So many sensors, so little time

Choosing a water content sensor from the many sensor types available may leave you feeling confused and overwhelmed. Every measurement method has strengths and limitations, but which method is right for you?

It’s not just about the sensor anymore

In this 20-minute webinar, Dr. Colin Campbell demystifies the differences between soil water content measurement methods. He explores the scientific measurement theory and the pros and cons of each method. He also explains which technology might apply to different types of field research, and why modern sensing is about more than just the sensor.

Learn: 

• Measurement theory behind the gravimetric method, capacitance, time-domain reflectometry (TDR), time-domain transmission (TDT), frequency-domain reflectometry (FDR), resistance sensors, and more
• Which technology applies to different field situations
• What factors matter when choosing a sensor type
• Why some methods are not research-grade
• How modern sensing is about more than just the sensor
• How to determine a good price-to-performance ratio for your unique application

Next steps

• View the slides
• Water content method comparison charts
• Water content vs water potential
• Hour-long, in-depth webinar on water content methods
• TEROS 12 soil moisture sensor FAQs
• Request a ZENTRA Cloud live demo

Questions?

Our scientists have decades of experience helping researchers and growers measure the soil-plant-atmosphere continuum.

• Talk to an expert
• Request a quote

Presenter

Dr. Colin Campbell has been a research scientist at METER for 19 years following his Ph.D. at Texas A&M University in Soil Physics.  He is currently serving as Vice President of Environment. He is also adjunct faculty with the Dept. of Crop and Soil Sciences at Washington State University where he co-teaches Environmental Biophysics, a class he took over from his father, Gaylon, nearly 20 years ago.  Dr. Campbell’s early research focused on field-scale measurements of CO₂ and water vapor flux but has shifted toward moisture and heat flow instrumentation for the soil-plant-atmosphere continuum.

Transcript

BRAD NEWBOLD
Hello, everyone, and welcome to Soil Moisture 102: Water Content Methods Demystified. Today’s presentation will be 20 minutes followed by 10 minutes of Q&A with our presenter, Dr. Colin Campbell, who I’ll introduce in just a moment. But before we start, a couple of housekeeping items. First of all, we want this to be interactive. So we encourage you to submit any and all questions in the questions pane. And we’ll be keeping track of these to answer during the Q&A toward the end. Second, if there’s anything that you missed, or like us to go back or whatever, don’t worry, we are recording this and we’ll be sending around the recording and links to the slides within the next three to five business days or so.

COLIN CAMPBELL
Alright, let’s get started. Today we’ll hear from Dr. Colin Campbell, who will compare water content sensor technologies, list the pros and cons of each, and discuss why modern sensing is about more than just the sensor. Dr. Campbell has been a research scientist at METER for 19 years following his PhD at Texas A&M University in soil physics. He’s currently serving as Vice President of METER Environment. And he’s also adjunct faculty with the Department of Crop and Soil Sciences at Washington State University, where he co teaches environmental biophysics, a class he took over from his father Gaylon, more than 20 years ago. Dr. Campbell’s early research focused on field scale measurements of CO2 and water vapor flux, but it shifted toward moisture and heat flow instrumentation for the soil plant atmospheric continuum. So without further ado, I’ll hand it over to Dr. Campbell to get us started.

COLIN CAMPBELL
Thanks, Brad. Today, I’m going to talk about in situ measurement techniques for soil moisture, specifically focusing on water content. But first, just a little bit more about me. As Brad mentioned, I have a PhD from Texas A&M University in soil science. And I spent the majority of my career developing instrumentation to make measurements in the soil plant atmosphere continuum. I also have the amazing privilege of teaching a class in environmental biophysics at Washington State University every spring. Interacting with others, especially students who are passionate about science and measurement, is delightful. Finally, as it relates to this discussion, I spent 1000s of hours installing soil moisture sensors and monitoring, interpreting and publishing data from field experiments. And in that process, I’ve learned a lot, some of which I hope to share with you today.

COLIN CAMPBELL
First, let’s start with a little story. And then in the late 1990s, reliable measurements of soil water content had just debuted on the market. There was a lot of excitement surrounding the opportunity to record changes in irrigation water and plant water uptake. Dr. Richard Stirzaker tells a story of himself and Dr. Phil Charlesworth, two colleagues at CSIRO in Australia, where they were so excited about seeing these amazing data that once they got their complicated system set up, they stayed in the field all day, just to see the irrigation water move down through the soil profile, which I think is pretty amazing.

