Posts tagged ‘water potential’

Apr 16

Do the Standards for Field Capacity and Permanent Wilting Point Need to Be Reexamined?

We were inspired by this Freakonomics podcast, which highlights the book, This Idea Must Die: Scientific Problems that are Blocking Progress, to come up with our own answers to the question: Which scientific ideas are ready for retirement? We asked METER scientist, Dr. Gaylon S. Campbell, which scientific idea he thinks impedes progress. Here’s what he had to say about the standards for field capacity and permanent wilting point:

Canola Field right next to an eroded soil cliff

A layered soil, a soil that has a fine-textured horizon on top of a coarse-textured soil, will hold twice as much water as you’ll predict from the -⅓ bar value.

Idea:

The phrase, “this idea must die,” is probably too strong a phrase, but certainly some scientific ideas need to be reexamined, for instance the standard of -⅓ bar (-33 kPa) water potential for field capacity and -15 bars (-1500 kPa or -1.5 MPa) for permanent wilting point.

Where it came from:

In the early days of soil physics, a lot of work was done in order to establish the upper and lower limit for plant available water. The earliest publication on the lower limit experiments was by Briggs and Shantz in 1913. They planted sunflowers in small pots under greenhouse conditions, letting the plants use the water until they couldn’t recover overnight, after which they carefully measured the water content (WC). The ability to measure water potential came along quite a bit later in the 1930s using pressure plates. As those measurements started to become available, a correlation was found between the 15 bar pressure plate WCs and the WCs that were determined by Briggs and Shantz’s earlier work. Thus -15 bars (-1.5 MPa) was established as the lower limit of plant available water. The source of the field capacity WC data that established a fixed water potential for the upper limit is less clear, but the process, apparently, was similar to that for the lower limit, and -⅓ bar was established as the drained upper limit water potential in soil.

Briggs and Shantz planted sunflowers in small pots under greenhouse conditions, letting the plants use the water until they couldn’t recover overnight, after which they carefully measured the water content (WC).

Damage it does:

In practice, using -15 bars to calculate permanent wilting point probably isn’t a bad starting point, but in principle, it’s horrible. Over the years we have set up experiments like Briggs and Shantz did and measured water potential. We have also measured field soils after plants have extracted all the water they can. Permanent wilting point never once came out at -15 bars or -1.5 MPa. For things like potatoes, it was approximately -10 bars (-1 MPa), and for wheat it was approximately -30 bars (-3 MPa). We found that the permanent wilting point varies with the species and even with soil texture to some extent.

Of course, in the end it doesn’t matter much as the moisture release curve is pretty steep on the dry end, and whether you predict it to be 10 or 12% WC, it doesn’t make a huge difference in the size of the soil water reservoir that you compute.

However, on the field capacity end of the scale, it matters a lot. If you went out and made measurements of the water potentials in soils a few days after a rain, it would be an absolute accident if any of them were ever -⅓ bar (-33 kPa). I’ve never seen it. A layered soil, a soil that has a fine-textured horizon on top of a coarse-textured soil, will hold twice as much water as you’ll predict from the -⅓ bar value. On the other hand, if you’re getting pretty frequent rains or irrigation, that field capacity number becomes irrelevant. Thus, -⅓ bar may be a useful starting point for determining field capacity, but it’s only a starting point.

Why it’s wrong:

Field capacity and permanent wilting point are dynamic properties. They depend on the rate at which the water is being extracted or the rate at which it’s being applied. They also depend on the time you wait to sample after irrigation. Think of the soil as a leaky bucket. If you were trying to carry water in a leaky bucket and you walked slowly, the bucket would be empty by the time you get the water where you want it. However, if you run fast, there will still be some water left in the bucket. Similarly, if a plant can use water up rapidly, most of it will be intercepted, but if a plant is using water slowly, the water will move down past the root zone and out the bottom of the soil profile before the plant can use it. These are dynamic phenomena that you are trying to describe with static variables. And that’s where part of the problem comes. We need a number to do our calculations with, but it’s important to understand the factors that affect that number.

