Office Hours 18: Water potential—field measurements vs modeling

Office Hours 18: Water potential—field measurements vs modeling
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Soil water potential, also known as soil suction, is a key factor in understanding how water moves through soil. However, choosing the best approach for your application—whether modeling, field measurement, or lab measurement—can be a challenge.

Join METER scientists Chris Chambers and Leo Rivera as they answer submitted questions and explore when it’s best to use modeling, measurement, or a combination of both. In this discussion, they cover:

  • How to translate a soil moisture release curve to unsaturated hydraulic conductivity in the lab
  • The benefits and drawbacks of measuring water potential in both the field and lab
  • How salinity affects soil moisture measurements
  • The utility of saturated hydraulic conductivity for crop irrigation management
  • Preferred methods and instruments for measurement of soilless media
  • And more

Presenters

Leo Rivera operates as a research scientist and Director of Science Outreach at METER Group, the world leader in soil moisture measurement. He earned his undergraduate degree in Agriculture Systems Management at Texas A&M University, where he also got his master’s degree in Soil Science. 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 METER’s 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 at METER Group, the world leader in soil moisture measurement. He specializes in ecology and plant physiology and has over 15 years of experience helping researchers measure the soil-plant-atmosphere continuum.

Questions?

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

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Transcript:

BRAD NEWBOLD 0:00
Hello everyone, and welcome to Office Hours with the METER environment team. Today’s session will focus on soil water potential measurements in both the field and lab, and we’re shooting for about 30 minutes of Q and 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 questions on our website at metergroup.com, and then someone from our science and support team will get back to you with an answer via email. All right, with that out of the way, let’s get started. Today our panelists are application specialists, Leo Rivera and Chris chambers. Leo is a research scientist and director of science outreach at METER Group. He earned his bachelor’s and master’s degrees in soil science at Texas A&M University, and there he helped develop an infiltration system for measuring hydraulic conductivity used by the NRCS in Texas. Leo is the force behind application development in METER’s hydrology instrumentation, including the SATURO HYPROP and WP4C. He also works in R&D to explore new instrumentation for field measurements of water content, water potential and hydraulic properties of 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. Thanks for joining us guys.

CHRIS CHAMBERS 1:27
Happy to be here.

LEO RIVERA 1:28
Yeah, thanks. We haven’t done this in a while.

BRAD NEWBOLD 1:34
All right, let’s get started.

CHRIS CHAMBERS 1:36
Okay, first question, can you elaborate more on how to translate a soil moisture release curve to unsaturated hydraulic conductivity in the lab. And this is a question. This is a question after a webinar that we didn’t get to right?

LEO RIVERA 1:51
Yeah, it sounds like it is. Yeah, I that’s an interesting way to go, or route to go. You can’t just take a soil moisture release curve and go to unsaturated hydraulic conductivity. You have to take other measurements, and so that’s in the HYPROP when we’re making our soil moisture release curve measurements, we’re also making the measurements necessary to calculate unsaturated hydraulic conductivity through Darcy’s law and based on the tensiometers at two different heights and water change.

CHRIS CHAMBERS 2:25
And that’s right. So if you if even, you could do this in the field as well. If you’ve got a soil water retention curve and or soil moisture release curve and uh water water content at two points

LEO RIVERA 2:36
Yeah water content and water potential, at two different points you would need both, because you need to know that that change in water potential at the two different points, along with the change in water over that time period. And we’ve actually experimented with this a little bit and presented on this in the webinar a while back, and are continuing to do more work in this area as well.

CHRIS CHAMBERS 2:55
Right but if you’re in the lab and doing the work, especially if you’re interested in saturated hydraulic, hydraulic conductivity, you know, pop it on a KSAT and, yeah, unsaturated, you know it’s why we’ve got the HYPROP right,

LEO RIVERA 3:11
Yep, yep, exactly, yeah. So you have to use the right tools to get the measurements. There are ways to estimate these parameters through through through models, but those are purely estimates. They’re not, they’re not, you know, hard measurements, right.

BRAD NEWBOLD 3:26
Alright next question, what is your opinion on comparing water release curves generated in the field by the sensors with curves generated in the lab with pressure plates in core samples?

LEO RIVERA 3:38
Okay, that’s a tricky question.

CHRIS CHAMBERS 3:39
It’s a two parter, though, right? It is a two parter because field measurements and lab measurements complement each other for sure, so leaving the pressure plate out of it, which has some baggage. Let’s talk about field measurements, like when you’d want to generate a field soil moisture release curve, and you can do that if you have matric potential sensors and water content sensors, co located, and you actually get really helpful information from that. So the field is telling you what’s happening, you know, it gives you that spot, that point in time, in space, and it’s going to be messier than lab data, yeah, right?

LEO RIVERA 4:23
Yeah now the field it is, you know, you would definitely have to know how to clean up the field data to really get a good retention curve, especially when you’re looking at wetting versus drying curves. I typically recommend looking at the drying curve from field data, because you’re not dealing with response time as much there, it’s a more consistent response. Whereas wetting data can be a little messy when you’re trying to do retention curves, but I do absolutely think, like you said, Chambers, that they complement each other.