COLIN CAMPBELL
Today, a Google search will bombard us with options for measuring soil moisture, from sensors that show moisture with the dial, to sensors that can be monitored electronically using a simple microprocessor. The possibilities are endless. To someone who’s worked in soil moisture sensing for most of my career, these sensors can seem almost laughable, as a few probably don’t work. Others can measure water content, but are wildly inconsistent. And still others don’t even measure water content at all. They measure water potential, which is not the same thing. Facing all these choices is confusing and frustrating when you simply want the answer to the question of which sensor to use to get the most reliable, robust, accurate and publishable data. Sometimes Google can’t help you answer every question, but hopefully I can get you started here today. Honestly, there’s not enough time to hit on every point that’s relevant, but please look to short YouTube videos that further our discussion on our blog site, environmentalbiophysics.org. Your chat pane will pop up a link to this site during the seminar. And don’t worry if you miss it, we’ll also put this these and other links in a post seminar email we’ll be sending out to you.

COLIN CAMPBELL
Our first problem is that searching for soil moisture sensors on the internet is not specific enough. I do this all the time and maybe you do too. Moisture in soil could refer to two different things. The amount of water or its energy state. One is an extensive variable, the other intensive. If you’re interested in diving into this more, you can learn more about it in our chalk talk sessions will link in your chat pane. Water content simply refers to the amount of water in the soil by weight or by volume. On the left hand side, I’m showing you how to calculate the water content by volume. All in situ measurements are via volume measurements. And I’ve shown you how to calculate that here. And also a graphical representation of what that might look like in terms of percent of soil minerals, versus the amount of water, versus the amount of air in the soil. Water potential refers to the energy state of water in the soil. Generally, this is dependent on this surface adhesion of water molecules onto soil particles. As water decreases, this boundary layer that I’m showing here becomes more and more thin. And as it does that, the water molecules are bound more and more tightly to the surface of the that soil particle. This binding reduces the potential energy of that water and makes it less and less available. In soil moisture 202, another seminar we’ll teach, we’ll get into water potential measurements, but here we’ll just focus on measuring volumetric water content.

COLIN CAMPBELL
Before we start to explore methods to measure water content, we first need to talk about applications. All sensors I showed earlier measured at a single point, and that’s what we’ll be focusing on today. But it needs to be said that water content can be measured at a field, catchment, or continental scale using satellite based technology. It can also be measured over large areas using downwelling cosmic neutrons. Both of these techniques are extremely useful, but we don’t have time to get into them today. Instead, we’ll just focus on techniques that measure at a plot, treatment, or subfield scale. There are basically four methods behind these measurements, and they include resistance, dielectric permittivity, thermal conductivity, and thermalized neutrons. But by far, the most common are resistance and dielectric. So again, we’re going to focus down further on just these techniques during our discussion. But again, look to our blog site for upcoming chalk talk sessions on these other methods.

COLIN CAMPBELL
On the right hand side, I’m showing a satellite picture of one of my research sites located in Rush Valley, Utah. Our challenge here was comparing water use between treatments where precipitation, rodents, and prescribed burns were changing. Selecting the right technology was critical to showing the effect of these treatments on the balance of native to invasive species. And I’ll use this example just a little bit later, as we try to decide what soil moisture sensor would work best for this. Here are a couple of sensors from that Google search that I made and talked about earlier. Both of these measures soil water content, both of these and many other sensors you’ll see, do this by measuring the amount of water while they create a voltage difference across two electrodes and allow a small amount of current to flow between them, outputting a value of resistance or electrical conductivity. Since water is a very poor conductor, it is the ions in the water that carry the current from one electrode to the other. The idea is a good one, it makes sense that the resistance will go down as the amount of water in soil increases.

