Transcript:

 

Brad Newbold 0:01
Hello, everyone and welcome to office hours with the METER Environment Team. Today’s session will focus on soil moisture measurements, and we’re shooting for about an hour of Q&A with our experts Leo Rivera and Chris Chambers, whom I will introduce in just a moment. But before we start one housekeeping item. If you’re watching this video and you think of a question you’d like to ask our science experts, we encourage you to submit your question on our website at metergroup.com and someone from our science and support team will get back to you with an answer via email. All right, with all of that out of the way, let’s get started. Today our panelists are application specialists Leo Rivera and Chris Chambers. Leo operates as a research scientist and director of science outreach here at METER Group. He earned his undergraduate degree in agriculture systems management at Texas A&M, where he also got his master’s degree in soil science. And there he helped develop an infiltration system for measuring hydraulic conductivity used by the NRCS in Texas. Currently, Leo is the force behind application development in meters hydrology instrumentation, including the SATURO, HYPROP and WP4C. He also works in R&D to explore new instrumentation for water and nutrient movement in soil. Chris Chambers operates as the environment support manager and the soil moisture sensor Product Manager here at METER Group. He specializes in ecology and plant physiology and has over 12 years of experience helping researchers measure the soil plant atmosphere continuum. So thanks for joining us, guys. All right, let’s get started taking some questions. This first question here today is how difficult is the calibration of dielectric sensors?

CHRIS CHAMBERS 1:42
I wouldn’t call it difficult. It’s more like just kind of a process to work through, right? Yep. Every basically any soils lab, most, most science labs across the country are going to have everything you need, which is a drying oven. Precision balance, and…

Leo Rivera 2:04
Soil.

CHRIS CHAMBERS 2:05
That’s pretty much it right? Pretty much okay, soil and water. Yeah, so difficult. It is not. There are some pitfalls that can lead to bad data. And we have standard operating procedure which that we use, we share it online, you can go to METER group.com. and search for calibration and our procedure will come up and we’re, we’ve been doing it for a long time. We’re pretty good at it. But it’s not, it’s not a secret everyone should be able to do it.

LEO RIVERA 2:37
Ya know, I think the biggest challenge that people run into is following- following bad methods when it comes to calibrating your sensors. And of course, really, the big question is, what are we trying to ultimately get the outcome of because we’re, we’re talking about calibrating dielectric sensors, but the goal is to take our dielectric measurement, and convert that to a volumetric water content or capacitance measurement or capacitance. Yep, whatever your technique is, yep, whatever your approach is, to get that, but our goal is to convert that to a volumetric water content. And one of the big challenges that I often see is if you look in the literature, people, not homogenizing the soil getting a good, evenly mixed water content within the soil. Because you have to remember what your sensing volume actually looks like in the sensor. Or I see people doing field methods where they’re trying to calibrate in situ, which is even more challenging, because it’s so much harder to control things and actually get a proper reference measurement.

CHRIS CHAMBERS 3:36
Now, let’s go a little bit further down this rabbit hole is the rationale. No seriously, the rationale behind doing these is because A. they want to mimic the soil structure. Right, which if you’re going to homogenize it, you totally destroy the structure. B. and to get what people will actually see in the field from a satellite. Right. That’s why they want to use those two methods. Yeah. And so what what do you think the trade off is here? Why does the homogenized method give you so much make it worth sacrificing that, especially the structure for ,

Leo Rivera 4:15
Yeah. I think that’s a great question. I mean, ultimately, is what is going to be our bigger source of error in the calibration. And also, what is our ultimate? You know, why do we do soil specific calibrations? What are the pieces that we’re trying to correct for in that soil specific calibration? And when you look at the things that primarily impact the water content measurement, structure plays a role but density driver bulk density and the soil texture itself and especially if it’s most mineral soil calibrations from manufacturers perform fairly well, but oftentimes, you might wind up in a site that has unique characteristics that you’re trying to calibrate for.

CHRIS CHAMBERS 4:58
Or you can you can almost always improve on on the factory calibration for sure. At least a little bit.

Leo Rivera 5:04
Yep. Yep. Yep. But you don’t want to improve by using a bad calibration?

CHRIS CHAMBERS 5:10
Exactly. Yeah, we’ve seen that happen before.

Leo Rivera 5:12
Absolutely.

CHRIS CHAMBERS 5:13
So know your bulk density the field? Yeah. Right, because you’re gonna want to if you use the modernized method, you’re going to want to pack back to that density. Correct. And what you- and the reason why we- the real reason why we recommend the homogenized method, is because if you get, like wedding fronts, or drying fronts in an intact core and a soil, yeah, so you have an inherent spatial variability in your core, from reading to reading, right? Yep. And that is what frequently leads people astray. Because you’re weighing this entire thing, assuming it’s homogeneous.

Leo Rivera 5:54
Yeah.

CHRIS CHAMBERS 5:54
But it’s not and your sensor may be detecting something different than what you’re actually verifying with your with your gravimetric sample.

Leo Rivera 6:03
Yep. Yep, exactly.

CHRIS CHAMBERS 6:05
And that’s where we’ve seen horror stories.

Leo Rivera 6:08
Yes.

Brad Newbold 6:09
Our next question here, they are asking, Can you discuss how soilless organic media such as biochar, or cocoa quar effects dielectric sensor accuracy? Does the physical shape and size of the pores holding the water being measured affect the measurement since it affects the electrical path between the anode and cathode?