CHRIS CHAMBERS 4:23
That’s right, yeah. And the reach in your field data is going to be greater than just the small soil sample. Yeah. The other things happening in your field environment are going to affect it.

LEO RIVERA 5:01
Yeah, and we’ve done some work actually comparing field versus lab measurements. And, and you can actually get really complimentary data that actually compares quite well, or is also the times where you get data that doesn’t compare well, and that oftentimes has to do with somebody has to do a spatial variability depending on where you take your samples versus where your measurements are made. And, and also has to do with how you prepare your sample for versus for especially when you’re looking at repacked versus intact soil samples. But also, I mean, just to touch on the last part of this measurement, some of it also depends on the methods that you use. I mean methods like pressure plates have their limitations, especially when you’re trying to put you’re trying to push into the dryer range with contact issues and equilibrium issues, equilibrium, and so you have to be careful with that and.

CHRIS CHAMBERS 5:51
Yeah, what’s about the range minus 500 kPa, you really can’t go much drier than that in pressure plates?

LEO RIVERA 5:57
Yeah. So oftentimes people try to push them down to that one and a half bar or, sorry, 15 bar point and and we constantly see equilibrium issues. Really, once you start getting past about one bar is where you start to see equilibrium and contact issues.

CHRIS CHAMBERS 6:14
Because the equilibrium the sample has to equilibrate, equilibrate throughout the entire sample.

LEO RIVERA 6:21
Yeah, yeah. And sometimes, especially some samples, you get separation from the plate that causes issues. And yeah, there’s, there’s just a lot of things that can can go wrong there. So yeah, definitely have to be careful when comparing those methods.

CHRIS CHAMBERS 6:33
But the lab to get back to the lab just briefly, and there’s a ton of data on pressure plates historically, and bigger and bigger databases on the HYPROP but yeah, when you bring these samples into the lab, what you get, as opposed to just doing it in a field, is, is this, this intrinsic soil property, yep, yep, and not all of these other field variables that could be confounding your interpretation.

LEO RIVERA 6:56
Yeah, exactly.

BRAD NEWBOLD 6:58
Okay. Next question here is soil moisture affected by soil salinity, and if so, how can we measure the actual soil moisture and avoid the soil salinity effect?

LEO RIVERA 7:10
This is more of a technology question or issue than anything else. I mean, that’s right. Soil moisture, intrinsically itself, is not affected by salinity. It doesn’t change because of salinity.

CHRIS CHAMBERS 7:20
You can have the same volume, yeah, yeah. Of of water based on, based on the salts in the soil.

LEO RIVERA 7:26
Yeah, yeah. So I don’t know Chambers you deal with this more the technology.

CHRIS CHAMBERS 7:29
That’s right, the technology soil sensing, because we’re measuring the electrical properties of the soil, for most soil moisture sensors, then the electrical conductivity is one of those electrical properties, and it can change how you’re sensing, let’s just say dielectric, since that’s what most sensors use, or capacitance, it’s going to affect both. But many sensors are robust to a certain level. Our capacitance technology is solid up to about eight to 10 decisiemens per meter of saturation extract EC, so below that, you can effectively ignore the effects of salinity on your water content measurement. Once you start getting up around eight decisiemens per meter of saturation extract, then you’re gonna have to pay attention to the EC and possibly do a calibration.

LEO RIVERA 7:41
Yeah, yep. Now you know, beyond the actually measuring measurements of soil moisture, I still think it’s super important to understand that salinity, because it can impact how available that water is for plants, and stress that you can see in plants due to due to the salt and so.

CHRIS CHAMBERS 8:45
Particularly in estuarine or managed agricultural systems.

BRAD NEWBOLD 8:49
And then a follow up along those same lines, are there any experiments that check the soil salinity effect on soil water potential?

CHRIS CHAMBERS 8:57
Soil water potential is a different story. Yeah, you absolutely, if you break out the water potential equation for soils, then your osmotic potential is a component in that, and that’s very much affected by the salinity.

LEO RIVERA 9:11
Yeah, absolutely. The great thing is, is, if you know the salinity, it’s actually pretty easy to calculate the osmotic potential, right? So there’s a pretty simple relationship that you can use to make that measurement, or to make that estimate of osmotic potential from the salinity measuring. So that’s really neat to be able to actually do that, and especially important when you’re making measurements in the lab with two tools, one, like the HYPROP versus WP for C, where the HYPROP measures matric potential and WP 4c measures, both matric and total product potential. And so it’s important to understand that and how you actually make those measurements and how to correct for those things.

CHRIS CHAMBERS 9:48
And depending on your measurement technology, matric potential can often be independent from EC, so you might not get that measurement interference like in a tensiometer, the solutes pass but in and out of the ceramic so you really don’t get that effect on your measurement.

BRAD NEWBOLD 10:06
Yeah, all right. This next question we get quite often, I guess, in our webinars and other places, is there a difference between placing the TEROS 21 sensor horizontally versus vertically in measuring water potential?

CHRIS CHAMBERS 10:21
I’m a horizontal fan, no vertical fan yeah, that’s right, yeah. But really, it’s just about how water moves through the soil.