COLIN CAMPBELL
And on the right hand side, I’m just going to show you what happens as we charge a positive and negative plate and those ions move in the soil. However, one critical assumption is that the number of ions in the soil is relatively constant. If not, as the number of ions in the pore water increases, the ability for current flow to flow increases, while the amount of water has not changed. And I’m illustrating this in this diagram where you see more of those salt molecules, or ions, moving to their polar sides. This idea may be better illustrated using a simple example. For a sensor to be used for more than wet dry measurements, it needs to have a calibration that relates the sensor output, whatever that might be, in this case its resistance or its simple inverse, the electrical conductivity to volumetric water content. So I took a simple model of electrical conductivity and I tried to show here that for some very realistic values of saturation extract electrical conductivity. That means the amount of water in the soil once it saturated in that water, sorry, the amount of electrical conductivity after it squeezed out at saturation from the soil. You can see that the calibration can change more than an order of magnitude. That’s, although the sensors are quite cheap, react to changes in water content, and are simple to integrate into projects, there are only real use is around the house and for fun science fair projects. And any scientific pursuit, you simply couldn’t get any reliable measurements to estimate volumetric water content.

COLIN CAMPBELL
Dielectric sensors are a general category that measure the charge storing capacity of the soil. We’ll talk briefly about specific types of sensors in a minute, but first I want to talk about why a charge storage approach is more effective than one focusing on resistance. I’m again showing a drawing of what happens when we measure the resistance across water along with an electrical circuit diagram above it, of what a resistor and capacitor look like in parallel. And that’s basically the electrical equivalent of what we’re doing in soil. Here, although there may be a small amount of capacitance, the general behavior of this system would be that of a resistor. Now consider an ideal electrical circuit that simply acts to polarize water molecules between two electrodes here. You can see all the water molecules aligning in that field. Now this is done very, very briefly, and it will store a small amount of charge without causing the salt ions to polarize. This ideal measurement would be sensitive to changes in the amount of water but not to changes in the amount of salt. Some dielectric, and the electrical equivalent is above it, shown here as a capacitor, but some dielectric measurements can act more like the middle circuit diagram I’m showing where they incorporate some resistance into the measurement and are somewhat sensitive to things like changes in salt concentration.

COLIN CAMPBELL
So why is dielectric an effective measurement of water in a porous soil matrix? Let’s talk about that now. Each material has a unique ability to store electrical charge. This is referred to as its dielectric constant. The dielectric scale gives a value of one to air arbitrarily, it’s just assigned, then relates other materials to that value. Soil is a mixture of solids, liquids, and gases, that all have different dielectrics. But in general, they all have low dielectric values compared to water. Thus, the dielectric sensor simply measures the changing charge storing capacity of the soil. And since water and air are the only things that change significantly by volume, we can relate that to volumetric water content, just as I’m showing you on the scale.

COLIN CAMPBELL
So here’s the same scale with the volume percentages of the soil mixtures, equated to dielectric values. I actually showed these earlier in the discussion. So here’s a soil that has relatively low amount of water, only about 1% going up to 10% in this figure and all the way up to 42% in this figure. I’m showing what that might look like in a soil matrix as well just so you can get a vision of how that might be. Now all the way over here at 80, we have water, which is 100% water. Of course that never happens in the soil, but it has a dielectric of 80 and is quite independent of these three cases. Since minerals are often around 50% of the total volume of the soil, the actual dielectric range of mineral soil is usually between 2 and 40. Although that’s just a general rule, and of course, would need to apply to specific situations in the soil. As we noted in with the resistance sensor, a key, or those sensors, a key characteristic of a useful soil water content sensor is an accurate measurement of volumetric water content. That’s pretty obvious.

COLIN CAMPBELL
Here I’ve graphed a relationship between the dielectric of the soil and its volumetric water content. Also like resistance, dielectric sensors are not perfect at predicting the volumetric water content. However with these sensors, things that affect performance, do so with a much smaller effect. In this graph, with the dielectric on the x axis and the volumetric water content on the y axis, I’m showing how the difference in soil bulk density affects the calibration. As you can see here, the effect is there, but it’s relatively minor. Of course, bulk density is not the only thing that will change the calibration. Things like soil type, yes, salinity as I mentioned, clay percentage, and sensor to soil contact can also affect accuracy, among other things. However, many of the high quality sensors available had developed technology to avoid most of these challenges, which we’ll talk about on the next slide.