LEO RIVERA 6:28
Yeah, I think that anode and cathode references is probably more so in line with resistance based measurements, which isn’t completely accurate for all sensors. But that’s I’m probably digressing into something that’s not really…

CHRIS CHAMBERS 6:43
No, I think it is, I think it is relative is trying not to when you think of anode and cathode, you know, what comes to mind is a one directional flow of electrons. Right? Can you have that? That’s kind of the mental model that comes to my mind anyway.

Leo Rivera 6:59
Yeah.

CHRIS CHAMBERS 7:00
And the sensors are emitting an electromagnetic field and alternating the charge at a high frequency, right?

Leo Rivera 7:07
Yep.

CHRIS CHAMBERS 7:08
So that electromagnetic field in the soil is, is kind of a key in it, it is affected by the substrate that it’s in the amount of water content in the soil. So it- it does change the shape. But in the end, it comes down to kind of a sampling question, doesn’t it?

LEO RIVERA 7:29
Yeah, I think it comes down to a sampling question. I also do think there is, depending on especially, its let’s say you’re amending soil with some of these materials, like biochar especially, there’s two things that can happen with that one. Biochar inherently has a different dielectric permittivity. Than soil. And some I’m if I’m not mistaken, when you amend with biochar, sometimes you bring along some some solutes, that that come with that can increase the electrical conductivity. So depending on your approach, you could see some issues there. But really, I think, ultimately, it comes down to the what the dielectric permittivity of the material is that you’re adding. And if that differs a lot from soil.

CHRIS CHAMBERS 8:16
And if your sample is representative of how you’re going to use it, use the data, you know, install the sensors in the field. Yeah, as long as that’s representative, then you’re going to be in good shape. We do recommend a custom calibration with biochar cocoa coir, because it will be very different than a mineral calibration, and probably different than a factory soilless calibration.

LEO RIVERA 8:39
Yeah. And I think just to dive deeper into this question, I don’t think it really has anything to do with the physical shape, or the size of the pores that are created by these different materials. Because we’re creating electromagnetic fields. Those don’t really govern how the electromagnetic field behaves. But it’s really ultimately comes down to the properties of the material itself. And the dielectric properties of the material.

CHRIS CHAMBERS 9:02
I hear there’s, there’s more and more commercial biochar products out there. So it’s probably we’ll see more and more of.

LEO RIVERA 9:08
Yep, yep. It’s true. I’d be interested to see where that continues to go.

Brad Newbold 9:12
Next question is about tensiometers. They’re asking, can they measure the potential difference between two points with millimeter precision?

CHRIS CHAMBERS 9:22
The difference between two points and millimeter precision? This kind of is, it’s kind of inherent in the accuracy of the sensor, right?

Leo Rivera 9:32
Yeah.

CHRIS CHAMBERS 9:32
It’s really dependent on how precise your your attenti ometer is. Yeah. So what is the accuracy of like, let’s use a T-5 here as an example. So not a T-5,

TEROS 31

TEROS 31. Yeah, thank you.

LEO RIVERA 9:45
Great question. So the accurac- the res- let’s fo- resolution is .1…

CHRIS CHAMBERS 9:53
Hectopascals

Leo Rivera 9:54
Hectopascals. 1hectopascal is equivalent to one centimeter of tension for water potential,

CHRIS CHAMBERS 10:04
Which is a 10th of a kilopascal?

Leo Rivera 10:07
Yeah

CHRIS CHAMBERS 10:08
No, ten kilop- a 10th of the kilopascal. All right, so we’re gonna do math on the fly here. This could get ugly folks. Bare with us.

LEO RIVERA 10:13
So in terms of resolution, you could resolve kind of to that millimeter. But your accuracy of the sensors is about half. Half a hectopascaal I believe.

CHRIS CHAMBERS 10:27
So especially if you’re comparing two points,

Leo Rivera 10:29
Yeah.

CHRIS CHAMBERS 10:30
If you’re comparing two points, the the, the, it’s the error bars overlap, and you really can’t separate those two.

LEO RIVERA 10:38
And I don’t really know anything that would measure to that type of precision in soil. But my question is, do you really need that level of precision? If you’re trying to govern water, if you’re trying, and ultimately comes down what your goal is, are you looking at fluxes, and you’re trying to look at water movement in the soil? Absolutely. I think potentiometers can give you the precision that you need,

CHRIS CHAMBERS 11:01
Or at least give you the best answer possible.

Leo Rivera 11:03
Yep. Yeah, out of anything that you’re gonna get. Now, if you’re looking at making- making-making these major potential measurements or something like a TEROS 21, which is a solid matrix sensor, I wouldn’t do it. You don’t have the resolution, or the accuracy that you need to make that, especially when you’re talking about those small changes and looking at waterflow. That’s not something that’s going to work well for that. But tensiometers are the best tool available to try and infer these things that might happen in the soil, and what comes up looking at water movement.

Brad Newbold 11:36
Next question, how much can organic soil amendments influence soil moisture?

CHRIS CHAMBERS 11:41
Quite a lot!

Leo Rivera 11:42
Yeah. Yeah. It depends on on the, what you’re adding to the soil, and also how long it stays before it degrades.