LEO RIVERA 10:29
Exactly, yeah. I mean, the measurement of the sensor itself doesn’t change how you depending on the orientation, but what does happen is how the water moves through the soil is definitely impacted and and it can create a a gradient from the top side of the sensor versus the bottom side of the sensor that can cause issues with the measurement. And so, yeah, along with Chambers, I’m a proponent of the vertical installation, it just helps mitigate those issues and creates the least resistance to the flow of water through soil.

CHRIS CHAMBERS 11:01
And the main yeah, we say that after putting out videos of installing it horizontally the main. But the main thing to bear in mind is you don’t want it to be flat where water can actually pool on the surface of the of the sensor. Yep.

BRAD NEWBOLD 11:17
What is the utility of saturated hydraulic conductivity for crop irrigation management?

CHRIS CHAMBERS 11:24
Yeah, this is kind of a tricky one to answer, but really relevant to irrigation management, a lot of people think you put water on the soil and you’re immediately going to see it in your sensor, yeah, it’s immediately going to be available to the plants. And it takes time, yeah, and it can take a differing amount of time depending on the antecedent soil moisture conditions, yeah. So hydraulic conductivity is, is very important for for how water gets to plants. But no, I don’t think it’s very, very well considered an irrigation management at this stage?

LEO RIVERA 12:01
No, I would agree with your thoughts on that, and I don’t know how well, I don’t think, I personally don’t think it’s super well considered in irrigation management, but I do think it’s an important piece, not only in understanding how it’s getting down to the plants, but also at what rate you can actually apply irrigation, because depending on your soil type, that can change, and especially when you think about a lot of people moving to drip irrigation and things like that. Not only is saturated hydraulic conductivity important, but I also would argue that unsaturated saturated activity is extremely important, because that impacts how the water actually moves away from the drip lines and how it can get to the roots.

CHRIS CHAMBERS 12:37
And it has a different path that it has to take with less with less water available.

LEO RIVERA 12:42
Yeah yep. So in coarser textured soils, you could see a lot more vertical movement of that water coming in from drip irrigation, whereas in finer textured soils, it’s going to pull out more laterally and get actually pulled probably more into the root zone. So you need to understand that when it comes to irrigation timing and the rate that you actually apply.

CHRIS CHAMBERS 13:00
That’s right and we actually see this in practice, in sensors all the time, where, where there’s a substantial delay from water being applied to it reaching the sensors. And if the sensor is deep enough, you might not see it at all, yeah, because so many things are happening over a foot of soil, how quickly the water can move through, which varies, as we’ve mentioned, but also your evapotranspiration and uptake by plants. So it’s a very important variable for irrigation management. It’s just tricky to get the right number and interpret.

LEO RIVERA 13:33
Yep.

BRAD NEWBOLD 13:34
So this next question is a fun one. How does boiling retardation occur in a TEROS 31 sensor to extend its measuring range. Is this achieved by adding solutes to water, or is it due to specific properties and design features of the sensor itself?

LEO RIVERA 13:49
Yeah, it’s a great question.

CHRIS CHAMBERS 13:51
Yeah, this is all about the air, right?

LEO RIVERA 13:53
Exactly. Yeah. It’s all about removing as much dissolved air out of the water that’s going into the tensiometers as possible, but it also is a part of the design features of the TEROS 31 itself. The engineers that worked on this worked really hard to limit the nucleation points in the internals of the TEROS 31 because that’s one of those things that causes that air bubble to form is that there needs to be a nucleation point for that. So that means we have really clean inner shaft of the clean and smooth inner shaft of the TEROS 31 and where the pressure transducer sits, all of that cavity is designed to limit its ability to cause a nucleation point. And then it just comes down to degassing the water as good as possible.

CHRIS CHAMBERS 14:43
Exactly. And if you could see bubbles, then you need to stop and re-de-gas your your shafts and your pressure transducer wells. And this was, I can’t remember who explained it to me like this. It might have been Wolfgang, but. How, if you imagine trying to lift up a weight with a steel chain, you know, it can be a lot easier, but then, if you have a flexible rubber band or something that you’re trying to lift a weight with, each one of those air bubbles is going to expand and contract more depending on how much tension you’re putting on your water column. So it weakens your water column, and you will not be able to transfer as much force.

LEO RIVERA 15:24
Yeah, yeah. So it is, it’s, it’s, it’s a pretty neat thing that we’ve been able to do without, without adding anything to the water. It’s actually just about removing the air and allows those sensors to achieve, you know, in some cases, well beyond 250 kilopascals of measurement rate.

CHRIS CHAMBERS 15:40
I think you need to be a bit of an artist to do that. I’m not that meticulous enough if I get mine down to minus minus 120 yeah.

LEO RIVERA 15:50
Yeah I think with what we’ve done with the systems for degassing, it really makes it a lot easier to do this, versus some of the traditional ways we used to have to do this. So yeah, but it’s all about the tools.

CHRIS CHAMBERS 16:05
Now, tell me, let’s, let’s take that a step further, because it’s TEROS 32 you cannot do that, correct? Because it’s going to get air entry at about minus 85 yeah, right, yeah.