Rye field

What do we do now:

What I hope we can do is better educate people. We should teach that we need a value we call field capacity or permanent wilting point, but it’s going to be a dynamic property. We can start out by saying: our best guess is that it will be -⅓ bar for finer-textured soils and -1/10 bar (-10 kPa) for coarser-textured soils. But when we dig a hole and find out there is layering in the profile or textural discontinuities, we’d better adjust our number. If we’re dealing with irrigated farmland, the adjustment will always be up, and if we’re dealing with dryland or rain-fed agriculture where the time between water additions is longer, we’ll use a lower number.

Some Ideas Never Die:

One of the contributors to the book, This Idea Must Die, Dr. Steve Levitt, had this to say about outdated scientific ideas, and we agree: “I love the idea of killing off bad ideas because if there’s one thing that I know in my own life, it’s that ideas that I’ve been told a long time ago stick with me, and you often forget whether they have good sources or whether they’re real. You just live by them. They make sense. The worst kind of old ideas are the ones that are intuitive. The ones that fit with your worldview, and so, unless you have something really strong to challenge them, you hang on to them forever.”

Harness the power of soil moisture

Researchers measure evapotranspiration and precipitation to understand the fate of water—how much moisture is deposited, used, and leaving the system. But if you only measure withdrawals and deposits, you’re missing out on water that is (or is not) available in the soil moisture savings account. Soil moisture is a powerful tool you can use to predict how much water is available to plants, if water will move, and where it’s going to go.

In this 20-minute webinar, discover:

Why soil moisture is more than just an amount
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

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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Mar 16

The History and Future of Water Potential

I often hear researchers complain about the accuracy of our TEROS 21 water potential sensors. We still have room to improve, but we’ve certainly come a long way! People have been attempting to make water potential measurement in the field for over 100 years. The following is a brief overview of the evolution and history of water potential measurements over that time.

Pre-MPS-1 prototype.

Livingston Discs

The Livingston disc, developed in 1908, was one of the first attempts at determining water potential in the field. The Livingston Disc was actually a primitive, manual version of the technology used in our MPS6 ceramic disc. Here is how it worked: first, you’d weigh the dry disk, then put it in the soil and let it equilibrate. After that, you would dig it up and weigh it again. Using the water retention curve of the disc, you could then determine the water potential.

Gypsum block

In the 1940s gypsum block sensors were invented as the first solid matrix equilibrium technique for water potential. This method tried to continuously sense water potential with a simple electrical conductivity measurement in a solid porous (and naturally occurring) gypsum matrix. However, because naturally occurring gypsum doesn’t have a consistent pore size distribution and it degrades over time, the instrument was not very accurate.

In the 1940’s gypsum block sensors were invented as the first solid matrix equilibrium technique for water potential. Image: www.soilmoisture.com

Tensiometers

In the 1960’s a liquid equilibration technique called tensiometry was discovered that allowed water potential measurement with good accuracy even in the presence of positive pore water pressures. Tensiometers work extremely well in wetter soils with water potentials between 0 and -80 kPa and should be the choice for all wet soil applications, especially above -9 kPa where the MPS6 will not work (the air entry value for its ceramic is -9 kPa). However, when the soil dries out the water under tension in the tensiometer eventually cavitates, causing the output to be useless until they are refilled. Thus solid equilibrium techniques like the TEROS 21 are the best choice across the dry range.

Tensiometers are the most accurate way to measure water potential in the field in the wet range, but are limited to the plant optimal range of about -100 kPa and above.