COLIN CAMPBELL
Now, in my experience, you can’t avoid these things completely, but you can do a pretty good job of minimizing them. Dielectric measurement techniques are certainly not all created equal. In fact, some connect more like resistance sensors depending on the measurement frequency that they run at. Let me explain. I mentioned earlier that dielectric techniques polarize the water molecules while ideally avoiding polarizing the dissolved ions. Success depends on how quickly this polarization happens, or what we’ll call, its measurement frequency. At lower frequencies, sensors polarize the water and the salts, making them incredibly sensitive to salinity in the soil, and they act more like sensors down, this system down here, which I’ve been talking about. As measurement frequency increases to around 50 megahertz and above, this becomes more or less negligible, but depends on the quality of the circuit design that the sensor actually uses. But I guess the take home point is just because a $5 sensor on Amazon states that it uses capacitance, for example, or is trying to create that charge storing capacity measurement ability, it doesn’t mean it can avoid the many factors that extinguish sensor accuracy.

COLIN CAMPBELL
As I mentioned earlier, there are several types of dielectric sensors available. I’ve named a few of them here. Since I don’t have time to go into any one deeply, we’ll put a link to more information on each of these technologies in the chat bar right now and you can look for more in depth analysis and future discussions that will be linked on our post seminar email. I also spent a long virtual seminar a few years ago, talking about all these in depth, and you’re welcome to look those up in soil moisture 201 on YouTube.

COLIN CAMPBELL
The most common research grade water content sensors on the market fall into three general categories. Capacitance sensors use the soil as a capacitor element and use the soils charge storing capacity to calibrate to water content. Time domain reflectometry or commonly known as TDR measures the travel time of a reflected wave of electrical energy along a transmission line. The travel time is related to the charge storing capacity of the soil, and the volumetric water content. Interestingly, TDR contains a range of frequencies, not just a single frequency in the signal, which can help reduce errors from soil salinity. Frequency domain sensors also use the soil as a capacitor to measure the maximum resonant frequency in the electrical circuit and relate the resonant frequency to water content. All of these categories contain some sensors that perform well, and some sensors that do not. There are several comparison studies in the literature that may be useful to look at for more information. And I encourage you to go out there and look, but there’s more to consider.

COLIN CAMPBELL
The research site I showed at the beginning of the talk, we had four treatments repeated five times with sensors at multiple depths in each treatment. The goal of the study was to see how rodents and prescribed burns affect native and invasive species diversity with changing precipitation. The picture on the right is a satellite view of the field. Two challenging aspects of the project were the type of sensor to use to ensure effective installation, and how to accumulate data and provide it to all the stakeholders in the project. This is something we dreamed about for a long time as we researched other sites and tried to make a pilot study here to get all the data out and to provide it to the stakeholders.

COLIN CAMPBELL
First, we’re going to address the challenges of sensor installation. Here I’m showing a picture sequence of an installation tool for a TEROS 12 water content sensor. Using this method, sensors can be installed as several depths down to two meters in the soil. First you simply auger a 10 centimeter borehole into the soil. Using the installation tool, you use this plate here to set the sensor installation depth. Here is a close up of Leo doing that in the field. Next, you put the sensor into the carriage that holds the sensor tight as you lower it down carefully into the auger hole. As that plate comes to rest on the ground surface, you simply turn these two handles, and the sensor slides easily in the soil. One of the things that I love about this installation technique is that the tool is able to insert the sensor exactly perpendicularly into the soil with no wiggles that are common when I do it myself. So we can get a very good or near perfect installation every time. And with the tool, it’s so effective that I can quickly put in many sensors across several sites in not very long. In addition to installation, simple deployment and reliable data collection and visualization from the field is another essential thing to consider, and something that Dr. Stirzaker and Charlesworth, that I mentioned at the beginning of the presentation, could only dream about at that time. Tools that quickly get sensors connected in the field, and data delivered to stakeholders anywhere in the world, are key to making experiments successful. I’ve been enjoying the ZENTRA system from METER in this way, and I want to show some data we collected over the summer, and some amazing things that we observed.