LEO RIVERA 16:14
So there’s two issues. What one with the bigger tensiometers, why you can’t do that? Those cavities are actually too large to do that. And so that’s the beauty of the small inner shaft on the TEROS 31 is it, it it’s just more feasible to actually do that. The TEROS 32 cavities are too large the ceramic also has a different air entry point. So there’s, there’s things that limit that ability. But yeah with the micro or small tensiometers, like the TEROS 31 it’s a lot more possible to do this.

BRAD NEWBOLD 16:46
Okay, this next individual is asking for the TEROS 21 and 22 sensors. What is the measurement range that gives better precision? Do they require calibration or not?

LEO RIVERA 16:57
Yeah.

CHRIS CHAMBERS 16:58
So they are calibrated.

LEO RIVERA 16:59
They are calibrated, yep.

CHRIS CHAMBERS 17:00
And this really just depends on the physics of this, of the ceramic water retention curve, yeah,

LEO RIVERA 17:08
yeah. And I mean, obviously the range so we our primary calibration is from minus five to minus 80 kilopascal. So in that range, the sensor obviously is going to measure the best, because that’s our primary calibration range. We do some stuff beyond that to help kind of characterize the sensors. But once you get beyond that, then there are other factors at play that can impact like temperature and some of those things that can impact the accuracy a little bit. But it’s also really hard to characterize beyond that range.

CHRIS CHAMBERS 17:39
And it’s it’s because we have that inflection point in the in the retention curve of a ceramic where, you know, when things are pretty wet, it’s really forgiving if you get a small change in water content, yeah, the calculations to matric potential are pretty small, yep. But then, you know, a little bit of electrical noise at, say, minus 15 bar, and you get a small, excuse me, a small change in water content that turns into a large change in matric potential.

LEO RIVERA 18:08
Yeah, exactly. And so that’s where the dry range is much trickier to deal with and make good measurements. I mean, I think we do as good as possible right now in that range, and hope to find ways to make better measurements in that range. But, but, but it’s tricky, for sure. I mean, in the wet range, these sensors do the best, but the great thing is that they can measure beyond the light range and and then when things come back into the wet range, you don’t have to refill them. You don’t have to do anything they’re just ready to go.

CHRIS CHAMBERS 18:40
It’s a great tool, depending on the questions you’re asking. Yep, exactly.

BRAD NEWBOLD 18:44
All right. Next one. Do you have experiences measuring water potential in soilless media? What are the challenges? What is the best tensiometers? TEROS 21? TEROS 22? etc?

CHRIS CHAMBERS 18:57
I found the TEROS 21 and 22 to not be great in soilless media, I it’s mostly the contact, right?

LEO RIVERA 19:03
Well, there’s two things at play. It’s mostly the contact, but also most of the water held in most soilless medias is, is really, I mean less than minus 30 kPa, or more than higher than minus 30 kPa, so, so you’re, and oftentimes it’s, it’s in the zero to minus 10 kPa range. Once you get beyond that, then it actually you start to see a lot less water. Yeah, there’s not, not that much water left. And so it’s really because of the way the water is retained in the media that most of it’s in that very wet range. And tools like the tensiometers are just much better one at getting good contact.

CHRIS CHAMBERS 19:39
I’ve seen them work pretty well. In yeah, soilless media, yeah.

LEO RIVERA 19:42
I mean, we’ve done countless number of measurements on soilless media with the HYPROP, and that uses tensiometers, and it does a great job of making measurements in that media. But that’s also what’s shown us, that the bulk of the water is held in the very wet range, right? So, yeah, tensiometers just have the precision to make measurements in soilless media, and they’re just a better tool for that.

CHRIS CHAMBERS 20:04
A little easier to shape into the media as well to get in contact.

LEO RIVERA 20:07
Yeah, exactly, yeah. And the nice thing is that most of these one because they stay in the right range, cavitation is less of an issue unless you screw up and let it get too dry.

CHRIS CHAMBERS 20:18
Well, in which case you might have other problems.

LEO RIVERA 20:20
Yeah, exactly. So, yeah, I don’t know. I’m a big proponent of tensiometers for soilless media. I think they’re the best tool available for that.

BRAD NEWBOLD 20:28
All right, here’s a fun one. Can you talk about conditions where you might see positive pressures in conditions less than saturated?

CHRIS CHAMBERS 20:38
Hmm, that’s a puzzler.

LEO RIVERA 20:41
Yeah? Well, I can think of a few scenarios, and it’s, I don’t think these are super common, but if you do have a wetting front moving through and there’s trapped air in the soil, that wetting front can induce pressure, okay, in that soil, in that soil. But again, this is being going to be pretty fast, really near to saturation too you would, yeah. And so I think these are pretty rare conditions, but it’s not and even in in saturated field conditions, there’s always a chance that there’s still some trapped air in the soil.

CHRIS CHAMBERS 21:11
Yeah and you think structure might play a role in this, for sure, where you got a mix of, like, macro pores and then some micro pores to yeah, really mess with it?

LEO RIVERA 21:20
Yeah, for sure that. I think that’s where that will will definitely play a role. But outside of that, I can’t think of I’m sure there are conditions that we’re missing, like confined with difference between some confined aquifers and unconfined aquifers. But the, I mean, we’re doing pretty complex stuff right there.

CHRIS CHAMBERS 21:39
It should be fairly rare and dissipate pretty quickly. Yeah, yep.