The Evolution of Ceramic Discs

We learned with the gypsum blocks that one of the challenges in solid matrix water potential measurement is finding a material that will create the same water retention curve every time. In quest of this goal, the ceramic discs in sensors like the TEROS 21 have taken years of development. Because of the limited range of the tensiometer, we wanted to develop a water potential sensor that could measure over a larger range. The hardest part about developing that ceramic was getting a variety of pore sizes so the instrument could read said wide range of water potentials. This started years ago in the lab of Dr. Gaylon Campbell at Washington State University where his technician, Kees Calisendorf, experimented over a long period of time to come up with the perfect recipe.

The MPS1 was our original matric potential sensor released in 2001. It allowed for long-term monitoring in the field because, unlike gypsum, the ceramic did not degrade over time.

Even after we found a consistent ceramic, there were still outliers. So creating a calibration method was essential to making an accurate sensor. The first challenge was to be able to store calibration points in the sensor, which required a microprocessor. The second, and more difficult task, was to establish a method to calibrate large numbers of sensors at once. We tried many different approaches like pressure plate, equilibration over salt solutions, and even centrifugal force, but nothing worked. Finally, in a discussion with our partner, UMS, we discovered the key. We now can accurately calibrate 50 sensors at a time in only 12 hours. Still, even with these advanced techniques, we only have a sensor with an accuracy of plus or minus 10%, but considering the history of how hard it’s been to develop consistent ceramic, this accuracy is exciting for the range that we can get.

The MPS 2 was our second matric potential sensor which offered two-point calibration and a temperature sensor, improving accuracy.

What’s Next?

Now that we’ve created a reliable calibration method, we can turn our attention toward further improving the sensor measurement range as well as its accuracy. Testing different ceramics, or other porous media, may hold the key to a solid equilibrium technique sensor reading all the way to 0 kPa, eventually replacing the need for tensiometers in the field.

The two key innovations in the MPS6 (now called TEROS 21), released in 2014, are the addition of a microprocessor to the sensor and fast, accurate equilibration at multiple points.

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Feb 11

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Founders of Environmental Biophysics Series: John Monteith

We interviewed Gaylon Campbell, Ph.D. about his association with one of the fathers of environmental biophysics, John Monteith.

John Lennox Monteith, image:agrometeorology.org

Who was John Monteith?

John Monteith was a professor at the University of Nottingham in England and one of the founders of modern environmental biophysics. He pioneered the application of physical principles in the study of how plants and animals interact with their immediate environment. He started his career at Rothamsted Experimental Station in Harpenden, England and was hired as professor at Nottingham in the early 1970’s. He went on to spend time at the International Crops Research Institute for Semi-Arid Tropics (ICRISAT) in India. He published a textbook that has been a foundation for Environmental Biophysics, called Principles of Environmental Physics. He was elected a member of the Royal Society of London, which is the highest scientific distinction a person can receive in the UK. He was also a member of the Royal Meteorological Society and was its president in 1978. These societies are both sponsored by the crown, and he told me on the occasion that he was installed as the president of the Royal Meteorological Society, the queen attended and he sat by her at dinner. He is known for the Penman-Monteith equation that has become the basis for guidelines for estimating irrigation water requirements used by the FAO (Food and Agriculture Organization of the United Nations).

How did you meet him?

As an undergraduate, I knew of John because I worked for a professor at Utah State University (Sterling Taylor), who was measuring water potential in soil using thermocouple psychrometers. I was keenly interested in the subject, so Dr. Taylor gave me a paper on thermocouple psychrometers to read, published in 1958 by Monteith and Owen, written while John was at Rothamsted. John’s work there was influential in developing instrumentation which formed the foundation for Wescor, METER, and several other companies.

When Prof. Monteith’s book came out, it was pretty exciting for me, because it had everything in it that I was trying to teach as a professor of Soil Science. I wrote to John in 1977 inquiring about the possibility of doing a sabbatical there, and he wrote back immediately and arranged for us to come. Amazingly, he and his technician met our big family at Heathrow airport and loaded up the whole crew, including our many duffel bags, into a university minibus. A couple of our bags were missing, and John picked them up from the railway station in Nottingham and delivered them to us the next day. I have often marveled that such a busy and important man would take the time to care for us like that.