COLIN CAMPBELL
In an irrigated wheat field located in southern Idaho in the United States, we deployed six soil moisture measurement profiles, including TEROS 12 water content, temperature, and electrical conductivity sensors. And we track those readings over the summer to try to help the grower improve the variable rate irrigation recommendation for a center pivot irrigation system. Although it wasn’t the purpose of this experiment, our observations of water content trends were pretty amazing. First, let’s look at the water content at the highest depth we measured, the 15 centimeters. Now to just give you a little primer we’re going to talk about 15 centimeters, then 45 centimeters, then sensors installed at 65 centimeters. And during this experiment, our research colleagues were talking to the grower and telling him when to turn on and turn off the center pivot to try to optimize moisture for this wheat. So the orange traces on this graph show the irrigation or precipitation events during the summer from June 10 to August 18. And there on the right, you can actually see the scale there and these precipitation events are in millimeters. As a note the irrigation water was turned off in mid July. This was to help the wheat mature to prepare for harvest in early August. The blue areas marked our optimal water content region. At first glance, as we look there at those, at the blue data, we see that the grower is trying to keep his field within those marks. But there are some wobbles in the data that seem to be temperature sensitivity as they go up and down and what you probably perceive to be a diurnal basis. But as you look more closely at the data, you realize that when the plants no longer take up water during August, those wobbles disappear.

COLIN CAMPBELL
This is something I thought was pretty interesting, and it strongly suggests that it is daily plant water uptake, and not temperature response, that’s causing those wobbles. We observe the consistency of the sensors themselves as they all dried down to similar water contents at the end of the summer. That gives us some confidence that they’re all sensing similar things. Now, this field was actually 700 meters in diameter and so these six sites were measuring in, we’re not in exactly the same soil, and certainly weren’t very close to each other. But we did observe similar behavior that would be dependent on things like evapotranspiration of the area and the center pivot irrigation was applying water. Now if we drop down to 45 centimeters, we see a similar behavior. When the water isn’t changing, the lines are fairly smooth, but when they start to decrease, many are showing the same wavy behavior as the plants take up the water, it appears that there’s more available water at 45 centimeters at some sites, which is kind of interesting. But the sites that dry down more significantly, they show that they are uptaking water in the same behavior with that wavy signal as we saw on the last slide.

COLIN CAMPBELL
Now consider the sensors installed at 65 centimeters, I want to pay a little bit more attention to this slide. These show very little drop in volumetric water content during June and even into mid July. Subsequently, they all show this wavy pattern. Remember that the irrigation water was turned off about the 18th of July and so it’s apparent that the wheat is going down further in the soil to take up water during its growth cycle. We can convince ourselves that the wobbles in the data that start in July and move into August, are not temperature induced as we look at the smoothness of the line in June and consider the next slide that I’m going to show you now, and look at the temperature at the same depths. Now I want to mention that the colors on each of these depths correspond. So now we’ll look at the temperature, I graphed it with a scale that’s only from 10 to 20 Celsius, so it’s very zoomed in. And look at this pink line right here. The pink trace, which shows the temperature at site 141, almost doesn’t change at the end of July.

COLIN CAMPBELL
Now look back at the water content curve during that time. Clearly there’s changes in this pink line during that time that aren’t represented by temperature. These sensors are performing well enough, in fact, that we can see daily water uptake, as I mentioned in the first slide, by the plants, as some of them increase even in water content during the night, where it’s my opinion that the heavy pull of ET is relaxed and the xylem stream seems to snap back into place. I think this is pretty amazing to look at these data. And I get more confident that they’re happening as I think they are, by the smoothness of the data when the plants aren’t taking up water. One other thing that you can see, where I’ll gently move through these three slides, is the kind of time series water uptake at the three depths. So we here have water being taken up in early July, then we move to mid July, and now down at 65 centimeters late July and into August, water being taken up. So these data help us see how the plants are accessing moisture at multiple depths. While there are a staggering number of water content sensor choices out there, choosing one specifically for your measurement needs may be more simple than it appears.

