Posts from the ‘Soils’ Category

Sep 4

Examining Plant Stress using Water Potential and Hydraulic Conductivity

Many scientists rely on water potential alone to measure plant water stress. Leo Rivera, a METER soil scientist, shows how a two-pronged approach, using hydraulic conductivity as well as water potential, can make those measurements more powerful.

Green tomato plant with three bright red tomatoes

Measuring hydraulic conductivity in nursery plants shows why plants are stressed.

Soil moisture release curves can give you incredible detail about water movement, allowing you to understand not only that plants are stressed, but WHY they are not getting the water they need.

Recently, we ran into a mystery where this method was useful. Growers at a Georgia nursery noticed that plants growing in a particular soilless substrate were beginning to show signs of stress at about -10 kPa water potential, which is still really wet. They wanted to know why.

We decided to create the unsaturated hydraulic conductivity and soil moisture release curves for the substrate (using the Wind Schindler technique [HYPROP lab instrument]) and found that it had a dual porosity curve: essentially, a curve with a “stair step” in it. The source of the “stair step” can be explained by considering the substrate, which was made up of bark mixed with some other fine organic materials. In the bark material there were a lot of large and small pores, but no medium-sized pores (this is called a “gap-graded” pore size distribution). This gap in the pore size distribution reduced the unsaturated hydraulic conductivity and caused the stress. Even though there was available water in the soil, it couldn’t flow to the plant roots.

Nursery seedlings

That would have been pretty hard to understand without detailed hydraulic conductivity and soil moisture release curves—curves with more detail than most traditional techniques can provide. Our measurements showed that unsaturated hydraulic conductivity can have a major effect on how available water is to plants. Our theory about the soilless substrate was that as the roots were taking up water, they dried the soil around them pretty quickly. In a typical mineral soil, the continuous pore size distribution would allow water to flow along a water potential gradient from the surrounding area to the soil adjacent to the roots. In the bark, the roots dried the area around them in the same way, but the gap in pore size distribution created low hydraulic conductivity and prevented water from moving into the soil adjacent to the roots. This caused plants to start stressing even though the substrate was still quite wet.

We were pretty excited about this discovery. It shows that water potential, though critical, may not always tell the whole story. Using technology to measure the full soil moisture release curve and the hydraulic conductivity in one continuous test, we discovered the real reason plants were wilting even when surrounded by water. In the past, it took three or four different instruments and several months to take these measurements. We can now do it in a week. For more information about creating these kinds of curves, check out the app guide: “Tools and Tips for Measuring the Full Soil Moisture Release Curve.”

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Aug 28

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Low Impact Design: Sensors Validate California Groundwater Resource Management

Michelle Newcomer, a PhD candidate at UC Berkeley, (previously at San Francisco State University), recently published research using rain gauges, soil moisture, and water potential sensors to determine if low impact design (LID) structures such as rain gardens and infiltration trenches are an effective means of infiltrating and storing rainwater in dry climates instead of letting it run off into the ocean.

Body of water with rain droplets hitting the surface

Can Low Impact Design Structures store rainwater?

Low Impact Design Structures

Global groundwater resources in urban, coastal environments are highly vulnerable to increased human pressures and climate variability. Impervious surfaces, such as buildings, roads, and parking lots prevent infiltration, reduce recharge to underlying aquifers, and increase contaminants in surface runoff that often overflow sewage systems. To mitigate these effects, cities worldwide are adopting low impact design (LID) approaches to direct runoff into natural vegetated systems such as rain gardens that reduce, filter, and slow stormwater runoff. LID hypothetically increases infiltration and recharge rates to aquifers.

Infiltration and Recharge

Michelle and the team at San Francisco State University, advised by Dr. Jason Gurdak, realized that the effects of LID on recharge rates and quality were unknown, particularly during intense precipitation events for cities along the Pacific coast in response to inter-annual variability of the El Niño Southern Oscillation (ENSO). Using water potential and water content sensors she was able to quantify the current and projected rates of infiltration and recharge to the California Coastal Westside Basin aquifer system. The team compared a LID infiltration trench surrounded by a rain garden with a traditional turf-lawn setting in San Francisco. She says, “Cities like San Francisco are implementing these LID structures, and we wanted to test the quantity of water that was going through them. We were interested specifically in different climate scenarios, like El Niño versus La Niña, because rain events are much more intense during El Niño years and could cause flash flooding or surface pollutant overflow problems.”

Sensors Tell the Story

The research team looked at the differences in the quantity of water that LID structures could allow to pass through. Michelle says. ”The sensors yielded data proving LID areas were effective at capturing the water, infiltrating it more slowly, and essentially storing it in the aquifer.” The team tested how a low-impact development-style infiltration trench compared to an irrigated lawn and found that the recharge efficiency of the infiltration trench, at 58% to 79%, was much higher than that of the lawn, at 8% to 33%.