A sunflower field in Karnataka, India

What was he like as a colleague?

He was a humble man in a lot of ways. After he passed away, one of his colleagues wrote in and told about some of the experiences he’d had with John in India. India has a pretty hierarchical society, and it’s not uncommon for somebody who is in a position of authority to take advantage of that. John was in charge of one of the big groups within ICRISAT, and the thing that impressed his colleague was that whoever came into John’s office was treated with great respect, whether it was the cleaning person or the lab technician. If they had come to see him, they got the same treatment and the same respect that the director of the lab got.

We worked on a lot of projects together, but the proposal we submitted that was funded was one on improving thermocouple psychometry. I wrote up the paper, but he had written the proposal and provided the funding for the work. I put him down as an author on the paper, and when I got ready to submit it, he went over the paper just as if he were an author and then crossed his name out. He said he hadn’t contributed enough. Well, he contributed way more than most authors do, but he had a set of standards that he expected himself to meet and his contributions to that paper hadn’t met those standards. He was pretty amazing that way.

How did he get to be a part of the Penman-Monteith Equation?

Penman was head of the research group at Rothamsted Experimental Station which Monteith joined, following graduation. Penman was already an established researcher by the time Monteith got there, and the Penman equation was already well known. But, Monteith worked with that equation, and in my opinion, improved it substantially. He never wanted to take credit for that. He always claimed that Penman already understood the things he had added, and he never did call it a Penman-Monteith equation, always referring to it as the Penman equation. But I have never read things of Penman’s that indicated that he had anywhere near the depth of understanding of the equation that Monteith had. To my way of thinking, it’s completely appropriate that his name is associated with it.

What was John’s secret to accomplishing all he did, and how can scientists today emulate his meaningful career?

His gift was the gift of clear thinking. I gave a talk about him a while ago entitled “Try a Straight Line First.” John hated the complexity of modern computer models for crop growth because he couldn’t easily see the end from the beginning in those models. He had the ability to look at a problem, no matter how complex, and just reach in and grab the essence of that problem and show it to you. He used to talk about Occam’s Razor and not multiplying complexity. Einstein was supposed to have said, “Everything should be as simple as possible, but not simpler.” John was always able to find a simple way to look at problems. It may have been a complex process to get there, but once he was done, you had something that you could manipulate. I think simplicity and uncluttered thinking would be the thing to emulate.

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Feb 2

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Learn to Measure Water Potential at a Bodentag

One of the best parts of my job is the opportunity I get to teach others about the science and technique of measurement. For more than 10 years, I have participated in seminars and workshops all over the world to do just that. But, a couple of months ago, I had my first opportunity to work with my good friend Georg von Unold (METER Ag) to do a Bodentag workshop, German-style. I learned a lot from my experience, and I think the participants did as well.

UMS’s Georg von Unold with his backhoe, digging a permanent soil observation pit in the Black Forest

A Bodentag (meaning “soil day” in German) is an unusual opportunity for the attendees to get practical hands-on teaching and training from the people who understand soil and environmental instrumentation. In a typical conference, you will not get a chance to do things under field conditions. Instead of sitting in a conference room all day, a Bodentag starts with presentations to set the stage with the theory and principles of measurement, but quickly moves to the lab and field to get the participant’s hands dirty. With the diversity of measurements required for today’s multidisciplinary research, there is great value in structured field installation familiarity.

Our trip to Freiburg was a great example of how a Bodentag works. Preparation started early in the morning the day before as Georg used his large Mercedes Sprinter van full of equipment to tow his Bobcat excavator for more than five hours on our drive from Munich. When we got there, we were directed to a nearby site in the Black Forest where we used the excavator to dig a permanent soil observation pit (Georg’s gift to the institute there), complete with a stairwell that allowed people to go and inspect the pit face and install sensors. We prepared other stations to get people to install soil sensors with minimum impact, cut out intact soil columns for a field lysimeter, and remove intact soil cores.