COLIN CAMPBELL
From our discussion it’s evident that relying on a resistance based technique will not yield good results, despite the attractive price and simple integration into a measurement project. Changing salinity in the soil driven by salinity fertilizer, even soil type, will often result in perplexing information coming out of the sensors and frustration on your part. Dielectric based sensors are a far better choice but still need careful consideration. Not all sensors are created equally. Although there are a few different approaches to measuring the dielectric or charge storing capacity of the soil. Studies show the performance is more closely related to individual sensor qualities like measurement frequency, and circuit design, than to specific measurement technology, such as capacitance or TDR. In general, higher frequency measurements result in higher quality data, but also higher sensor costs. One might say that true value in a sensor comes from the optimization of the balance between performance and price. I’d like to thank you for spending time with me today, I realized I wasn’t able to do any deep diving into some of the interesting details due to our limited time. And I don’t want to take too much of your time and be respectful of it. But I hope you will join me and some of my colleagues as we complete some of those efforts on our blog. Now back to Brad.

BRAD NEWBOLD
Awesome. Thanks, Colin. We have a few minutes here for questions. Again, we’d like you to enter any and all questions in the questions pane. We’ll try to get to as many as we can, if we definitely will not get to all of your questions. If we do not get your question, we do have them recorded and we will be able to get back to you, Colin or one of his colleagues will be able to answer your questions via email. So definitely submit all your questions, and we will get to them. So there’s a couple that that popped up Colin, that I think, questions about applications in some very different environments. The first one is asking about sensors and applications in nurseries, so in potting. And so he asked which sensors and applications are adequate for measuring water in the individual potted plants? And then in the whole nursery?

COLIN CAMPBELL
Great, this is a great question. I spent a lot of time measuring water content in nurseries and honestly, my favorite sensor for doing that is that sensor I was showing you, the TEROS 12. And the reason is that in a nursery situation, in the individual pots, we’re really wanting to be able to both measure the water content, which is important, but maybe even more importantly, the nutrient situation because we’re almost always in some kind of engineering material, a perlite peat mix, you know, a bark, a sunshine potting soil mix something that’s unique to a situation, and in that situation, we need a good balance of fertilizer to make sure the plants have enough nutrients to grow, but are not overwhelmed by them. And that combination of water content and electrical conductivity is critical to get that balance right. Now, one of the challenges here and one of the things I’ve seen as I’ve gone around is people kind of saying, well, I want to put a measurement in every single pot, and I can’t spend enough money to do that. And my reply to that is, I mean, and they suggested that time hey, why don’t I just get one of these resistance sensors that’s only $3 off Amazon and then I can do this in every pot and then I can just throw them away? Well, more data is not necessarily better. Having a single, or a few measurements in each irrigation zone that’s accurate is going to help you much more than inaccurate measurements all across the pots. Because you can only control a zone with a switch on or off, you can’t control anything to a specific pot. Now often in nursery operations people are out there anyway observing plants and if there’s an issue with watering in a specific pot, it often won’t be caught by a sensor but rather someone visualizing it, or it could be caught by an infrared camera or some other technique.

BRAD NEWBOLD
Okay. On the other hand moving from pots. Are there any special considerations for installing in dry or rocky soils, such as in, this question was asking about the Mojave Desert.

COLIN CAMPBELL
Okay. Yeah, so dry or rocky soils, those create a challenge. One of our research sites, we’re up at 3000 meters on the Wasatch plateau and there’s a lot of rocks there. My recommendation for that is first to get a robust sensor that’s not only going to be able to be pushed effectively in the soil, but also last in time. That Mojave like many places is going to see a lot of temperature swings. The other the other consideration you need to make is that in those cases, like on the plateau, we weren’t actually able to use our installation tool, I simply had to auger and use my hand to go down and find places to insert in the soil where there was basically enough room between the rocks to get the needle so the sensors, so it just takes a little bit more time when you have a lot of rocks. I’ve been impressed in several situations, we were installing in Portugal, with many stones in the soil, but we actually were able to use the augering tool and installation tool to install there and we simply just needed to feel as we rotated those arms, whether we had an easy insertion or if we were hitting a rock, and if we were hitting a rock, pull the tool out, pull the sensor back out, and try again.

BRAD NEWBOLD
Okay. I think we have time for a couple more. We’re going a little bit over but that’s okay. One person was asking about our particular TEROS sensors and how’s the repeatability from sensor to sensor within our TEROS line?

COLIN CAMPBELL
That’s a great question. I think for many, many years, and I started making soil moisture sensors almost 20 years ago when I arrived here at METER, Decagon, and repeatability was a concern of ours. But right then we were really focused on trying to make a sensor that would make high quality water content measurements. In our most recent release of the TEROS 10, 11, 12, we’ve developed technology where each and every sensor can be calibrated to an electrical standard. And the repeatability of that TEROs 10, 11, 12 line is now exceptionally good. You saw the data that I used from our experimental site at the variable rate irrigation field, and the quality of those, the repeatability of those sensors was excellent. And I’ve seen this time after time, whether it’s up on the plateau, it’s down in this variable rate irrigation, in some of our test sites here around METER, in every case we’ve seen excellent repeatability in the field and during our lab testing. So I’m super happy that we finally were able to get to focusing on some of these other things besides calibration, which looks really good, to other things like sensor to sensor repeatability.

BRAD NEWBOLD
Great. I think we’ll do one more. And I’m going to kind of squish two together again, going back to installations with the sensors. One person was asking why in particular with the TEROS 12, is the installation vertical as opposed to horizontal? And another person was asking, how do you deal with roots growing in around or installing it around roots?

COLIN CAMPBELL
Okay, so a couple questions about vertical versus horizontal and then roots. It was our goal for many years to figure out how to make this installation process more simple. As I mentioned, at the beginning of the seminar, I’ve installed these sensors in soil since my graduate days where I installed several Campbell Scientific sensors out in rice fields, and grew to love those sensors. And really, as we anticipated how to do a better job, and how to really make your time in the field effective, moving, you know, installation processes from hours to just minutes in that irrigation field that I talked about, we installed each site in about an average of 35 minutes. But installing down a borehole if we try to do that in a horizontal pattern, we’re not going to be able to get the good soil the sensor contact. And so this is why we decided to do them vertically. It’s a great question and we continue to think about how to adapt our designs for more accurate measurement. For example, maybe you want to install them horizontally, because you really do want the measurement at let’s say, 50 centimeters and not 48 and not 52, and I can understand that. And as we continue to develop our ability to make better and better water content measurements, more and more accurate measurements, that certainly is on the list is how to install them horizontally rather than vertically. But right now, our best method is really that vertical installation tool, which I’ve really grown to love.

COLIN CAMPBELL
Ah, I almost forgot Brad, sorry, I saw you looking at me. What about roots? Now you know, in that, and I forgot to mention this in the data that I showed, where we have those wobbles in the data that were really from the relaxation of the xylem and the water kind of moving and filling the roots. What I believe is that we probably had roots growing around those sensors. And you saw in the data that not every sensor perform in the same way. And so whether the water actually moved outside the roots into the soil, or the roots were growing around the sensor, and we simply just I mean, we didn’t know about it, I don’t know the difference. And so what do we do about roots growing around the sensors? Well, I can say not much because we install the sensors into the soil and then allow the roots to be there if they want to be. In terms of calibration I think there may be a very minor issue there but I’m not all that concerned about it. If that’s something you’re really concerned about, there are techniques to try to limit root growth around objects. They’re things you can treat the sensor with, I suppose, or things that you can do. I’ve seen it done in other instrumentation I’ve worked with and so that’s a possibility but I actually kind of like having the roots in the general area because I know that the water is being taken up and my guess is the roots around the sensor are similar to roots in their bulk soil. Although I have seen situations where, and you probably have too, where roots will grow down, and they find some object in the soil, and it seems like they gravitate toward that object and kind of fill and mat against that surface. And it may be true that that happened here in the data I’m showing, but the students dug those sensors up, and didn’t report any challenge like that, but it’s something we could pay attention to in the future.

BRAD NEWBOLD
Great. Thanks, Colin. That will wrap it up for us here. We did go a little over. Thank you for sticking around. We hope that you enjoyed this discussion, this presentation, as much as we did. We appreciate all of your questions. We only got to about five or six of them and we were inundated with dozens more. Again, if we did not get to your questions, we do have them recorded and we’ll be able to get back to you and email via email and answer your questions. Either Colin or someone else who is on our team will be able to answer your questions. Also, as you log out, please consider answering the short survey that will appear after the webinar is finished. That will help tell us what kind of webinars you’re interested in for us to cover in the future. And so look, again, look for the recording via email. We’ll be sending it out in the next couple of days, along with links to the slides. And from all of us here METER, have a great day. Thanks.