors use a ceramic disc as the porous medium. The quality of the sensor depends on the specific qualities of the ceramic.
Accuracy is limited by the fact that each disc has a somewhat unique moisture characteristic. Uniformity in the ceramic material yields greater accuracy but significantly limits the range. Custom calibration of each individual sensor improves accuracy dramatically but is time consuming. Recent innovations in calibration technique may offer better commercial calibration options.
Range is limited on the wet end by the air entry potential of the ceramic. Ceramic-based sensors can only start measuring water content/potential when the largest pores in the ceramic start to drain. On the dry end, range is limited by the total porosity contained in small pores that drain at low water potentials.
The heat dissipation sensor measures moisture content of the ceramic by measuring its thermal conductivity. Using a ceramic cylinder containing a heater and a thermocouple, it measures baseline temperature, heats for a few seconds, and then measures temperature change. By plotting the change in temperature vs. log time, it determines the moisture content of the ceramic. Moisture content is translated into water potential using the moisture characteristic of the ceramic disc. Note that because the sensor is heated, it must be powered by a system with large power reserves (e.g., Campbell Scientific data logger or equivalent).
Unless it is individually custom calibrated, the heat dissipation sensor has only moderate accuracy.
On the very dry end, there is a lot of sensitivity in the thermal conductivity curve, which gives heat dissipation sensors extended usefulness in the dry range (-1 to -50 mPa). On the wet end, the heat dissipation sensor is limited by the air entry potential of the ceramic.
Dielectric matric potential sensors measure the charge-storing capacity of a ceramic disc to determine its water content. They then use the moisture characteristic of the disc to convert water content to water potential.
Because they use a dielectric technique, the sensors are highly sensitive to small changes in water. Like all ceramic-based sensors, matric potential sensors require custom calibration for good accuracy.
Dielectric matric potential sensors are low power and maintenance-free.
Without calibration, the sensors have an accuracy of just ±40% of the reading. However, a recent, custom-calibrated version of the sensor promises an accuracy of ±10% of the reading.
Dr. Colin Campbell’s webinar covers water potential instrument theory, including the challenges of measuring water potential and how to choose and use various water potential instruments.
Download the “Researcher’s complete guide to water potential”.
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