Georg standing in the finished soil observation pit

The day of Bodentag, participants listened to two hours of lecture/presentations in the morning followed by both lab and field practicum sessions. During the field practicum, attendees could do actual installations of sensors into pit faces. This was useful because there were several researchers there who had Black Forest research sites, and they could look at and ask questions about the challenges of the rocky soil pervasive in that region. We used augers to dig holes to install Decagon sensors so everyone could see how that was done. Georg had one of his Smart Field Lysimeters out there and did a half-field installation. He showed them how to dig the Smart Field Lysimeter down into the soil, scrape the soil off, and actually collect a monolith right there.

After the outdoor practicum session, we went back to the lab where we broke up into small groups. There, people had an opportunity to go see laboratory instrumentation while learning some best practices for making measurements. In mine, people were using the WP4C water potential instrument to figure out the permanent wilting point of the soil that we brought. Attendees also got some careful training on the Hyprop to measure the wet end of the moisture release curve as well as learning about the KSAT, a METER instrument which measures saturated hydraulic conductivity. Because Bodentag is an opportunity to share ideas, we also got a chance to see the multi-step outflow instrumentation developed over the past 20 years by the Forest Research Center there in Freiburg that they use to create soil moisture characteristic curves.

2014 Bodentag attendees

At the end of the day, everyone was exhausted, and we still had a five-hour drive left to get back to Munich. But, everyone had a great time, and the students and researchers who were there learned enough so they could be confident when using an instrument to get the data they need in an experiment. It was a unique opportunity for me to see how to put together a great educational experience, and I am excited to try one here in the U.S. sometime soon: especially if I can run the excavator again!

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Dec 22

Measuring Osmotic Sap Water Potential

Sometimes networking with new scientists at conferences and workshops can pay dividends in terms of new ideas. Steve Garrity and I recently attended and taught practicum sessions at the PEPg (Plant Environmental Physiology group) Ecophysiology Workshop. The mission of this workshop was twofold: to invite the world experts on plant physiology measurements to come and lecture, and to invite the manufacturers to teach about instrumentation and provide hands-on training.

Workshop participants check the water potential of soil with a UMS T5 mini-tensiometer.

With three sessions per day using METER instrumentation and only two of us, neither Steve nor I could teach about leaf water potential using the WP4C chilled mirror dew point instrument. So, we asked another scientist who is an expert in plant water relations to teach it for us. Not only did he do a great job of teaching about measuring leaf water potential using a hygrometer, but he also inspired us to take another look at how to make this measurement as we learned about its importance to his research (to learn more about how to do this, watch our virtual seminar).

He’s developed a procedure where you can freeze the leaf and break all of the cells so you are left with the cell water (the symplastic water).

Later in the conference, this same scientist gave a talk about the importance of osmotic potential. He’s developed a procedure where you can freeze the leaf and break all of the cells so you are left with the cell water (the symplastic water). He was able to squeeze that sap out and test it in a thermocouple psychrometer, where he established a relationship between how tolerant plants are for drought and what their osmotic sap water potential (turgor loss point) was. We have made many of those sap measurements but had not used them in this manner. That’s really interesting to us at METER because we were unaware of this relationship, and we have now found another use for osmotic potential measurements in leaves.

We would never have realized this new idea without the help of our colleague. Meeting with other scientists at conferences and talking over ideas can be really important. Have you ever struck gold in terms of coming up with new ideas for research, funding, or inventing new research tools at a conference you’ve attended?