CHRIS CHAMBERS 11:51
Its distribution in the soil.

Leo Rivera 11:53
Yep. Yeah.

CHRIS CHAMBERS 11:54
So let’s, let’s kind of unpack it and go into how they affect how they impact both water movement through the soil, which is interesting in the soil moisture, because we’re looking at an effect on porosity, right? Generally, they’re, they’re lighter than the density of, you know, less dense and soils. So you could see higher saturated hydraulic conductivity, right?

Leo Rivera 12:18
Yep.

CHRIS CHAMBERS 12:18
In general, in general, more water holding capacity with the higher porosity. Yep. And in the end, a custom calibration is probably going to be, it’s probably going to be a good idea for organic soil amendments, depending on how much you’re adding.

LEO RIVERA 12:37
Yeah, I think if you’re adding more than 5% of the volume of soil, then it’s probably something where you’d want to add, look at a custom calibration. But I mean, there’s a lot of ways that these different organic soil amendments, in fact, impact how water is retained in soil. There’s quite a bit of literature out there on there, looking at these different things, and we will see people spend a lot of time looking at that, especially looking at how it impacts the retention curve of the soil. But, yeah, it just depends on the kind of similar to our first question or an earlier question about the coco coir and biochar right and type things. Yep.

Brad Newbold 13:20
Question here. Is it possible to determine the soil hydraulic characteristics, field capacity, permanent wilting point KSAT from soil water content readings?

CHRIS CHAMBERS 13:31
No.

Leo Rivera 13:31
Yup.

CHRIS CHAMBERS 13:32
Next question. Wait, you said yep? Are we disagreeing here?

LEO RIVERA 13:32
No, no, I agree. I said nope,

CHRIS CHAMBERS 13:36
Yes or no?

Leo Rivera 13:37
No.

CHRIS CHAMBERS 13:37
All right,

Leo Rivera 13:38
By themselves, no.

CHRIS CHAMBERS 13:39
That’s right. You need additional information.

Leo Rivera 13:41
Yep. Yep. If you can combine your water content with a water potential reading, then you can start to get at some of those things, especially fuel capacity, permanent wilting point, if you have those two measurements together,

CHRIS CHAMBERS 13:51
Especially water potential, okay. Hang on here. Because if you go to methods of soil analysis. Which is a key book, then there is a method for determining field capacity. Is there not?

Leo Rivera 14:02
Yes.

Am I wrong?

Yes, you’re not wrong.

CHRIS CHAMBERS 14:05
Okay. So, but yeah, the point is, it’s kind of subjective, right? Exact because you’re waiting for, for the soil to drain to a certain degree, right? It’s something like water stops dripping out of your bucket or whatever.

Leo Rivera 14:21
Yeah.

CHRIS CHAMBERS 14:22
And I always found that terribly subjective. And like subjective errors are some of the some of the worst ones to try and account for, even if you have matric potential.

Leo Rivera 14:33
Yeah.

CHRIS CHAMBERS 14:33
And your field capacity is at -35 kPa instead of -30 kPa. You know, it’s at least you’re gonna get a consistent a consistent reading out of that every time instead of a different water content at

Leo Rivera 14:48
Yeah.

CHRIS CHAMBERS 14:50
fuel capacity. Also, well,

LEO RIVERA 14:52
We’ll put a question our traditional view of field capacity, but that’s probably a bigger conversation than this, this office hour session. But some of the one that is a more complex value to get out is going to be your hydraulic conductivity, you need lots more, you need a lot more information. Now, having said that, if you have two points, you have your water potential and your water content readings. And you know the distance between those two points, and you know, your your general fluxes of water, you can just use Darcy’s equation to calculate your hydraulic conductivity and get a curve, but it is going to be the it’s going to be prone to error, especially because hydraulic conductivity, a big part of that is flow through macropores. And oftentimes, that is missed in some of these in situ measurements. So it can be challenging to do that.

And so soil water content alone gives you half of the water state picture.

Leo Rivera 15:05
Yeah,

CHRIS CHAMBERS 15:06
Right. You’ve got you’ve got the volume. Which, you know, with a little additional- additional information, you can get the mass,

Leo Rivera 15:28
Yep.

CHRIS CHAMBERS 15:29
Right. So we have the mass, but we’re missing the energy state part of the equation, which is necessary if we’re going to get those other those other values.

LEO RIVERA 16:11
Yeah. Yep. So I think to sum it up you can try and get some of these things through with these measurements. It’s a lot easier if you have the, the water content and water poten- potential measurements together. But also, you need to understand where your sources of air from that will come.

CHRIS CHAMBERS 16:15
Right, exactly.

Brad Newbold 16:29
All right, a basic introductory question here. Why is it important to find the volumetric water content reading instead of gravimetric water content? What does the volumetric water content reading mean? How does it help us in real life?

CHRIS CHAMBERS 16:44
I don’t think that’s that basic.

LEO RIVERA 16:47
Yep, I’m with Chambers, it actually is a more complex question than than it comes off as

CHRIS CHAMBERS 16:53
Right. The sensors, the reason why we work with volumetric water content a lot is because that what the sensors give you. And the reason it’s volumetric water content and not gravimetric is because the volume, the total volume that you’re reading with a sensor is largely undefined. Right? So that volume that you’re reading, that volume of that measurement, volume of influence, can change a little bit, when you put it in soil, when there’s a capacitive load on it, it’s going to shrink or get bigger. So but we can measure the volume of water per unit volume of soil, that’s what we can do. So to, you kind of have to know the bulk density to convert back to gravimetric water content with that. Even though the mass of the water might be what we really, really want, in many cases, the volume of water with in situ soil- with in situ soil sensors is what we can actually measure.