BRAD NEWBOLD 21:41
This one is asking, How does organic matter content affect water potential? Is this something that could be calibrated to an existing water release curve?

CHRIS CHAMBERS 21:53
I mean, you want to do the water release curve with your organic matter in there, right?

LEO RIVERA 21:57
Exactly, because you need to characterize the impact that it has. It does impact it, yep, because we know that increasing organic matter in soil increases its water holding capacity, therefore it’s going to impact, or impact the water potential and the and the moisture soil moisture release curve. But it’s, I’m sure there’s ways to model it with with pedo transfer functions, but it’s really, if you can measure it to understand what that impact is.

CHRIS CHAMBERS 22:22
It’s going to be highly variable depending on the organic matter too. Like if you have fine roots that are breaking down, it’s changed over time. If you’ve got coarse material that’s that’s more resistant to decomposition, then it really you just want to take the sample, do your measurements with the organic matter content, and be aware if you’re getting large changes in in that content.

LEO RIVERA 22:23
Yeah, yeah.

CHRIS CHAMBERS 22:24
All right, here’s another one, hydrocarbons. Do they have an effect on sensors? And do your sensors operate sufficiently in contaminated soils?

CHRIS CHAMBERS 22:59
It’s kind of tricky.

LEO RIVERA 23:00
Yeah. I mean, I think it depends I get this is getting beyond my area. It depends how much carbon, for sure, do can they contaminate sensors? I mean, if you’re getting a large amount of hydrocarbons coming in.

CHRIS CHAMBERS 23:16
You know, I think it depends a lot on what it is. A lot of them are hydrophobic, in which case, yeah, you know if you’re gonna, if you’re going to change the properties of the soil or coat the sensor, I think for capacitence sensors, they should be pretty neutral for the measurement itself, yeah, since that’s emitting an electromagnetic field into the soil, they’re definitely going to have a change on the electric, an effect on the electrical properties though, yeah. So it depends on, it really depends on how much and what the nature of them are. It’s a bit, it’s a bit, it’s a bit broad to just address hydrocarbons in general.

LEO RIVERA 23:56
Yeah, that’s an interesting point that, you know, I didn’t think about the fact that they, they can be hydrophobic. I mean, beyond the impact on sensors, it impacts how water is retained in soil, because that wetting angle change, wetting angle changes, and it’s going to have a pretty big impact on on how watch retained and moves through soil.

CHRIS CHAMBERS 24:15
It’s definitely going to, yeah, that’s right, you’re going to have different soil properties, for sure.

LEO RIVERA 24:21
Yeah.

BRAD NEWBOLD 24:22
How about this one? Can sensor accuracy be affected in any way by the type of field crop?

CHRIS CHAMBERS 24:27
I think your interpretation of the sensor information is going to be more affected than sensor accuracy. Yeah. It’s kind of fun to go down this rabbit hole, like, what are the factors that affect sensor accuracy? Wait, what type of sensors are we talking about right now, soil moisture, or, like, water content, or what matric potential?

LEO RIVERA 24:48
I think we could talk about both. Let’s talk about both. I Yeah. I mean, I can’t think of many situations where the sensor itself is affected, unless you have a huge root ball that is maybe forming because of it.

CHRIS CHAMBERS 25:00
The sensor is going to detect the water, yeah, in plants, just like it affects the water in soils. Yeah, right. So it basically, you’ve got water carrying organic matter in the soil, yeah? But your water content, unless you have, like, really high, really high biomass in the soil, really high root mass, right there. The only thing you’d really have to worry about for accuracy is your calibration. Yeah, right. So you could conceivably have to do a custom calibration.

LEO RIVERA 25:36
Yeah, which I think is still rare, because you think about the dielectric properties of most organic material is not that different than soil. It’s right, pretty close. That’s right. And so I can’t think of many situations where you would like you would have to have a pretty significant increase in organic matter to need to do a custom calibration. To me, though, I think it’s not really what it does to the sensor, but it’s about where you need to measure. To me, I think you think about the different field crops, what I’m concerned about is, what are you what is your rooting depth, and where are the critical measurement zones that you need to be measuring.

CHRIS CHAMBERS 26:12
Right placing your sensor appropriately to answer the questions you’re asking, yeah, and then, you know, interpreting your soil moisture data, like, at what level is the plant happy? Yeah. Where are your set points? Where are your critical you know the zones you want to avoid for sure, so it should affect your interpretation more than it affects the accuracy of the sensor itself.

LEO RIVERA 26:39
Yeah, yeah. Agreed.

BRAD NEWBOLD 26:40
Talking about calibration. So what happens if a sensor is calibrated wrong? For instance, the sensor gets changed from one soil to another. Also, who calibrates the sensor? Is it METER Group, or can the end user change the calibration based on needing to move the sensor? And is it necessary to calibrate every time before use?

LEO RIVERA 27:00
That’s a fully loaded question, right there.

CHRIS CHAMBERS 27:02
Yeah, it is, and really relevant, because calibration is one of the big things that can affect your accuracy, which can infect your interpretation.

LEO RIVERA 27:12
Yeah, and I think we need to break it down into two different aspects of calibration. There’s the calibration of the sensors themselves to make them measure consistently among different sensors.

CHRIS CHAMBERS 27:24
And that’s your that’s the responsibility of your manufacturer.

LEO RIVERA 27:27
Yep, and so that’s where we spend a lot of time trying to improve those processes, to have as consistent sensors as possible.

CHRIS CHAMBERS 27:34
And make it testable with a verification clip you can use in the field, so that you know whether or not your sensor is properly calibrated by the factory, yeah.

LEO RIVERA 27:43
And then the other spec, the other aspect of that, of course, is the soil specific calibration.

CHRIS CHAMBERS 27:50
And that’s converting the electrical properties of your soil to a water content, yeah, yeah, exactly.

LEO RIVERA 27:57
And I mean, of course, that can have an impact on the measurement, and those can be bad calibrations too, for sure, and we’ve seen users do really bad calibrations, and so it’s definitely something you want to be careful. At METER Group, we have two soil specific calibrations. We have our field like our mineral soil calibration, and we have a soilless media calibration.

CHRIS CHAMBERS 28:18
And in general, the mineral calibration will get you within about point 03, meters cube plus or minus in your typical mineral soil. Yeah, we get this question once in a while, like, what is your mineral soil? What? What do you mean by mineral soil? Like, when do we use that calibration? Yeah, and basically anything that falls in your texture, classes of sand, silt, clay, is is going to be appropriate for that calibration.

LEO RIVERA 28:43
Yeah, and we have a pretty good soil library that we use for our soil specific calibration that fits into that general mineral calibration. But There absolutely are conditions where the you may need a custom calibration.

CHRIS CHAMBERS 28:55
Exactly, volcanic soils, pumice folks have wanted to to use it there, and absolutely needed a custom calibration. Yeah, I’ve seen some, some really, really low density soils come across that where our standard calibration didn’t make any sense. Like, some soil is less than one gram per centimeter cube. Yeah, in density, are definitely going to need a soil specific calibration. On the other end, it’s really dense clay’s, maybe some heavy clay’s, right?

LEO RIVERA 29:28
Yeah I mean some like, I mean, these have to be pretty heavy clay’s, because we do use clay as a part of our calibration. But there are some sites where you have upwards of 70-80% clay. That’s where you might need soil specific calibration to address some of those behaviors that that aren’t quite captured in the typical mineral calibration.

CHRIS CHAMBERS 29:50
That’s right, and it’s good to bear in mind that there’s an immense variability of soils in our biosphere, across the planet, and having one you. Equation to capture that entire variability. It’s not a valid expectation. Let’s call it not a reasonable expectation. Yeah, agreed. So occasionally you are just going to have to do a custom calibration, and you can actually generally improve expect to improve your accuracy if you do a custom calibration, yeah, although there’s some soils that that the calibration fits extremely well.

LEO RIVERA 30:25
Yeah, yeah I mean, one part I would stress there is, is, if you’re going to do this, follow good methods for doing this. Homogenize your soil. And we have resources out there that cover how to do this, or SOP’s online. Yep, please follow those because, like I said earlier, we’ve seen some really bad calibrations as well, and sometimes you can do yourself a disservice by doing a bad calibration and assuming that it’s going to help your measurement and it actually makes things worse.

CHRIS CHAMBERS 30:54
Yep, exactly.

BRAD NEWBOLD 30:55
Getting into a lab question, what would be the performance of the HYPROP in measuring soil water retention curves and calculating hydraulic conductivity at the near saturation range.

LEO RIVERA 31:08
Okay, well, there’s two parts of this. I mean, they’re asking about one, the retention curve versus hydraulic conductivity. And there’s two different aspects of that. The retention curve phenomenal. I mean, that’s obviously, that’s where the HYPROP thrives is in measuring that near saturation down to about minus 100 kPa range of the retention curve. Hydraulic conductivity is a different beast.

CHRIS CHAMBERS 31:30
Hydraulic conductivity blows my mind once in a while, just just imagining how the different ways that water can move through a soil based on its soil conditions. Like, if you’re saturated, it’s pretty simple, right? Yeah, it’s going to move one direction. Yeah, it’s going to have a pretty consistent conductivity. And you know, your pressure heads are going to make a big difference in the water flux. But unsaturated, that gets like, if you’re looking at an unsaturated conductivity between, like, minus two megapascals, yeah, and say minus 100 megapascal Or no, say minus 100 kilopascals, yeah, you’re gonna get very different connectivities, right?

LEO RIVERA 32:13
Oh, yeah, yep, yeah. And one of the challenges with making this measurement is that, especially when it comes to measuring it with the HYPROP, is that we need a difference. In the rate of change between the upper and lower tensiometer that is detectable. So for the retention curve, that doesn’t matter, because then we’re just measuring that the average matric potential in the core. But to do the hydraulic conductivity measurement and to follow Darcy’s law, we need a measurable rate of change difference between the upper and lower tensiometer. And for some soils near saturation, that doesn’t happen for a while, like you don’t actually see that they just track with each other. And but in finer textured soils, we actually can push that measurement further up into the range, because you start to get deviations earlier on in the measurement. And so so that’s where the answering this question is a little tricky, because it is dependent on soil type, and how that how that rate of change differs, and so it can be a little more challenging for some soils to get right near saturation to get that hydraulic conductivity measurement.

CHRIS CHAMBERS 33:16
Now, how well do models work for unsaturated conductivity based on like texture, class, or?

LEO RIVERA 33:21
They work better than saturated hydraulic.

CHRIS CHAMBERS 33:24
Good to know.

LEO RIVERA 33:24
Yeah, because once, when you start getting into the unsaturated hydraulic conductivity measurements, you do get to a point where structure is playing less of a role and it’s more so the movement in between the inner particles. And so that if you are using models that take into account bulk density, they do a much better job than they do at that than they do at measuring saturated hydraulic conductivity.

CHRIS CHAMBERS 33:24
Like your tortuosity from your extra class, yeah, and then your macro pores really don’t make a difference.

LEO RIVERA 33:39
Exactly, yeah, at that point, the macro pores aren’t what’s contributing. There is a point. I mean, there’s a point, and depends on the size of the macro pores, but you can use models to estimate this, and they do, you get a better estimation for models for unsaturated hydraulic conductivity because of that than you do for saturated hydraulic conductivity. Cool, yeah.

BRAD NEWBOLD 34:12
All right, what is the quickest and easiest way to install a water potential sensor in the field?

CHRIS CHAMBERS 34:18
Matter transporter.

LEO RIVERA 34:21
Yeah. I mean, if that was feasible,

CHRIS CHAMBERS 34:23
All right, yeah, we wouldn’t. Probably wouldn’t have this problem if we had that technology, would we?

LEO RIVERA 34:26
No.

CHRIS CHAMBERS 34:28
So this depends on the sensor selection, right?

LEO RIVERA 34:31
Yep, yeah. And then it also depends on depths that you’re wanting to measure at. In some cases, if you’re trying to do deep measurements, I you know, to some extent, depending on the soil type, the 21 can be better if you have good conditions where the install tool will work, which we know, there are some conditions where the install tool doesn’t work.

CHRIS CHAMBERS 34:48
The 21 can be tricky. It can be tricky. It’s gotta its form factor isn’t, yep, isn’t made with installation. Ease of installation in in mind.

LEO RIVERA 34:57
No, but that’s one of the reasons we worked on tools like the TEROS 22, 22 yeah, is because it really does simplify that. Now, of course, the TEROS 22 has those limitations too, but you can, especially if you’re measuring within the upper meter.

CHRIS CHAMBERS 35:11
Within the upper meter, drill a hole, yeah, push it in. Drill another hole. Push your next one in at a different depth, yeah, and you’re ready to go.

LEO RIVERA 35:19
Yeah. So that is something that’s neat about the TEROS 22 is you can take a drill out there with a masonry bit on it, drill your hole takes probably less than a minute, and you just push the sensor into the ground.

CHRIS CHAMBERS 35:30
Yeah, there are conditions when we were at National Soil Moisture network last year, and so where were we? Logan, and it was hit that Caliche layer, yeah, hammer drill, that still, yeah, was kind of fun, but we got, we got into there, yeah, and, yeah.

LEO RIVERA 35:49
I mean, some soils are gonna cause challenges, rocky soils, Caliche layers.

CHRIS CHAMBERS 35:52
I mean, every single, yeah, every single device you’re trying to install in soils, you’re gonna get challenges, yeah.

LEO RIVERA 35:57
But in many sites, especially if you’re these are sites that are cultivated, sites where you’re growing less likely you’re going to run into any major issues like that, because typically those are not optimal growing sites. But you can pretty easily install those sensors. And what’s really nice about those sensors is, if you need to remove them at the end of the season, the TEROS 22 something, you can also remove at the end of the season.

CHRIS CHAMBERS 36:19
Exactly, especially if you’ve got it going all the way to the surface, you can just grab it and pull it out.

LEO RIVERA 36:26
Yep.

BRAD NEWBOLD 36:27
This next question is asking, How does sensor equilibration time affect the temporal resolution of water potential measurements in dynamic field conditions?

CHRIS CHAMBERS 36:36
Really, this is going to have the biggest impact when you install the sensor, right?

LEO RIVERA 36:40
Yeah, for sure. I mean, I think when you install the sensor into some cases, if you have really flashy events, and some some sensors might have a slight delay, like the solid matric sensors, like the TEROS, 21 and 22.

CHRIS CHAMBERS 36:52
But largely it’s going to be a function of your soil hydraulic conductivity, yeah. And then the ceramic hydraulic conductivity, yeah, if it’s different than the soil, yeah.

LEO RIVERA 37:02
I mean, for the most part, the ceramic has fairly similar behaviors to soil.

CHRIS CHAMBERS 37:07
Because we have, we’ve composed it such that it’s very similar to soil in its core structure. Yeah. The big difference is, is soil structure. But again, unless you have, like, macro pores where water is just flying through it, the sensor is going to respond very similarly, yeah, to the soil.

LEO RIVERA 37:27
And if this is ever something you’re really worried about, and you have really fast, flashy events that you need to measure, sensors like the tensiometers have much have incredibly fast response times, basically seconds, yeah. And so if that is the thing that you’re worried about, then I would use tools like the tensiometers to capture those events. But in most cases, I mean, there’s, there are some applications where I think that’s important.

CHRIS CHAMBERS 37:50
And if it’s too dry for tensiometers, then your matric potential sensor, like the TEROS 21 or 22 is going to be perfect.

LEO RIVERA 37:56
Right, and when you’re in those dry conditions, your rates of change are not that fast. So, so those, yeah, those sensors are going to be have no issues with that.

BRAD NEWBOLD 38:04
All right. Next one. In what ways do field conditions such as freeze, thaw cycles, root growth or microbial activity interfere with long term water potential measurements?

CHRIS CHAMBERS 38:14
I mean, this is why you take long term water potential measurements, right? So that you can observe how these are happening and what’s happening, you don’t always know what is happening in there, so these can frequently appear like confounding factors. But if you’re one, if these are any of your research questions or things you want to control in the field, this is why you do field measurements rather than lab measurements, yeah, because you cannot capture variability from any of these things in the lab, unless you’ve got a very elaborate, long running experiment.

LEO RIVERA 38:44
Yeah, no, because a lot of these things that we’re talking about here definitely impact, over time, how water is retained in soil and what that water potential looks like. I think the only other thing I would add to this is there are some situations where it can impact your ability to make measurements, especially if you’re using something like tensiometers. Because tensiometers do not like freezing conditions, nope. And so you would have to empty them during those freeze thaw periods until it’s at a safe point where you’re not worried about the sensor freezing and getting damaged. But in these conditions, tools like the TEROS 21 and 22 you can leave them out. They can handle freeze thaw cycles with no problem, and you can continue to make measurements during those periods. But outside of that, yeah, I mean, a lot of these are factors that just actually impact how water is retained in soil.

BRAD NEWBOLD 39:29
Okay, next question, how can real time water potential data from field sensors be integrated into decision support systems for irrigation scheduling?

CHRIS CHAMBERS 39:39
Basically be an excellent tool to do that, right? Yeah, especially, we have something like zentracloud, where your data is near real time, you can pull it in from a bunch of different sources, get alerts and visualize the history of the data as well.

LEO RIVERA 39:57
Yeah, no. I mean, I think that these sensors can play such a good role in building out these decision support tools for irrigation, because ultimately, if you have the data, you can make better decisions on when and how much you need to irrigate, and these sensors can help you with that.

CHRIS CHAMBERS 40:16
Even if you don’t know where to start, the sensors are going to give you an objective view of the water status, exactly, and you can watch how your crops respond over time, yep, see what your yields are like over time, and take a lot of the guesswork out of what’s what’s affecting your yield and and fruit quality.

LEO RIVERA 40:37
Yeah. I mean, if you know what the optimal ranges for water potential are for the crop that you’re trying to grow. Then you have the measurements in the data you need. Like, okay, these are my bounds. I need to stay in these bounds. Get a text message when it leaves them, Yep, yeah, and especially if it’s if, I mean, not only on the dry end, but there’s bounds on the wet end too, that sometimes many crops are sensitive to being too wet, and that can impact you as well and these give you both sets of information.

BRAD NEWBOLD 41:02
Yep, all right. Last question here, how do field measured water potential values compared to those predicted by pedo transfer functions, and what are the implications for model reliability?

LEO RIVERA 41:13
Yeah, my favorite topic.

CHRIS CHAMBERS 41:16
So the functions are built on this information, right?

LEO RIVERA 41:18
Exactly pedo transfer functions are, are basically a large database of data that they’re pulling from to take in the properties that you put into them, to estimate what either your retention curve is, what your hydraulic conductivity is, it’s, it’s, that’s all it is is. It’s just pulling in a huge data set to try and estimate, based on the parameters that you input, what these variables could be, and they do a decent job if you put in the right parameters. And so it’s all about what you put in, but at the end of the at the end of the day, it’s still an it’s just an estimate. It’s great. It gives you an idea of what’s going on. And if you can’t make the measurements, you know at least it’s helpful to understand, right? But, but again, there are still so many things that pedo transfer functions don’t take into account, or they’re trying to improve those pedo transfer functions to take them into account, like organic or, sorry, like organic matter, soil structure, all these things that also play a really big role that aren’t as well accounted for in pedo transfer functions. And so that’s why I think measurements are so critical. It’s really actually, really actually characterize these properties.

CHRIS CHAMBERS 42:25
And do some ground truthing on the models as well. As you build a model, test it against how it performs with real values from the field.

LEO RIVERA 42:32
Yeah, yep. And the you know, the more data you collect, then you can maybe build a more robust tool set of pedo transfer functions to then estimate some of these parameters from other data sets in the future, but you still need a really strong data set, model or data set to build that model, and the input parameters are also very critical, because if you’ve gone really basic and just using soil texture for your pedo transfer function, you’re going to get a pretty basic answer.

CHRIS CHAMBERS 43:03
Yeah, just like your face.

LEO RIVERA 43:08
You can cut that.

CHRIS CHAMBERS 43:11
You don’t have to though!

BRAD NEWBOLD 43:13
All right. That wraps it up for us. Thank you again for joining us today. We hope that you enjoyed this discussion, and thank you again for all the great questions. Also, if you have any questions we didn’t answer, please contact us via our website at metergroup.com finally, 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.

 

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