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Dec 17

Dr. Gaylon S. Campbell Author Interview

METER’s founder, Dr. Gaylon S. Campbell was born in Blackfoot, Idaho, and grew up on a dry farm in Juniper, Idaho. He went to school in Logan, Utah, finally attending Utah State University where he received a B. S. in Physics in 1965 and an M. S. in Soil Physics in 1966. He was granted a Ph. D. in Soil Physics from Washington State University in 1968. He became an officer in the U. S. Army in 1969, doing meteorological research at White Sands Missile Range, New Mexico. In 1971 he returned to Washington State University as Assistant Professor of Biophysics and Assistant Soil Scientist. There he taught and did research in Environmental Biophysics and Soil Physics until 1998. Since 1998 he has worked as vice president, engineer, and scientist at Decagon Devices, Inc (now METER). He has written three books, over 100 refereed journal articles and book chapters, and has several patents. Today we are interviewing him about his book, An Introduction to Environmental Biophysics.

Dr. Campbell is the author of An Introduction to Environmental Biophysics

Where did you get the knowledge to write the book?

I was hired to teach Environmental Biophysics at Washington State University in 1971, and when I looked around for a textbook to go with the class, there weren’t any that fit very well. I knew what I wanted to teach in the class, and some of the principles were in books that were available, but a lot weren’t. So I started writing up notes to hand out to the students and then improved them over time.

One of the important sources of knowledge for my book was John Montieth’s book, Principles of Environmental Physics. Its first edition came out in 1973. It’s a wonderful book. I didn’t know about it until one of my students brought it into class and let me borrow it overnight.

I went home and started reading it. I read it all night, and by morning I’d finished it. I have read some novels that could keep me awake all night, but that’s the only science book I ever read that could do it.

I was really excited about his approach because it was perfect for what I wanted to do in the class. However, it was at a different level than I needed, so I went ahead and developed my own notes, but his book certainly was an important source.

I started writing up notes to hand out to the students and then improved them over time.

How difficult was it to understand the theory behind what you were writing about?

When I’d take a class in school, I felt like I never understood what was in that class until I attended the next class. Then when I got a bachelor’s degree, I thought, I hope nobody expects me to know something just because I have this degree, because I don’t feel like I know anything. I hoped when I earned a masters degree that it would be better, but I got there and thought, oh boy, I still don’t know anything. It was probably when I took my prelim exam that I finally felt confident enough that I could be a soil physicist if I had to.

But I was wrong about that. I really didn’t understand physics very well, even then. It was when I had to teach it that the real understanding came. When I understood it well enough to lecture about it was when I felt like I had really mastered the theories and understood them at the level that I wanted to.

I suppose that came one piece at a time. In the beginning, I certainly didn’t understand things as well as I did later on. And that still happens today. I learn things that I hadn’t understood before. So I guess when you ask how hard it was: it was an ongoing process. Even when somebody’s already laid it out for you, it doesn’t mean you’re going to understand it. But when you lecture about it and write about it, those are the processes that help to deepen your knowledge and understanding.

When you lecture about a subject and write about it, those are the processes that help to deepen your knowledge and understanding.

The subject is extremely complicated, but people are always saying how easy it is to understand environmental biophysics from your book. How did you bring it down to the level of the students?

When I was in the Army, the philosophy they had was, “If the student hasn’t learned, the teacher hasn’t taught.” That was not the philosophy that you normally encountered at the university. Many professors complained often about how lousy their students were. I never found it to be that way. I always thought my students were getting better and better.

I think it comes down, to some extent, to the philosophy the teacher has. We often see teachers come in and fill the board with equations and wonder why their students don’t understand them. But it’s likely the teacher hasn’t looked at it from the standpoint of the students. The student is going to gain understanding by the same path the teacher did. Professors work and work to put together a wonderful picture of things, and once they have that wonderful picture, they tend to want to dump the whole thing on the student. But students can’t assimilate the whole picture all at once. They have to go step by step too.

If people wanted to learn from your book, what is the best way to get the principles down?

It’s no accident that there are lots of both worked examples and problems for students to solve. I don’t think you can learn physics without solving problems, and so the best way to do it is to look through the ones that we’ve solved in the book and then look through the problems we give at the end of the chapters and solve them. That, I think, is the best way to get there.