LEO RIVERA 17:56
Yeah. Yep. And, you know, it’s really this is an interesting question, because depending on the industry that you come from, you actually might gravitate more towards gravimetric water content. Engineers tend to use gravimetric water content more than volumetric water content. But I would argue, you know, this question I the love the last part of this question, how does this help us in real life. Gravimetric water content by itself can be hard to interpret what it actually means. Because it has to be referenced to like, you don’t know how much mass of soil you have in an area. But you can always relate to like, I know, in this unit area, I have this volume of water,

CHRIS CHAMBERS 18:36
And your inputs and outputs in the system are going to be in generally a linear, a linear water. So your precipitation, you’re going to look at it in millimeters of water, your drainage, you’re going to look at in millimeters of water. So if you have centimeters of water, which is really easy to convert to millimeters of water in the system, then your storage is on the same unit scale as your fluxes.

Leo Rivera 19:01
Yeah.

CHRIS CHAMBERS 19:02
And that’s really convenient.

LEO RIVERA 19:03
Yep. The only area that I’ll add that there can be some challenges with volumetric water content is in expansive soils, that change volume, but that’s a whole different. You gotta remember, that’s where I spent a lot of time. But the expansive sales bring about their own challenges that are well beyond…

CHRIS CHAMBERS 19:23
And they’re pretty common too, so you’re gonna encounter them.

Leo Rivera 19:26
Yeah.

CHRIS CHAMBERS 19:28
But unless you live on the Palouse.

LEO RIVERA 19:31
Yeah. Everything works well, is easy on the Palouse. But ultimately, I think volumetric water content just gives you a much better reference to understand what water content means.

CHRIS CHAMBERS 19:42
And if you have the bulk density, you can go back and forth between them.

Leo Rivera 19:44
Exactly.

Brad Newbold 19:46
Is there a soil temperature slightly above freezing that you would recommend using as a data quality threshold or just use zero degrees Celsius and then check if there were any remaining rapid drops and the volumetric soil moisture dielectric observed that need to be filtered out as well.

CHRIS CHAMBERS 20:03
Like the freezing frozen soils, is that what we’re talking about?

LEO RIVERA 20:06
Yeah, I think that what they’re getting at is, you know, what temperature would they would you use as a QAQC threshold for your water content.

CHRIS CHAMBERS 20:14
I don’t like using temperature, I don’t like using temperature at all, for this A, you’re gonna be slightly, it’s going to be slightly negative freezing point and soils. And you see this over and over, it’s not going to be too negative, -2, -3, the, the lowest freezing point I’ve ever, in the datasets that I’ve looked at, are about minus getting close to -5. So generally, your freezing points going to be a little bit negative, and your water content. Unless it’s bone, dry soil, and there’s very little water in there, it’s going to be pretty clear, it’s going to be a pretty clear threshold for when you’re wa- for when your soils freeze. Unless you’re in this middle state in between freezing, you know, if you’re during during a state change, then things can get really messy and hard to interpret. If you have frozen soils, it’s going to look like there’s zero volumetric water content, or there’s going to be like 3 or 4, water 3 or 4% volumetric water content, it’s going to be really dry.

LEO RIVERA 21:20
Yeah. And the reason that that freezing point shifts down is because of the solutes that are present.

CHRIS CHAMBERS 21:28
Solutes in in the soil surface.

Leo Rivera 21:30
And that that brings down the freezing point of the water. And that varies because you might have different levels of solutes in different areas, different different salts even at different times. So it does- it is challenging, but like you said, it’s pretty apparent when freezing occurs. That’s right now after that, it can be a little

CHRIS CHAMBERS 21:49
Yeah,

Leo Rivera 21:50
A little dicey.

CHRIS CHAMBERS 21:51
And there’s. Yeah. So just state changes are always difficult. Yeah. For this, I hate using a temperature filter might be really handy, just as a flag.

Leo Rivera 22:04
Yeah.

CHRIS CHAMBERS 22:04
I think you could use zero. And if you get below zero, then just kind of flag the data to look at. But it’s it’s really hard to get, it’s really hard to get a- to nail that point down to this is frozen, and this is not frozen. Just from the temperature.

Brad Newbold 22:26
This individual is asking is the TEROS 21 water potential sensor sensitive to temperatures? I found a strong daily change in TEROS 21 data in concert with temperature changes. Is there a possible way to correct the temperature effect?

LEO RIVERA 22:40
I can start this one off. So yes, we know that the TEROS 21, both the gen one and the gen two are sensitive to temperature, especially as the water as the water potentials go lower. And there’s two factors that impact that one is the gen one, the center itself, the electronics were a little more sensitive to temperature changes. But the ceramic, as you get to lower water contents, there’s less water in the ceramic. And so as temperature changes, there, you see a bigger impact from temperature on that measurement on that matric potential measurement.

CHRIS CHAMBERS 23:21
But why why is that important? Because you know, you get hardly any temperature sensitivity in the wet end.