Rain Gauges Yield Surprises

Though it wasn’t part of the researchers’ original plan, they used rain gauges to measure precipitation, which yielded some surprising data. Michelle comments, “We were just going to use the San Francisco database, but it became necessary to use the rain gauges because of all the fog. The fog brought a lot of precipitation with it that didn’t come in the form of raindrops. As it condensed on the leaves, it provided a substantial portion of the water in the budget, and that was surprising to me. The rain gauge captured the condensate on the funnel of the instrument, so we were able to see that a certain quantity of water was coming in that is typically neglected in many studies.”

Future El Niño Precipitation

Michelle also found some really interesting results regarding El Niño and La Niña. She says, “I did a historical analysis to establish baselines for frequency, intensity, and duration of precipitation events during El Niño and La Niña years. I then ran projected climate data through a Hydrus-2D model, and the models showed that with future El Niño intensities, recharge rates were effectively higher for a given precipitation event. During these events, in typical urban settings, water runs off so fast that only these rain gardens and trenches would be able to capture the rain that would otherwise be lost to the ocean. This contrasts with a La Niña climate scenario where there’s typically less rain that is more diffuse. Most of that rain may eventually be lost to evaporation during dry years. So using sensors and 2D modeling we have validated the hypothesis that LID structures provide a service for storing water, particularly during El Niño years where there are more intense rainstorms.”

Michelle’s research received some press online and also was featured in the AGU EOS Editor’s spotlight. Her results are published in the journal Water Resources Research.

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Aug 22

Author Interview: Soil Physics with Python

The new book Soil Physics with Python: Transport in the Soil-Plant-Atmosphere System written by Dr. Marco Bitteli, Dr. Gaylon S. Campbell, and Dr. Fausto Tomei presents concepts and problems in soil physics as well as solutions using original computer programs.

Soil Physics with Python

In contrast to the majority of the literature on soil physics, this text focuses on solving, not deriving, differential equations for transport. Numerical methods convert differential equations into algebraic equations, which can be solved using conventional methods of linear algebra. Here, Dr. Campbell interviews about this update to his classic book Soil Physics with BASIC.

Why did you write the first book, Soil Physics with BASIC?

Soil physics classes were always frustrating for me because you would spend time writing fancy equations on the chalkboard, and in the end, you couldn’t do anything with them. You couldn’t solve any of the problems because, even though they involved difficult mathematics, the math was still so simplified that it didn’t apply to anything that went on in nature.

When I taught my first graduate soil physics class, I determined that we were going to be able to do something by the time we finished. Luckily, in the mid-1970s, personal computers were being developed, and I realized this was the answer to my problem. Numerical methods could solve any problem with any geometry in it. It wasn’t limited to problems that fit the assumptions needed to derive a complex differential equation. I could write computer programs that simplified the mathematics for the students and teach them how to solve those problems using numerical methods. By the end of the semester, my students would have a set of tools that they could use to solve problems in the real world.

Did this book come from class notes or some other source?

I wrote two textbooks and they both came the same way. When I first started teaching, I had a textbook that was inadequate, so I began writing notes of my own and handing them out to the students. After two years, I turned these notes into An Introduction to Environmental Biophysics. Soil Physics with BASIC came about by the same process, but I enlisted the help of my daughter, Julia, to type it up. It was in the early days of word processing so entering equations was quite difficult. It all went well for her until chapter eight, which was a nightmare of greek symbols. After she finished slogging for days through the material, we somehow lost the chapter. She retyped it, and we lost it again, making her type it three times! We didn’t have spreadsheets then either, so the figures were all hand-drawn by my daughter, Karine.

Red soil in the desert with trees and brush around

Marco [Bitteli] has added two and three-dimensional flow problems, so you can model whole landscapes and water behavior in an entire terrain.

What does Soil Physics with Python add to the conversation?

First, it updates the programming language. BASIC was a language invented at Dartmouth and intended to be a simple teaching language. It was never supposed to be a scientific computer language. Python (13:26.) is a newer language, and there are many open source programs for it, making it a better language to use for science.

Secondly, the old book had one-dimensional flow problems in it for the most part, but Marco [Bitteli] has added two and three-dimensional flow problems, so you can model whole landscapes and water behavior in an entire terrain.

In addition, Dr. Bitteli describes the process and analysis of soil treated as fractals as well as soil image analysis. There are a lot of extensions and updates that weren’t in the original book.

Will it be accessible across all disciplines?

To some extent, different disciplines speak different languages. A soil physicist talks about water potential, and a geotechnical engineer talks about soil suction. Thus, there may be some translation of discipline-specific terms, but it’s intended to be a book that people in the plant sciences can use along with people in the soil sciences.

Dr. Marco Bitteli earned his PhD at Washington State University and was Dr. Campbell’s former student. This book is a product of their continued collaboration. Dr. BBitteli is now a professor at University of Bologna, the oldest university in operation in the world. Soil Physics with Python is available at Amazon.com.

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Aug 7

New Medium Scale Soil Moisture Measurement Technique

Between dielectric soil moisture sensors with a volume of influence measured in liters and remote sensing systems which measure soil moisture on the scale of kilometers, there is a gap—a gap Dr. Larry Murdoch of Clemson University has been working to fill. In this post, read about the DELTA (Displacement Extensometer for Lysimetric Terrain Analysis), an instrument that measures water content measurements over an area with a 25 m radius.

Close up picture of cracked and dried soil

Dr. Murdoch became interested in how much water content was in the vadose zone (the unsaturated soil above the water table). He wondered if he could use a strain measuring technique to quantify it.

A New Idea:

Dr. Murdoch was a graduate student in structural geology and geomechanics in the mid-1980s, working on the mechanics of hydraulic fractures in soil. He developed techniques for environmental “fracking” to clean up contaminated soil, long before the recent applications by the oil industry that have caused fracking to become a household word. Fracking causes movements in soil, and Dr. Murdoch developed methods for measuring those movements in order to monitor fracture displacement. This led to work on sensitive borehole extensometers that could measure small strains in rock during well testing.

In the course of his hydrology work, Dr. Murdoch became interested in how much water content was in the vadose zone (the unsaturated soil above the water table). He wondered if he could use the strain measuring technique to quantify it. He decided to bore a hole into the vadose zone and insert a simplified extensometer device that could measure the strain as the soil expands and contracts. This would allow him to gauge the weight change of the overburden. Then, because other mass changes are relatively minor compared to the water in the soil, that weight change would enable him to determine water content.

Since soil compresses more than bedrock, Dr. Murdoch developed a method where he inserted two anchors and cylinders that are pressed up against the soil borehole. In the middle of these cylinders is a fiberglass rod held tight by the bottom anchor which is able to move inside the top anchor. The anchors move up and down from the stress on the soil, and this movement is transferred to the rod where it can be measured with a high-resolution displacement transducer.

Diagram of the DELTA (Displacement Extensometer for Lysimetric Terrain Analysis)

Dr. Murdoch’s device is so sensitive that when it is buried 6 m, it will register clear strain signals as his student walks over it. The weight of a person causes around 50 nanometers of displacement at the Clemson Field site, but the instrument itself can resolve displacement approaching 1 nanometer. And the diameter of measurement on the surface is about 4 times the depth. So if you install the system at 7m, you’d be measuring about a 25 m diameter circle on top.

Like almost all other water content techniques, the challenge is removing all other confounding factors that affect the measurement. It has been said that all sensors are temperature sensors first. Not surprisingly, one thing that causes errors in the system is temperature, though Dr. Murdoch’s team has dealt with that by getting the system deep in the soil and putting the electronics near it so the temperature change is small. Barometric pressure also produces cyclical loading of soil mass and requires correction over a range of periods. And, since the calculation of water content requires an estimate of the soil elasticity, changes in soil moisture also may affect the measurement. Considerable work has been done and significant progress has been made in dealing with these and other issues with the extensometer approach.

picture of a field with a barn in the distance and the ski orange and grey

An advantage of the system is its ability to be buried. In order to plow, for example, all you have to do is pull the sensor up, take off the top plastic casing, and cap it, and the grower can drive a plow over the top.

Strengths:

The amazing thing is that Dr. Murdoch’s system can resolve less than a millimeter of rain water falling on the soil surface, and it can match trends over time. In addition, you can easily calibrate the system by getting your 190-pound student to walk over the top of it and then checking that the compressibility of the soil matches that weight.

Another advantage of the system is its ability to be buried. In order to plow, for example, all you have to do is pull the sensor up, take off the top plastic casing, and cap it, and the grower can drive a plow over the top. Finding the installation can be challenging, so it must be located by precision GPS or survey equipment prior to burial. But, if done correctly, the site can be monitored for long periods of time.

Though not yet a final technology, the Delta extensometer did correlate well with point measurements of water content and shows a lot of promise. The instrument was developed with funding from the National Science Foundation. Colby Thrash, a grad student at Clemson, has done much of the recent work. Dr. Murdoch’s team will publish a paper describing the technique soon in Water Resources Research.

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Jul 31

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Water Potential Versus Water Content

Dr. Colin Campbell, soil physicist, shares why he thinks measuring soil water potential can be more useful than measuring soil water content.

A horsetail plant showing possible signs of guttation where the water potential in the soil overnight is high enough to force water out of the stomates in the leaves.

I know an ecologist who installed an extensive soil water content (VWC) sensor network to study the effect of slope orientation on plant available water. He collected good VWC data, but ultimately he was frustrated because he couldn’t tell how much of the water was available to plants.

He’s not alone in his frustration. Accurate, inexpensive soil moisture sensors have made soil VWC a justifiably popular measurement, but as many people have discovered, a good hammer doesn’t make every soil water problem a nail. I like to compare water potential to temperature because both are considered “intensive” variables that define the intensity of something.

People often try to quantify their own environment, because those measurements define comfort and happiness. Long ago, they discovered they could make an enclosed glass tube, put mercury inside, and infer this intensive variable called temperature from the changes in the mercury’s volume. This was an obvious way to define the comfort level of a human being.

People discovered they could make an enclosed glass tube, put mercury inside, and infer an intensive variable called temperature.

They could have measured the heat content of their surroundings. But they would have discovered that while heat content would be higher in a larger room and lower in a smaller room, you would feel the same comfort level in both rooms. The temperature measurement helps you know whether or not you’d be comfortable without any other variables entering into the equation.

Similar to heat content, water content is an amount. It’s an extensive variable. It changes with size and situation. Consider the following paradoxes:

A soil with fairly low volumetric water content can have plenty of plant-available water and a soil with high water content can have almost none.

Gravity pulls water down through the profile, but water moves up into the soil from a water table.

Two adjacent patches of soil at equilibrium can have significantly different water content.

In these and many other cases, water content data can be confusing because they don’t predict how water moves. Water potential measures the energy state of water and thus explains realities of water movement that otherwise defy intuition. Like temperature, water potential defines the comfort level of a plant. Similar to the room size analogy for temperature, if we know the water potential, we can know whether plants will grow well or be stressed in any environment.

sand with plants poking out and a blue sky in the background

Soil, clay, sand, potting soil, and other media, all hold water differently.

Plants don’t understand the concept of a content in terms of “comfort” because soil, clay, sand, potting soil, and other media, all hold water differently. Imagine a sand with 30% water content. Due to its low surface area, the sand will be too wet for optimal plant growth, threatening a lack of aeration to the roots, and flirting with saturation. Now consider a fine textured clay at that same 30% water content. The clay may appear only moist and be well below optimum “comfort” for a plant due to the surface of the clay binding the water and making it less available to the plant.

Water potential measurements clearly indicate plant available water, and, unlike water content, there is an easy reference scale. We know that plant optimal runs from about -2-5 kPa which is on the very wet side, to about -100 kPa, at the drier end of optimal. Below that plants will be in deficit, and past -1000 kPa they start to suffer. Depending on the plant, water potentials below -1000 to -2000 kPa cause permanent wilting.

So, why would we want to measure water potential? Water content can only tell you how much water you have. If you want to know how fast water can move, you need to measure hydraulic conductivity. If you want to know whether water will move and where it’s going to go, you need water potential.

Learn more

Soil moisture is more than just knowing the amount of water in soil. Learn basic principles you need to know before deciding how to measure it. In this 20-minute webinar, discover:

Water content: what it is, how it’s measured, and why you need it
Water potential: what it is, how it’s different from water content, and why you need it
Whether you should measure water content, water potential, or both
Which sensors measure each type of parameter

Many questions about water availability and movement are best answered by measuring water potential. To find out more, watch any of the virtual seminars below, or visit our new water potential website.

Download the “Researcher’s complete guide to water potential”—>

Water Potential 101: Making Use of an Important Tool

Water Potential 201: Choosing the Right Instrument

Water Potential 301: How to Push Your Instruments Past their Specifications

Water Potential 401: Advances in Field Water Potential

Find out when you should measure both water potential and water content.

Take our Soil Moisture Master Class

Six short videos teach you everything you need to know about soil water content and soil water potential—and why you should measure them together. Plus, master the basics of soil hydraulic conductivity.

Watch it now—>

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Jul 24

Lessons from the Field – Sensor Installation Considerations

In the Midwest, government incentives are sometimes provided to convert marginal lands to switchgrass, a leading choice for bio-energy feedstock production. Michael Wine, a researcher at New Mexico Tech, wanted to investigate whether switchgrass’s deeper root systems would affect the water cycle both during and after crop establishment. In the first stages of his investigation, he learned that many factors need to be considered when determining the optimal location for sensor installation.

Image of Switchgrass in a field with a gray sky in the background

Switchgrass (Image: Plant and Soil Sciences E-library)

Aquifer Recharge

Wine used Gee passive capillary lysimeters to determine deep drainage under natural vegetation, wheat, and switchgrass in order to improve our understanding of both the baseline water cycle and the water budget associated with a switchgrass monoculture in Woodward, Oklahoma. He put the lysimeters and some soil moisture (capacitance) sensors into the Beaver-North Canadian River Alluvial Aquifer to look at recharge, but ran into challenges with sensor installation from the start.

Climate Considerations

One thing Wine learned was that biofuels aren’t very successful in his research location– there wasn’t enough water to support switchgrass. He says, “Most places here may have no precipitation recharge for a great many years. But there are sites, such as sub-humid environments, where you could get a whole lot of infiltration in a very short time.” In hindsight, Wine says he “would have increased his use of preliminary data to more efficiently determine the frequency of recharge events.”

Using Preliminary Data to help Site Instrumentation

Wine learned that it’s important to think about the time constant of your system when siting instrumentation and that preliminary data are crucial. He says, “Before sensor installation, I did a chloride mass balance which helped me determine where I should install the lysimeters.” He had been planning to put them at watersheds at the USDA-ARS Southern Plains Range Research Station, but the chloride mass balance showed there hadn’t been a recharge event at that site in the past 200 years. So he chose to install the lysimeters at the USDA-ARS Southern Plains Experimental Range, located in the Beaver-North Canadian River Alluvial Aquifer, a site with coarser soil and higher permeability.

Wine also thinks numerical modeling could have been useful in determining placement. “In siting the instruments, numerical modeling would’ve been a big help because we could have predicted the likelihood and frequency of recharge events. So I think preliminary data, numerical modeling, and environmental tracers can all help in terms of where to place these research devices.”

a baby calf walking towards the photographer with other cows, who are collectively walking through a field

After long absences, Wine often had to repair damage caused by cattle.

Proximity to Research Site

Another challenge was that the researchers were located in Stillwater, Oklahoma, far from their research site. The experiment was protected by fences, but after long absences, Wine often had to repair damage caused by cattle. “I really need to hand it to these instruments that can be trampled numerous times by cows and the battery compartment filled up with water,” Wine says. “They just needed to be dusted off, dried out, new batteries inserted, and they worked great.” Wine adds that researchers need to consider the distance between their office and their research site because in his case, the cows would have been less of an issue if it had been a fifteen-minute drive instead of three hours each way. He adds, “Selecting a nearby research site would have allowed us additional flexibility in our experimental methods; for example, with a nearby site we could have more easily conducted artificial rainfall simulations if a particular year turned out to be too dry for natural recharge events to occur.”

Proper Siting of Equipment Makes a Difference

Once Wine determined the correct placement of his instruments, he was finally able to get some interesting data. He says, “There are large pulses of focused recharge that do occur in certain places, and we quantified one of those pulses following a storm with one of the lysimeters. We’ve got about a year’s worth of data. Since we installed lysimeters at adjacent upland (diffuse recharge) and lowland (concentrated recharge) sites, we succeeded in observing large differences between the recharge fluxes at these nearby sites.” Wine’s plan is to see if he can replicate the results of the lysimeter experiment using numerical modeling, because he says, “the data look reasonable, but I’d like to confirm the measurements due to the cows playing havoc with our site.” Wine is excited as these lysimeters are yielding the first direct physical measurements of diffuse and concentrated groundwater recharge in the Beaver-North Canadian River Alluvial Aquifer, and he’s optimistic that his numerical modeling will match this unique time series of direct physical measurements of groundwater recharge.

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Jul 5

A History of Thermocouple Psychrometry

Dr. Gaylon S. Campbell gives a short history on his involvement in the development of thermocouple psychometry:

A psychrometer measures wet and dry bulb temperatures of air in order to determine the relative humidity or vapor pressure.

The Original Psychrometers:

I started working with psychrometers in Sterling Taylor’s lab when I was a sophomore at Utah State University in 1960. A psychrometer measures wet and dry bulb temperatures of air in order to determine the relative humidity or vapor pressure. In a conventional psychrometer, a thermometer bulb is covered with a wet wick and measured to find the wet bulb temperature. A thermocouple psychrometer is used to measure the wet bulb temperature of air equilibrated with soil or plant samples. When a plant is at permanent wilting point, its relative humidity is close to 99%, so the whole range of interest for soil and plant measurements is between 99 and 100% RH. The measurements need to be very precise. To make a wet bulb we couldn’t use a wick. We made thermocouples from 0.001” chromel and constantan wires. We cooled the measuring junction of the wires by running a current through it (cooling using the Peltier effect), condensed dew on the wires through the cooling, and then read the wet bulb temperature by measuring the thermocouple output as the water evaporated. We needed to measure temperature with a precision of about 0.001 C.

Diagram of isopiestic psychrometer used to measure the water potential of plant tissue. Image: 6e.plantphys.net

A New Idea:

The original psychrometers we used in Dr. Taylor’s lab were single junctions mounted in rubber stoppers and placed in test tubes in a constant temperature bath. They were calibrated with salt solutions. Typically, before we could finish a calibration, we would break the thermocouple, so we never got data on soils. I found that frustrating, so had the idea of putting the thermocouple in a sample changer which would hold 6 samples. The sample changer went in the constant temperature bath. When it was equilibrated, we could make 6 readings without taking it out or opening it up. Calibration was done in one try, and we could start running soil or plant samples right away. This was a huge improvement. Our lab was one of a very few who could even make those measurements, and we could make them six at a time. That was about 1964.

Two New Businesses Born:

Later, when I was a graduate student at WSU, I started building soil psychrometers for my own research. Other researchers wanted them, so I taught Marv Sherman, a vet student friend to do the manufacturing, and we sold the psychrometers to whoever wanted them for the cost of his time plus materials. There was a sizable and growing demand when he and I graduated, and no one to carry on. My brother Eric came for my graduation. We asked him if he would like to take over the psychrometer business, and he said yes. We sent him home with some instructions and the materials we had left from Marv’s work. Eric built the business himself and then sold it to Wescor, where he and my brother, Evan became employees. I contributed ideas and helped Wescor grow for a few years, but Eric and Evan were not satisfied there and wanted to start a business of their own. We came up with the idea of them building a laser anemometer, and that was the start of Campbell Scientific.

Decagon’s retired SC10/NT3 thermocouple psychrometer

More Improvements:

When we were on sabbatical in England in 1977-78 I had access to a small machine shop and a machinist who was willing to make things for me. The sample changer psychrometers up to this time all had to be used in carefully controlled constant temperature water baths. However, the soil psychrometers that my brother, Eric, sold at Wescor worked fine with no temperature control. I suspected it would be possible to make a sample changer that didn’t need a constant temperature bath. I made some sketches and the machinist made it for me. It had places for 10 samples, had a large aluminum block to hold the rotor with the samples and the thermocouple, and stood on 3 legs. It worked fine without any temperature control.

I showed the new sample changer to my brothers at Campbell Scientific, and they set up and machined a couple of them. CSI didn’t have much interest in selling psychrometers, though, so Decagon began as a way for my children to earn money for college by selling the thermocouple psychrometer sample changer. The name Decagon came both from the 10 people in our family when we started and the 10 samples in the sample changer.

Thermocouple Psychrometry Fades into History:

Decagon (now METER) began selling the thermocouple psychrometer system in 1982 and updated the user-intensive calibration and measurement system to a much simpler one in the mid-1990s. Automation, speed, simplicity, and accuracy soon tipped the scales in favor of a dewpoint technique for measuring water potential, and the system was retired and replaced by a chilled mirror hygrometer (WP4C) in 2000. However, Dr. Campbell believes that thermocouple psychrometers may still have a role to play in measuring water potential. If you’re interested in water potential, check out our water potential pages. It puts many of our best water potential resources in one place and contains sections on theory, measurement methods, and history.

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May 30

This Idea Must Die: Using Filter Paper as a Primary Method for Water Potential

In a continuation of our popular series inspired by the book, This Idea Must Die: Scientific Problems that are Blocking Progress, Dr. Gaylon S. Campbell relates a story to illustrate the filter paper method, a scientific concept he thinks impedes progress:

Folded old paper sitting on a wooden table with a gold antique pocket watch and pen laying in top

There are times when our independent verification turns out to be like the clock and the whistle, and we end up inadvertently chasing our tail.

I remember listening to a story about a jeweler who displayed a big clock in the front window of his store. He noticed that every day a man would stop in front of the store window, pull out a pocket watch, set the watch to the time that was on the large clock, and then continue on. One day, the jeweler decided to meet the man in order to see why he did that. He went out to the front of the store, intercepted the man, and said, “I noticed you stop here every day to set your watch.”

The man replied, “Yes, I’m in charge of blowing the whistle at the factory, and I want to make sure that I get the time exactly right. I check my watch every day so I know I’m blowing the whistle precisely at noon.”

Taken aback, the jeweler replied, “Oh, that’s interesting. I set my clock by the factory whistle.”

The Wrong Idea:

In science, we like to have independent verification for the measurements we make in order to have confidence that they are made correctly, but there are times when our independent verification turns out to be like the clock and the whistle, and we end up inadvertently chasing our tail. I’ve seen this happen to people measuring water potential (soil suction). They measure using a fundamental method like dew point or thermocouple psychrometry, but then they verify the method using filter paper. Filter paper is a secondary method—it was originally calibrated against the psychometric method. It’s ridiculous to use a secondary method to verify an instrument based on fundamental thermodynamics.

Tunnel looking up from the bottom with square holes in the sides going up to the top

Geotechnical engineers use natural material such as soil and rock in combination with engineered material to design dams, tunnels, and foundations for all kinds of structures.

Where the Filter Paper Method Came From:

Before the development of modern vapor pressure measurements, field scientists needed an inexpensive, easy method to measure water potential. I.S. McQueen in the U.S. Geological Survey and some others worked out relationships between the water content of filter paper and water potential by equilibrating them over salt solutions. Later, other scientists standardized this method using thermocouple psychrometers so that there was a calibration. Filter paper was acceptable as a kind of a poor man’s method for measuring water potential because it was inexpensive, assuming you already had a drying oven and a balance. The thermocouple psychrometer and later the dew point sensor quickly supplanted filter paper in the field of soil physics. However, somewhere along the line, the filter paper technique was written into standards in the geotechnical area and the change to vapor methods never occurred. Consequently, a new generation of geotechnical engineers came to rely on the filter paper method. Humorously, when vapor pressure methods finally took hold, filter paper users became focused on verifying these new fundamental methods with the filter paper technique to see whether they were accurate enough to be used for water potential measurement of samples.

What Do We Do Now?

Certainly, there’s no need to get rid of the filter paper method. If I didn’t have anything else, I would use it. It will give you a rough idea of what the water potential or soil suction is. But the idea that I think has to die is that you would ever check your fundamental methods (dewpoint or psychrometer) against the filter paper method to see if they were accurate. Of course they’re accurate. They are based on first principles. The dew point or psychrometer methods are a check to see if your filter paper technique is working, which it quite often isn’t (watch this video to learn why).

Which scientific ideas do you think need to be revised?

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May 9

Founders of Environmental Biophysics Series: Sterling Taylor

Gaylon Campbell’s first experience with environmental measurement came in the lab of Dr. Sterling Taylor at Utah State University, where he was asked to make water potential measurements in order to understand plant water status. What he learned with Dr. Taylor became the start of four scientific companies and gave Dr. Campbell the tools and the confidence to become one of the world’s foremost authorities on physical measurements in the soil-plant-atmosphere continuum. Here’s what Dr. Campbell had to say about his association with Dr. Taylor:

Sterling Taylor 1918-1967 Image: dlscience societies.org

Who was Sterling Taylor and why is he considered one of the Founders of Environmental Biophysics?

Sterling Taylor was professor of Soil Physics at Utah State University. He did his undergraduate work at what was Utah State Agricultural College, and earned his PhD at Cornell University. He worked on both theoretical and practical problems in soil physics. His practical work focused on research in the area of plant-water relations and irrigation management. Dr. Taylor worked out water potential limits for both maximum and reduced growth rates of crops. The irrigation limits tables that he put together are still used in today’s handbooks. His theoretical contributions were on linked transport and applications of non-equilibrium thermodynamics to soil physics, which he was working on at the time of his death. Dr. W. H. Gardner, a soil physicist of the time, called the amount of work Dr. Taylor and his students did “unparalleled” and noted that attendees at regional conferences often had to carry Taylor’s “weighty reports” home as overweight baggage.

Attendees at regional conferences often had to carry Taylor’s “weighty reports” home as overweight baggage.

What was your association with him, and how did he influence your life and your science?

Sterling was a kind of second father to me and to many other young scientists. He loved to help boys and teach them what their potential was. At that age, I didn’t have any idea that I could do anything in science. The first assignment he gave me was to set up an experiment to measure the simultaneous movement of salt and water in soil. I had no idea what I was doing, and it was a challenging project. It would be challenging for me to do it right now! But he’d give me ideas about how to do the next thing, I’d try to do it, and eventually I got some data that he thought was useful. He did some analysis of it, and that’s how I learned to measure electrical conductivity and salt concentration in water and soil. Sterling’s lab is also where my brother Eric and I learned how to make thermocouple psychrometers and other instruments for environmental measurements. Those insights led directly to the start of Wescor and Decagon. Campbell Scientific, Juniper systems and others eventually came from those beginnings.

Dr. Taylor was also a very patient man. He made a precision constant temperature bath out of an old washing machine. It had an agitator in the middle to stir the water while cooling it with coils around the outside of the tub. It was a wonderful setup, and he took a lot of pride in how well it worked. He came into the lab one day while I was making some modifications to it. I was drilling a hole through the outer jacket around the Freon(™) coils where the refrigerant ran. He said, “Now be careful if you’re drilling holes through that thing so you don’t hit the coils”. And I said, “Yes, I’m being careful.” But I wasn’t. The coils were a couple of inches apart, and I thought, There’s no way I’m going to hit one. I didn’t even get a ruler. I just eyeballed it, drilled a hole, and hit the tube dead on. I couldn’t have hit it more perfectly if I’d measured as carefully as I could. All the refrigerant came hissing out, and I thought he would hear it over in his office. He probably did hear it, but he didn’t come out to see what was going on. One of the hardest things I ever did in my life was to go in and tell him I’d drilled a hole in his refrigerant tube. He just said, “Well…I guess we’ll have to get some new refrigerant.” He was just patient, and knew how to work with young people.

Student Examining a Textbook Reading the Pages at a Desk in a Classroom

I made a career choice to be a teacher and have students.

But that wasn’t the only way he influenced me. As it came time for graduation he gave me some advice that had an enormous impact. Once when I was trying to choose between soil physics and medical biophysics he said “do you want to be a little duck in a big puddle or a big duck in a little puddle?” I decided on the little puddle. On another occasion, I was wondering what kind of soil physics position would be best. One of his former students had gotten a job at an experiment station near Kimberly, Idaho, and I thought that would be ideal. He observed, “Those can be fun jobs, but if you go to a position like that you just don’t have any offspring.” That resonated with me, and I thought, “I would like to have offspring.” So I made a career choice to be a teacher and have students. It was wonderful to have had that kind of advice at that critical time.

What do you think we missed because he died so early?

It’s interesting to think about scientific contributions and other types of contributions people make. One of my sons gave me a book of science cartoons, and one of those cartoons shows a couple of scientists talking together. One of the scientists says to the other, “Isn’t it sad to think that everything we come up with now will be disproved in 20 years?”

It just shows you what a transient thing our work is. We think it’s so important, but the important contributions that Sterling made were the numbers of people that he influenced so profoundly. I’m not the only one he was a second father to. Sterling Taylor had a huge family of students. Many went on to prestigious institutions like CalTech (California Institute of Technology), making important contributions over their careers. And they trace it back to Sterling’s influence on them.

How can scientists today emulate the great man that he was?

I think it would be to not take science so seriously but to take interactions with their fellow travelers seriously. There is a quote by Clayton Christensen from an article in Harvard Business Review on how to emulate what Sterling Taylor was. Christensen says, “I’ve concluded that the metric by which God will assess my life isn’t dollars but the individual people whose lives I’ve touched. I think that’s the way it will work for us all. Don’t worry about the level of individual prominence you have achieved; worry about the individuals you have helped become better people. This is my final recommendation: Think about the metric by which your life will be judged, and make a resolution to live every day so that in the end, your life will be judged a success.”

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May 2

2 Comments

Small Company, Big Mission: The Phoenix Mars Lander & TECP Sensor

On May 25, 2008 NASA’s Phoenix Lander successfully landed on the surface of Mars and used a robotic scoop arm to deliver regolith samples to the suite of instruments on the deck of the Lander—with one exception. The Thermal and Electrical Conductivity Probe (TECP), designed by a team of Decagon (now METER) research scientists, was mounted on the knuckle of the robotic arm and made direct contact with the regolith. It measured thermal conductivity, thermal diffusivity, electrical conductivity, and dielectric permittivity of the regolith, as well as vapor pressure of the air.

But, that’s starting at the end of the story. The fact is that TECP almost didn’t get started. After seeing a thermal properties needle at the American Geophysical Union meeting in San Francisco, Mike Hecht (project leader on the Mars Environmental Compatibility Assessment (MECA) instrument suite) encouraged his colleague Martin Buehler to call Decagon (now METER) to see if we’d be willing to participate in the Phoenix Lander project. When Martin called one Friday afternoon, announcing that he was from JPL and wondering if we would be willing to fly our sensor on the Phoenix Lander, I was instantly intimidated. I knew JPL was associated with NASA, and I couldn’t imagine why they would be calling Decagon. I always thought there was a fundamental relationship between NASA and Lockheed Martin, Northrop Grumman, and other major companies that did NASA work. I told him that Decagon, which was much smaller in those days, didn’t have the capacity to develop instrumentation for space flight. He suggested they come up for a visit and at least consult with us on what they would need to do to obtain this measurement. The following Monday, we were talking Martian science and inexorably hooked on the idea of joining the team.

I knew JPL was associated with NASA, and I couldn’t imagine why they would be calling Decagon.

Deciding to put one of our sensors on Mars did nothing to lessen the intimidation factor. But, working with Mike and his team at JPL/NASA taught us that doing amazing science can be an inspiring and collaborative effort. I’d always imagined NASA as a group of uber-scientists and engineers sitting in glass offices dreaming up and executing great projects that would be impossible for mere mortals. The reality is that sending something to Mars and having it do real science requires the combined effort of thousands of smart, dedicated people who are not that much different from the rest of us.

This idea was really brought home when we finally visited JPL. Although the things they were doing were amazing and on a much grander scale, they weren’t that much different from the things we do at Decagon. They had testing facilities, development facilities, production facilities, and support personnel all working together on projects, just like us. However, the projects were pretty amazing. We watched the robot arm being tested in a lab for the ability to dig martian soil analogs. We observed an ice probe working in a 55-gallon drum trying to prove it could melt its way down through the thick Martian polar ice caps. We were mesmerized by prototypes of Mars rovers being programmed and executing maneuvers on Martian surface analogs.

It was fun to discover who the Jet Propulsion Lab is and how enjoyable it is to collaborate with people that are thinking about new applications of technology. This collaboration also benefitted METER’s thermal properties instrument because the mathematical models we developed for Mars made this sensor much more accurate and effective. The Mars project expanded both the depth of our understanding and the breadth of our perspective. Even so, it was fun to find out that scientists who work at JPL have to put their pants on one leg at a time, just like all of us.

Watch this virtual seminar where Dr. Mike Hecht talks Mars, poetry, and Decagon’s (now METER’s) involvement in the Mars Phoenix Lander Mission.

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Measuring and Modeling the Environment

Posts from the ‘Soils’ Category

Low Impact Design Structures

Infiltration and Recharge

Sensors Tell the Story

Rain Gauges Yield Surprises

Future El Niño Precipitation

Why did you write the first book, Soil Physics with BASIC?

Did this book come from class notes or some other source?

What does Soil Physics with Python add to the conversation?

Will it be accessible across all disciplines?

A New Idea:

Strengths:

Learn more

Take our Soil Moisture Master Class

Aquifer Recharge

Climate Considerations

Using Preliminary Data to help Site Instrumentation

Proximity to Research Site

Proper Siting of Equipment Makes a Difference

The Original Psychrometers:

A New Idea:

Two New Businesses Born:

More Improvements:

Thermocouple Psychrometry Fades into History:

The Wrong Idea:

Where the Filter Paper Method Came From:

What Do We Do Now?

Who was Sterling Taylor and why is he considered one of the Founders of Environmental Biophysics?

What was your association with him, and how did he influence your life and your science?

What do you think we missed because he died so early?

How can scientists today emulate the great man that he was?