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Dec 15

Despite Drawbacks, Scientific Collaboration Pays Off

Though collaboration can fuel innovation and increase the relevance and complexity of the scientific questions we study, I’ve noticed it does have its ups and downs. The highs and lows we’ve run into on our research projects may help others avoid some of the pitfalls we experienced as many diverse groups tried to learn how to work together.

Researchers discussing science at the Lytle Ranch Preserve, a remarkable desert laboratory located at the convergence of the Great Basin, Colorado Plateau, and Mojave Desert biogeographical regions.

There can be bumps in the road when collaborating with companies who want to test their product. Being at the forefront of innovation means that untested sensors may require patience as you work out all the bugs together. But from my perspective, this is part of the fun. If we are late adopters of technology, we wouldn’t get to have a say in creating the sensors that will best fit our projects as scientists.

Collaborating scientists can also sometimes run into problems in terms of the stress of setting up an experiment in the time frame that is best for everyone. During our experiment on the Wasatch Plateau, we had six weeks to get together soil moisture and water potential sensors, but our new GS3 water content, temperature, and EC sensors had never been outside of the lab. In addition, we planned to use an NDVI sensor concept that came out of a workshop idea my father Gaylon had. We’d made ONE, and it seemed to work, but that is a long way from the 20 we needed for a long-term experiment in a remote location at 3000 meters elevation. In the end, it all worked out, but not without several late nights and a bit of luck. I remember students holding jackets over me to protect me from the rain as I raced to get the last sensor working. Then we shut the laptop and ran down the hill, trying to beat a huge thunderstorm that started to pelt the area.

Desert-FMP Researchers at the Lytle Ranch Preserve

Other challenges of scientific collaboration present organizational hardships. One of the interesting things about the interdisciplinary science in the Desert FMP project is the complexity of the logistics, and maybe that’s a reason why some people don’t do interdisciplinary projects. We are finding in order to get good data on the effects of small mammals and plants you need to coordinate when you are sampling small mammals and when you’re sampling plants. Communicating between four different labs is complicated. Each of the rainout shelters we use cover an area of approximately 1.5 m2 . That’s not a lot of space when we have two people interested in soil processes and two people interested in plants who all need to know what’s going on underneath the shelter. Deciding who gets to take a destructive sample and who can only make measurements that don’t change the system is really hard. The interesting part of the project where we’re making connections between processes has required a lot of coordination, collaboration, and forward-thinking.

In spite of the headaches, my colleague and I continue to think of ways we can help each other in our research. Maybe we’re gluttons for punishment, but I think the benefits far outweigh the trouble we’ve had. For instance, in the above-mentioned Desert FMP project we’ve been able to discover that small mammals are influential in rangeland fire recovery (read about it here). We only discovered that piece of the puzzle because scientists from differing disciplines are working together. In our Wasatch Plateau project, my scientist colleague said it was extremely helpful for him to be working with an instrumentation expert who could help him with setup and technical issues. Also, we’ve been able to secure some significant grants in our Cook Farm Project (you can read about it in an upcoming post) and answer some important questions that wouldn’t have occurred to either one of us, if we hadn’t been working together. In addition, solving problems that have cropped up in our projects has spurred us on to a new idea for analyzing enormous streams of data in near-real time. (read about it here).

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Dec 3

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TDR versus Capacitance or FDR

When we talk with scientists at conferences they often want to know the difference between TDR versus capacitance or FDR. We’ve written a paper about this in our app guide that has been pretty popular, but it can be difficult to find on our website. Here is an introduction and a link if you are interested in learning more.

TDR Sensor Installation (Giulio Curioni, School of Civil Engineering, Univ. of Birmingham)

Capacitance and TDR techniques are often grouped together because they both measure the dielectric permittivity of the surrounding medium. In fact, it is not uncommon for individuals to confuse the two, suggesting that a given probe measures water content based on TDR when it actually uses capacitance.