Leo Rivera 23:27
Yep.

CHRIS CHAMBERS 23:27
Between zero and minus 100 kPa. It’s, you’re gonna see like little tiny little squiggles.

Leo Rivera 23:32
Yeah.

CHRIS CHAMBERS 23:33
Right? And then same temperature change at, say 15 bar minus 15 bar. And you could see 100 kilopascals swing.

Leo Rivera 23:43
Yeah.

CHRIS CHAMBERS 23:44
For the same temperature. Now explain that.

LEO RIVERA 23:46
So I think there’s a couple of factors. I mean, we talked about the sensor component. But the other piece is that water potential is a temperature dependent measurement. And so some of those things that you’re seeing is actual temperature influence on water potential matric potential itself. But it’s really hard to interpret that. And so Walthert, there’s a paper by Wathert and Schleppi, that does a pretty good job of normalizing that to a temperature that you can use. But, but, you know, I, if you can remove the sensor component of it, I think there’s value in knowing what that pattern is. Because we do know that water potential is is dependent on temperature as well.

CHRIS CHAMBERS 23:46
And the soil water retention curve makes a difference there too, right? Because you’re at a different region of the soil water of the ceramic.

Leo Rivera 24:39
Yes.

CHRIS CHAMBERS 24:39
Characteristic curve.

LEO RIVERA 24:40
Yeah, yeah. And so that there’s a lot that goes into into some of that.

CHRIS CHAMBERS 24:48
One of the main things to remember with the TEROS 21 is that it doesn’t measure matric potential. It measures water content. And so the relationship between water content and matric potential for the ceramic we haven’t really well characterized. But a small change in water content at one part of the curve isn’t going to be the same change in matric potential, depending on the water level.

LEO RIVERA 25:14
Yeah. Yeah. So there’s definitely some challenges with that, I think there’s still quite a bit of work to be done into understanding that, that Walthert and Schleppi paper is actually a really great paper. And, and they do a really good job of of coming up with some methods of handling that.

CHRIS CHAMBERS 25:30
And of course, the TEROS 21. gen two is so much better.

Yeah.

For that.

LEO RIVERA 25:33
Yeah. And so that the other factor has been working on improving the electronics and the capabilities of the sensors didn’t make those corrections. And it’s something that I think we’re going to continue to push on as we as we continue to develop these sensors.

CHRIS CHAMBERS 25:44
Good.

Brad Newbold 25:45
All right. This person’s asking, what is the best method to correct for temperature sensitivity of sensors? Does a temperature correction depend on the measurement principle of the sensor, for example, time domain or frequency domain, reflectometry, capacitance, resistance, etc?

Leo Rivera 26:02
Great question.

CHRIS CHAMBERS 26:03
Oh boy. There’s a couple parts of this right?

Leo Rivera 26:05
There is, yeah.

CHRIS CHAMBERS 26:06
There’s the effective temperature on dielectric itself.

Leo Rivera 26:08
Yep.

CHRIS CHAMBERS 26:09
Right. That’s pretty well characterized, correctable?

Leo Rivera 26:11
Yep.

CHRIS CHAMBERS 26:12
And then there’s the effect of temperature on the electronics.

Leo Rivera 26:15
Yeah.

CHRIS CHAMBERS 26:16
And that’s not I don’t think that’s terribly technology dependent. What do you think?

LEO RIVERA 26:23
I- the only way I think it’s technology dependent is if the sensor has built in correction, for temperature or not. So for example, analog sensors, typically are not going to have a built in temperature correction, on the on the on the measurement itself, digital sensors, depending on the manufacturer, it’s a lot easier to build that in. And so you, you’ll probably find that most digital sensors have a temperature correction applied to the sensor.

CHRIS CHAMBERS 26:23
The five most use sensors for research are going to have that.

Leo Rivera 26:42
Yeah, yeah. And so. So as long as you’re using, you know, the more commonly used research grade sensors should have a correction for that. So then it really just comes down to understanding the impact of temperature on water content itself and dialect- and dialectric permittivity. But also, I think it also comes down to understanding that some of those fluctuations that you see, could be daily fluctuations in water movement in, in the profile as well. And I think it comes down to looking at when your peak-

CHRIS CHAMBERS 27:23
So hard to pin down though.

Leo Rivera 27:24
It is. Yeah, I mean, I always, if I’m looking at those data, I’m gonna look at my peak in temperature and my peak in water content. If those coincide perfectly, it’s probably more of a temperature dependence thing. But if there’s an offset, then some of that actually has to do with those those daily kind of hydraulic flexes that happen, as you get a pull on the water from evapotranspiration, and then a relaxation in the evening, that allows the water to kind of shift back down a little bit, which we have seen observe those type of data.

CHRIS CHAMBERS 27:57
Give a flag, like what percent change in water content per degree C, that sets you off the kind of trips you’re…

LEO RIVERA 28:03
Oh, that’s a great question. Yeah. Probably. This is going to be just off the top of my head.

CHRIS CHAMBERS 28:12
I totally put you on the spot.

Leo Rivera 28:13
You did. That’s great. I mean, probably more than per degree C.

CHRIS CHAMBERS 28:19
We would have over 10 degrees C

Leo Rivera 28:21
Over 10 degrees. C, probably 2%. Maybe

CHRIS CHAMBERS 28:26
That’s pretty big, isn’t it? Yeah. That’d be that’d be a red flag.

Leo Rivera 28:30
Yeah, I think that’d be a pretty big red flag. 2-3%

CHRIS CHAMBERS 28:33
In the TEROS 12. Maybe not the EC five?

Leo Rivera 28:35
Yeah, yeah. Yep. And that goes back to the analog versus digital.

CHRIS CHAMBERS 28:39
Exactly.

Leo Rivera 28:39
So yeah, it’s probably somewhere around that 2-3%. Cool range across 10 degrees C. Yeah.

Brad Newbold 28:50
All right. We’ve got a field question. Here are their installation strategies to avoid cracking around the sensor for clay soils? By the time the soil cracks, I would imagine it’s too dry to reinstall,

CHRIS CHAMBERS 29:02
you know, on installation, if you’re installing in August in a clay, it’s gonna be a rough day. I sympathize. Some, it’s it’s just really hard to work around. You know, if you’re installing on in dry clays and you see a crack, I think you should move, move to a different spot, you’re probably not going to be able to, there’s just, there’s just going to be the risk of having that crack right there. Yeah. It’s less of a risk with our TEROS line of centers than it was has less of an impact. It’s much easier to avoid with TEROS than it was in our older sensors. But yeah, the the real key here is not having any air gaps near your pins.

Leo Rivera 29:52
Yeah.

CHRIS CHAMBERS 29:53
It’s a little bit forgiving if you get further away from the sensor. When you look at it, think about the fact that air has a dielectric of one, and water has a dielectric of 80. So any cracks are just going to kind of push your soil moisture sensor data towards the extremes towards the low extreme when it’s super dry towards the high extreme when it’s super wet.

Leo Rivera 30:18
Yeah.

CHRIS CHAMBERS 30:19
When it’s saturated. So try again, if you get, if you get a crack on installation, move it a little bit, either a different side of either a different side of your borehole or a different part of your trench. But stay away from the cracks that you can.

Leo Rivera 30:38
Yeah, I agree with that. And I also think that one of the approaches that you can take to minimize, and I say, minimize, you’re not going to get rid of this issue. Clays- shrink swell, soils are a challenge no matter what, but minimizing site impact. So the site disturbance upon your installation really helps minimize that shrinkage. For example, if you dig a big trench to install your sensors, there’s a big possibility that that trench is going to experience a lot more volume change over time than some of that native soil might. And so the borehole method, I just, that’s why I love the borehole installation method, because especially for these types of sites, is that it helps avoid some of those issues. The other thing is, you’re not always going to know if- especially like in true vertisols, where you have those slick and slides from the- and the cracks that form from that shrink swell behavior. If you install in the wet season, you might not you might not realize that you’ve installed on a crack. And so the best thing to do is have replication in those sites, because that way, you can use that as a QAQC for each other

CHRIS CHAMBERS 31:49
What do you think, is it best to install in spring or fall then? Install when it’s dry?

Leo Rivera 31:49
Personally, I, I would still say it’s better to install in the spring. Just because of the challenges of installing in clays outside of that period.

CHRIS CHAMBERS 32:03
You’re gonna it’s gonna go a lot easier on your gear, it’s gonna be a lot easier on your sensors, yeah, you’re probably going to get better contact. Yeah. And then it’s just keep an eye open for signs of cracks.

Leo Rivera 32:15
And it is- you can see in the data, if that’s occurred, because you see a pretty- 1, you see, you can see like a really rapid change in water content that coincides with that crack opening. And when periodic wetting events come in, you’ll see really, it’s going to be really flashy, it’s going to be a quick spike, in water content, and then a quick dip in water content.

CHRIS CHAMBERS 32:36
You can see it in your data. But I’ve never once had the conversation with someone where it looks like a crack developed and they’re happy.

Leo Rivera 32:43
No, they’re not gonna be happy.

CHRIS CHAMBERS 32:45
It just doesn’t happen.

Leo Rivera 32:46
Unless you’re like me, and you get excited by soils cracking. Yeah, but no it is the only way to really best I mean, for me, it’s, it’s replication. Just the more measurements, the better. To help cover some of that variability that you’re gonna see from the soils cracking

CHRIS CHAMBERS 33:04
That’s a good answer.

Brad Newbold 33:06
With a field of 100 meters square, how would you distribute? And how many soil sensors would you consider to have reliable soil moisture data? What if there are two different soil types?

CHRIS CHAMBERS 33:17
Now this is a classic sampling problem.

Leo Rivera 33:19
Yeah.

CHRIS CHAMBERS 33:20
Regardless of the field size, it really comes down to soil soil type and hydraulic characteristics.

Leo Rivera 33:30
Yeah.

CHRIS CHAMBERS 33:30
Right? And soil type doesn’t quite capture it all. If you say two soil types, like you’ve got, you know, just a clay in one spot, and a Palouse silt loam in another spot, right, you’ve got two types, but they’re going to behave very differently depending on their slope, their, not so much their aspect, but whether you’ve got a high spot or a low spot, whether water’s draining into it. So classic sampling problem, but not necessarily very easy to answer. Because you have to capture the variability of your field to answer your research questions.

Leo Rivera 34:16
Yeah, it is, this is one of the more challenging questions that we get a lot. And it’s really, it’s hard to determine without having more background information on the site. One thing I do think that is promising is utilizing satellite data to try and characterize-

CHRIS CHAMBERS 34:35
To identify beyond just the soil maps.

Leo Rivera 34:37
Yep, yeah, soil maps are super helpful. Getting your your elevation changes, understanding your landform changes across that site, unless it’s perfectly flat. But even then, there are things that happen below ground that you’re not seeing at the surface.

CHRIS CHAMBERS 34:48
That’s right.

Leo Rivera 34:49
But satellite data actually, I mean, we’re we’re spending some time working on that. That’s actually what we have some of our some of this fellowship students that we’re working with looking looking at. And that might be a powerful tool to use. But still you need background information to help make these decisions. It’s just another source of form of background information.

CHRIS CHAMBERS 35:11
Let’s simplify this question a little bit. 100 meter squared field, and we’re on a side slope, there’s no top nor bottom, there’s two soil types in there. So no, no hydraulic difference other than the two soil types, right? Let’s simplify it a little bit. So a minimum of three, right? Because you’re gonna want to calculate a variance.

Leo Rivera 35:33
Yep.

CHRIS CHAMBERS 35:34
So a minimum of three sensors in each soil type, so that you can capture the variability because they will be different. So that’s 6 sensors, how many depths?

It depends

Leo Rivera 35:48
It depends.

CHRIS CHAMBERS 35:48
So this is where if you want to profile, if you want to capture the different layers, you know, and kind of where you put your deeper soils and your deeper sensors is going to depend on the soil type that you have and your soil horizons, and the questions you’re trying to answer. So you might want three sensors at your deeper depths in each soil type too.

Leo Rivera 36:12
Yeah. Just use a profile probe, and that’s-

CHRIS CHAMBERS 36:15
Or just use a profile probe and then you get it all! TEROS 54.

Leo Rivera 36:18
Yep. Yeah, that’s a tough one. But I agree with that, I think at least characterizing the two different soil types. I would still try to replicate a little bit across the soil types, maybe two to three replicates.

CHRIS CHAMBERS 36:30
Right.

Leo Rivera 36:31
But it really depends on what you can do. That’s not not always feasible. So.

Brad Newbold 36:36
next question, if the input voltage of the sensor fluctuates, will this affect sensor readings?

CHRIS CHAMBERS 36:43
That depends on your sensor. So we only have a couple of sensors now that I can think of that you don’t have voltage regulators. So something like the EC-5, if you change the voltage input, it will change the voltage output. So keep a consistent input. That’s our only soil moisture center for which the input voltage matters.

Leo Rivera 37:07
Yeah, I mean, there is an operational range that they need to be within. But as long as you’re within that operational range, then it really shouldn’t matter. But if you do dip outside of that, then then it will impact your measurement, even with the the digital sensors, because the circuitry needs a certain input voltage to operate. If you dip below that- is

CHRIS CHAMBERS 37:27
So if you go too low, it’ll, your data’s going to be crazy. So you’ll know

Leo Rivera 37:32
Yeah.

CHRIS CHAMBERS 37:33
If you go too high, it’s probably gonna fry your sensor and break it.

Leo Rivera 37:36
Yeah. So just operate within the specs.

Brad Newbold 37:39
How much is the soil water content reading affected by soil texture? Is there a standard method to calibrate the soil water content reading for different soil textures?

Leo Rivera 37:50
Yeah, I mean, we talked a little bit earlier about soil specific calibration operations, there absolutely is methods for that. But really, it comes down again, some of it actually comes down to the sensor that you’re using and the frequency that that sensor operates at. Some soil- sorry, some sensors, we do find to be more sensitive, especially as he increased clay content. That’s the main of the three sand, silt and clay. You’re when you increase your clay content, it can impact some measurements, some measurement techniques.

CHRIS CHAMBERS 38:27
Let’s back up a little bit. Yeah, let’s let’s just answer the first part of the question. How much is the soil water content reading affected by the soil texture all the way. So you cannot interpret soil moisture data if you just have soil water content, and no soil texture information. It’s- you can’t.

Leo Rivera 38:44
Well, that’s cause we’re trying to interpret what soil water content means

CHRIS CHAMBERS 38:47
You don’t know whether or not you have sand or clay. And you just have soil water content numbers, those numbers are going to be meaningless.

Leo Rivera 38:56
They’re meaningless, because there’s no basis of what that means in terms of other things, like plant available water and things like

CHRIS CHAMBERS 39:02
Exactly, yep.

Leo Rivera 39:03
Yeah, I absolutely agree with that. But when it comes to the sensor performance, it really depends.

CHRIS CHAMBERS 39:11
That’s right.

Leo Rivera 39:11
Yeah. But yeah, for sure. The disclosures comes down to background information. Tying water contents on this form meaningful like plant available water. Those- those all play a role.

CHRIS CHAMBERS 39:24
But it’s going to be super helpful, it’s going to be necessary for any treatment effects that you have. If you have two treatments, and you don’t know what soil type is in either one of them and they’re different, then you’re gonna completely confound your treatments. So soil texture matters.

Leo Rivera 39:41
Metadata,

metadata.

Sorry. Metadata is super valuable. Trying to interpret any type of data that you’re reading. If you don’t know the sight, the characteristics of those things, then those readings can be meaningless.

CHRIS CHAMBERS 39:53
Right.

Leo Rivera 39:54
So yeah, we know that as you change your there’s two things to happen when you change your frequency of operation, you either get impacts from texture, it’s clay content primarily because it impacts how the water polarizes when you emit that electromagnetic field. Or if you go in the other direction, it’s the EC. So some of the, then you become more sensitive to changes in the salinity of the soil. So it’s about finding that for that sweet spot. A lot of sensor manufacturer has spent a lot of time working on this, trying to develop tools for that some sensor manufacturers have different calibration equations for different soil types as well. So those are also good options. If you know you have a specific soil type, use the calibration that they they recommend using for that soil type.

CHRIS CHAMBERS 40:40
Exactly.

Brad Newbold 40:42
This looks like it is going to be our last question for this session. Can you build and trust water retention curves if you bury water content and water potential sensors at the same location and depth? Will it compare well with what one gets from the HYPROP?

LEO RIVERA 40:59
It’s a great question.

CHRIS CHAMBERS 41:00
I love it.

Leo Rivera 41:01
Yeah, I absolutely think you can. And I honestly think there’s even more information you get out of your in situ retention curves, especially when it comes to getting your scanning curves., then what you might get out of just the HYPROP measurement that is only on the drying leg of the retention curve.

CHRIS CHAMBERS 41:21
And it’s also the HYPROP is also your ideal measurement. Right? Yes, while you take lab measurements, you completely saturate it, which doesn’t happen in the field. So you, you it’s pretty common to have like a field method and a lab method.

Leo Rivera 41:39
Yeah.

CHRIS CHAMBERS 41:39
And where the lab method is going to kind of get at the intrinsic values of your soil.

Leo Rivera 41:45
Yeah

CHRIS CHAMBERS 41:45
Right? How your soil, your soil’s effect on the soil water retention curve, or the- or the-saturated hydraulic conductivity, where your field data is going to be messier. There’s going to be other things going on that aren’t controlled for and the HYPROP.

Leo Rivera 42:00
Yeah, I think there are some things that you have to think about for the QAQC of your data. When it comes to doing in situ retention curves, the response time of the two different sensors might vary a little bit. So that you might need to do some cleaning of the data to get rid of especially during really flashy events. Like if you have a sudden high intensity rainfall event, one sensor might respond faster than the other to that. So doing some some cleaning up of the data on that. I’ve seen some examples where people generated retention curves in situ from those sensors and didn’t do any cleaning of those data.

CHRIS CHAMBERS 42:34
Yeah.

Leo Rivera 42:35
But it’s pretty obvious when you see that.

CHRIS CHAMBERS 42:36
It kind of brings up a good point, though, you don’t have to use all 35,000 data points that you have.

Leo Rivera 42:40
No, you don’t.

CHRIS CHAMBERS 42:41
But, you know, anytime anytime you’re plucking data points from a data set it, it opens the possibility to be- to be biased.

Leo Rivera 42:49
Yep.

CHRIS CHAMBERS 42:50
Right?

Leo Rivera 42:50
Yep.

CHRIS CHAMBERS 42:51
So you want to have some some objective criterion in which to select what data you’re going to use?

Leo Rivera 42:57
Yep. Yep. But I’ve seen some really interesting retention curves. Actually, I think Andre Patrignani has done quite a bit of work on this himself. And he’s seen some really good results from doing these in situ measurements, and seen some really interesting results. And so I recommend keeping an eye out for some of the publications that are coming out from him. And we’ve done some work ourselves. But I think there’s more work to be done in this area, because I really think the technology is just now getting to the point where we can make really reliable measurements-

CHRIS CHAMBERS 43:32
Over pretty much the entire range.

Leo Rivera 43:34
Yep. So I think it’s gonna be really interesting to look at that. And I’m personally a field measurement person myself, I always find that lab measurements are in ideal conditions. And so when we can tie in those field measurements, it really helps us get a better understanding of actual field behavior of water dynamics,

CHRIS CHAMBERS 43:52
And they complement the lab measurements and not necessarily supersede it.

Leo Rivera 43:55
Yeah.

CHRIS CHAMBERS 43:56
Because you can separate what the soil’s contribution is to it in an ideal condition. Yep. As opposed to what’s actually happening in a more chaotic field setting with more confounding variables.

Leo Rivera 44:08
Yeah, exactly.

Brad Newbold 44:09
All right. Any final thoughts on soil moisture measurements, you guys before we close up here,

Leo Rivera 44:16
The more background data, the better metadata is critical when it comes especially to water content measurements by themselves. And so have more site information to go along with that. And also reach out to us we love talking about this.

CHRIS CHAMBERS 44:32
Yeah, seriously, if you can’t tell already. We do. But yeah, water content. It’s hugely important variable across, you know, any type of ecosystem. Yeah, it’s going to be a driver of what is happening in that system, whether it’s an agricultural system or a natural system, and it’s going to drive everything from water from fires to, to crop yields and crop quality.

Brad Newbold 45:03
All right, well, that’s gonna wrap it up for us today. Thank you again for joining us. And we hope that you enjoyed this discussion. And thank you again for all your great questions. And if you have any questions that we didn’t answer, please contact us via our website metergroup.com. Finally, you can subscribe to the METER Group YouTube channel and accept notifications to see previous episodes of office hours and to get notified when future videos are available. Thanks again, stay safe and have a great day.