10HS capacitance sensor

With that in mind, we will try to clarify the difference between the two techniques. The capacitance technique determines the dielectric permittivity of a medium by measuring the charge time of a capacitor, which uses that medium as a dielectric. We first define a relationship between the time, t, it takes to charge a capacitor from a starting voltage, Vi , to a voltage V, with an applied voltage, Vf. Read more….

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In this webinar, Dr. Colin Campbell discusses the details regarding different ways to measure soil moisture and the theory behind the measurements. In addition, he provides examples of field research and what technology might apply in each situation. The measurement methods covered are gravimetric sampling, dielectric methods including TDR and FDR/capacitance, neutron probe, and dual needle heat pulse.

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Nov 13

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Complex Scientific Questions Yield Better Science in Desert FMP Project

The Desert FMP project originated from a discussion between pretty divergent scientists: Rick Gill, a BYU ecologist, another scientist who works on soil microbes, a plant physiologist, and a mammalogist who researches small mammals.

Tree fire in Rush Valley

In an interview Rick said, “We started talking one day about the transformations that have occurred in the arid West over the past 100 years. One of the things we are really interested in is fire. How do ecosystems recover after fire? What’s the role of water in rangeland recovery? And the unique piece of this is: what’s the role of small mammals in this process? We may never have thought of that question, or the complexity of researching how all of our questions work together in a system, if scientists from different disciplines hadn’t decided to collaborate.”

Rush Valley research site. Five replications with four treatments: burned/unburned and small mammal/no small mammal. What’s interesting for us is that you can see that in the burned plots (the light brown) there are strong differences in the amount of the bright green plant—halogeton—that was present and it is systematically associated with the presence of small mammals. Here is the logic: In the spring, the presence of small mammals suppressed the cheatgrass and to some extent halogeton; in the absence of halogeton, cheatgrass ran wild. The cheatgrass transpired away all of the water and the halogeton that had germinated all died before it could flower.

As the experiment unfolds it is becoming clear that small mammals play a larger role in ecosystem recovery from fire than originally thought. The scientists have used their observations to hypothesize that small mammals eat the seeds and seedlings of two invasive species. This ends up setting the vegetation along a very different trajectory than when small mammals are absent following fire. Rick says, “We have discovered this complex but interesting interaction between water, fire, and small mammals. The first year after the fire, a really nasty range forb moved in called halogeton, which is toxic to livestock. Halogeton also accumulates salts in the upper soil profile that will cause failure in native plant germination. Cheatgrass has also moved in which makes the area more prone to fire as it connects the sagebrush plants with flammable material. But what’s interesting is in treatments where mammals were present, the densities of both halogeton and cheatgrass were much lower than where small mammals were absent.

Plot water potential comparison using matric potential sensors between Mammal (blue) and no mammal (red) over time. With no mammals to control cheatgrass, it depleted soil water availability below no mammal treatment and consequently halogeten was not able to grow.

“The other really important thing is that cheatgrass and halogeton have different growth patterns. Cheatgrass germinates in the Fall. It reaches peak biomass early in the growing season and then dies off leaving a blanket of dead, highly flammable vegetation. Halogeton germinates early in the growing season and remains relatively small until early Autumn when it bolts. These are things that will be really easy to pick up using NDVI sensors, which are sensitive to the amount of green vegetation within the field of view of the sensor. We are also using a system that we’ve designed to manipulate precipitation input. This will enable us to connect water availability to the success of two invasive plants that have negative impacts on rangelands. And with these same treatments we’re going to be able to tease out when in the year and to what extent small mammals are influencing the ecosystem by eating the seeds or the plant and at what stage.”

“Until I saw it in the field, the question of mammals being influential in rangeland fire recovery had never occurred to me. We only discovered that piece of the puzzle because scientists from differing disciplines are working together.”

Below are two virtual tours of the site: