Transcript

BRAD NEWBOLD 0:00
Hello everyone, and welcome to Office Hours with the METER Group team. Today’s session will focus on the proper installation of volumetric water content sensors, and we’re shooting for about 30 minutes of Q and A with our experts, Leo Rivera and Shaun Weldon, 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 then 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 Shaun Weldon. Leo is a research scientist and director of science outreach at METER Group. He earned his undergraduate degree and masters in soil science at Texas A and M University, where 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 and D to explore new instrumentation for water and nutrient movement in the soil. Shaun Weldon operates as the SATURO HYPROP and tensiometer Product Manager at METER Group. He has 13 years of experience helping researchers measure the soil, plant, atmosphere, continuum. And our first question of the day is, what is the best method to correct for temperature sensitivity of sensors? Does the temperature correction depend on the measurement principle of the sensor? TDR, FDR, capacitance, resistance, etc?

LEO RIVERA 1:36
Yeah, that’s a really good question. And, and I’m going to actually break that down into a couple of different parts, because when you’re thinking of temperature sensitivity, there’s two things that you need to think about right? There’s the sensitivity of the electronics of the sensor, and then there’s a sensitivity of dielectric permittivity to water. And first and foremost, you need to make sure that the sensors are temper themselves, have temperature correct electronics, and that can vary depending on the technology, yeah, but there aren’t really any sensors out there that correct the dielectric permittivity, or the impact that temperature has on the dielectric permittivity when then going from dielectric permittivity to water content. So that’s, I think, where things get a little more challenging, which we do know that dielectric permittivity is sensitive to temperature, but making that correction is not as straightforward as one might think.

SHAUN WELDON 2:29
Yeah, and I think that’s an important thing to remember, that it’s not just the sensors that are changing, that there’s actual changes happening in your soil and in the water there as the temperature changes. A lot of times, when we talk to people about it, they think that it’s just a part of the sensor electronics, like you said, but there’s actual different things happening as that temperature changes, yeah. And like you said, there are some technologies are a little bit more susceptible to those changes, things like TDR, those are more susceptible to those temperature changes. The electronics themselves, capacitance is a little bit less sensitive, and we also build in corrections for that electronic changes due to the temperature.

LEO RIVERA 3:10
Yeah, that’s correct, yeah. And so it’s, it’s important that that’s looked at, and just making sure that the sensor actually has corrections for temperature, resistance sensors are also probably gonna be quite a bit more sensitive to that than some of these other technologies. It’s just, it’s just the nature of the of the beast. We’re spending a lot of work right now actually trying to understand the upset end of that, understanding how dielectric permittivity changes with temperature. But it becomes pretty complex, because what you what we see is that it changes with soil type as well, and depending, especially as you go to finer textured, higher clay content soils, that you actually see a change in that relationship. And so it makes it not as straightforward as one would hope to do corrections.

SHAUN WELDON 3:50
Yeah. And another thing to remember with that is that when we’re doing these corrections for our sensors, we’re doing it in a very controlled environment where we can hold those temperatures for a long time. Yeah. When you actually have these in the field, you’re not able to maintain those kind of corrected, corrected conditions. So while we do it, while we do our best to try to come to account for that, there’s definitely going to be times when it’s going to be just harder to harder to account for because of the rapidly changing conditions.

LEO RIVERA 4:20
Yep, exactly.

BRAD NEWBOLD 4:21
Next question here, how do you deal with sensor degradation over time?

LEO RIVERA 4:27
I guess it really depends on the type of sensor you’re working with, right? Yeah. Ultimately, the goal is to have sensors that are going to be pretty robust over time, and we’ve spent a lot of time ourselves learning how to make sensors that are more durable. I ideally, we don’t see a lot of this with with current generation of sensors.

SHAUN WELDON 4:48
Yeah, and I think that’s one of the big inspirations for our TEROS line of sensors. You know, some of our older sensors, we did start to see degradation in the in the softer materials that we used with it, and that’s what inspired us. To create the more robust TEROS sensors. You know, currently, the place where we see the most degradation is not even really degradation, but the most susceptible part to that is the is the cable. Yep, and so good cable management is going to really extend the life of your sensors in that way, the actual sensor housing and electronics themselves, they’re so well protected now that, I mean, we’ve had some out for 10 years that have not shown any signs of damage or degradation. So I think just really making sure that you protect and take care of that cable, yeah, and your, your telemetry section is going to be the part that’s really going to extend the life of that.

LEO RIVERA 5:39
Yeah, yeah cable protection is key. I mean, for any installation, especially if you want something to last and ensuring rodents are going to get to it. I mean, you do see some UV, UV degradation of the cable over time. So protecting that, yeah, yeah, whatever you can do to protect the cables is really what’s going to prolong the life of your measurements.

BRAD NEWBOLD 5:59
Next question, how easy is it to move sensors between sites, and should we more carefully monitor ones that have been moved just to make sure that they’re functioning properly?

LEO RIVERA 6:10
Yeah. I mean, unfortunately, I’m not a big fan of moving sensors between sites, except for the types of sensors that are designed to be removed. That’s why we design tools like profile probes, right? And those are designed to be removed at the end of the season and moved if moved around, if there needed to be point sensors, it’s much more challenging to remove the sensors, yeah.

SHAUN WELDON 6:33
And I think most of the time, the issue that we see is, again, going to be cable damage. Without digging them up, if you’re pulling them out, you can stretch those cables. One thing that I like to recommend if you if you absolutely do have to move them, is to get one of the if for our TEROS sensors, get one of the verification clips, and before you install it in the next one, just run that verification clip and run it in air, just to make sure that you’re getting the readings that you would expect, just to ensure there’s no cable damage to that sensor before you install it in your next site?

LEO RIVERA 7:04
Yeah, no, that’s, I think that’s the key part. Is checking the sensors, and really, if you’re having to remove point sensors, taking care when removing the sensors, trying not to bend the needles, potentially damaging the cables. Because even if you don’t see visible damage to the cable, it could be that you’ve strained its connection within the housing, and you could see issues there. So it’s definitely something you have to be careful with. If it’s something where you know you need to move sensors, I would encourage using tools like profile probes and things like that that are are meant to be removed. Same goes for the for the water potential sensors, like we’re designing water potential sensors now that can be removed at the end of the season. So if you know this is something you’re gonna have to do, try to use the tools that are a little bit better suited for being moved.

SHAUN WELDON 7:48
Yeah, and also, do yourself a favor, and don’t put them down three to four meters and then dig all that out. Yeah, we run into that as well.

LEO RIVERA 7:56
I think at that point you might be better off just getting a new sensor. You’ll spend more money trying to dig out from from that depth than what it’s worth.

BRAD NEWBOLD 8:05
In this next question this individual is asking in one of your webinars, I thought I heard that one of the installation problems is not having the water content sensor properly leveled. Do they need to be leveled?

LEO RIVERA 8:17
Yeah, that’s an interesting question. I mean, that’s not really something for most sensors, except for the tensiometers, where the angle does impact the refilling. But for water content sensors, it’s more so about the measurement volume and what it’s actually measuring yeah, and that orientation.

SHAUN WELDON 8:36
I could see possibly if, if it’s at an angle where water could have flow to, right that that could be the problem. So it’s not that it has to be perfectly level, it’s just that we want to eliminate any possibility for preferential flow right on that sensor.

LEO RIVERA 8:52
Yeah, and that brings up another good point is some of the larger sensors bodies, when you do install them flat, they can also impede water movement through your profile, so that’s also an area, so sometimes, ideally, we might want to install them where it’s less of an impedance to water flow in the soil as well, especially for the water potential sensors like the TEROS 21. I always encourage that those the discs are oriented vertically on that because you can wind up with weird dry zones if you insert them differently, but outside that, the sensor is going to operate fine no matter what orientation you put it in but there are other factors that the installation can impact.

BRAD NEWBOLD 9:34
This next one is asking if the manual moisture is captured through gravimetric sampling, but the TEROS 12, for example, logs data through volumetric measures. How could this be converted?

LEO RIVERA 9:46
Yeah, I mean, it’s a pretty simple conversion if you know the bulk density of the soil, because volumetric water content is just your gravimetric water content multiplied by the bulk density of the soil. So that’s a really simple correction, but with the caveat, knowing that when you take gravimetric samples, there is spatial variability of soil moisture. So if you’re trying to directly compare with a point measurement of water content, you need to understand that there can be differences just due to the spatial variability. Yeah, yeah. And so just trying to avoid some of that we do see people often trying to do in situ, soil specific calibrations, which I don’t really recommend. We actually just published a paper with researcher at Kansas State where they talked about methods for calibrating sensors and a field validation with a form of gravimetric sampling, and that that was probably the best approach I’ve seen to doing this where they’re not calibrating, it’s just a validation, and their validation recommendations were really good. But yeah, outside of that, just always caution when doing those comparisons, yeah.

BRAD NEWBOLD 11:02
What are your thoughts on digging holes to install sensors in drip irrigated fields when it comes to water infiltration rates where soil was disturbed to install the sensors?

SHAUN WELDON 11:12
I think this is a great question. It’s something that a lot of people ask us, and so I think it’s really important in these kind of situations that you’re being careful when you fill that that hole back up, that you’re using the soil horizons that you pulled out, and you’re putting them back as closely as you can, both to the the horizons and the density that it was previously. There’s going to be a little bit of of I mean, anytime you install a sensor, you’re creating disturbance, so it’s going to change things a little bit, but especially with irrigated fields, because there’s wetting so often, the soil tends to settle pretty quickly, yeah, and you’ll get back to those pre-disturbance behaviors pretty quickly.

LEO RIVERA 11:54
Yeah, and one of the things I really you know, talk about this a lot, but using tools to help us minimize site disturbance, and this is where tools like the borehole installation tool have really helped with this, because traditional methods of digging larger pits or trenches to install sensors result in much larger disturbances to the site, whereas the borehole installation tool or those methods really minimize that. And like Shaun said, though it is really important to try and repack the soil in the same order and as close to the approximate density as possible, and probably ensuring that you use a tarp or container for the soil that you pull out so you don’t lose any of that soil, to ensure that you actually return the same amount.

SHAUN WELDON 12:35
Yeah, and that’s one of the things that the borehole tool, like has a big advantage over, say, a pit or a trench, is that you can keep those layers exactly how they came out.

LEO RIVERA 12:44
Yeah, so minimize site disturbance as much as possible, because, for sure, it can impact the measurements, but also, if this is a continuous crop, that you’re not disturbing the site using permanent installations and leaving them the site’s gonna kind of, you know, it’s gonna last longer, yeah, it’ll be. So those are methods. But in sites where you do need to pull sensors in and out at the end of the season, again, going to go back to using the sensors that are a little bit better suited for this because, like the the profile probes and also the water potential sensors that are designed to be removed really don’t create a lot of site disturbance, yeah, and so using the tools that are best suited for your application.

BRAD NEWBOLD 13:30
This individual is asking, are there 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.

LEO RIVERA 13:42
Yeah, cracking soils are a challenge. Always have been, always will be. Actually, was meeting with some folks from California that were doing some measurements, and they were finding that they were actually, I don’t know why I keep going back to profile probes, but they actually were getting better results in these cracking soils with their profile probes, which surprised me, to be honest, but I’ve looked at their data and it it did show that they were seeing less of an impact from the cracking soils, which is not quite sure why that is, but you know definitely when you install You want to install during an non cracking period of the of the soil, especially in high shrink, swell soils like vertisols, but, but, yeah, I don’t know what other thoughts you have there Shaun?

SHAUN WELDON 14:32
Well, I think one of the big things is it kind of goes back to that last question that we’re talking about. When you minimize the disturbance in there, you’re giving less opportunities for those cracks to form around the sensors specifically, you’re still going to get cracking on the surface. That’s just going to happen. But when you do, when you have larger disturbances, when you’re installing, you introduce new avenues for those cracks to go deeper and closer to the sensors, because that’s where the disturbance was. So using things like that, that might be why the profile probe does better, yeah. But using things like the borehole tool, bore hole installation tool, again, really helped to minimize that, and it can, it can lower the occurrence of those cracks as well.

LEO RIVERA 15:11
Yeah and one other thought that I have here is redundancy helps. So we have some sites down in Texas where we have multiple sensors installed in very expansive soils, and I have seen in some of those sensors where you can see when the crack formation occurs, because you see a very drastic drop in the water content as that crack opens, versus some of the other sites. But we have enough sensors out there where we’re in the in that same horizon we’re not measuring in that crack, so we can compensate for that, yeah, and so, so that helps but what we also find is, at the end of the season, when those cracks close back up, that sensor actually returns, returns to normal operation. And so it does, you know, they can kind of rebound from that, but I think redundancy is helpful here, especially in cracking soils.

SHAUN WELDON 16:01
Yeah, definitely.

BRAD NEWBOLD 16:02
If you install two soil moisture sensors in the same soil layer, but at different depths, and you know, the transpiration of your crop, is it then possible to quantify from the soil moisture content data, the water flux between those soil layers?

LEO RIVERA 16:17
Yeah, that’s a really good question. And actually something that we’re seeing more and more and more work being done, not that this hasn’t been done in the past, but more people exploring this. There’s a couple things that I think are important here. It is very helpful to know the flux, but you ideally need to know the flux in both directions and the way I would approach this, there’s two ways, Gaylon recently presented on this in a webinar where he used the salt balance approach to actually look at those fluxes. But another approach that I would also recommend taking is, if you have a water content and water potential sensor installed at the same depths, then you can actually look at your gradients and know which direction the water is moving. And then with the water content, you have the volume of water, yeah, and, and then you can actually do some of your fluxes, and I presented a little bit on this on a webinar a little while back, but, but those are the approaches I would take.

SHAUN WELDON 17:11
Yeah, I think it’s also important to make sure that you have more than just one set of sensors so that you can see if there is lateral movement as well you can start to see that, yeah, with with the multiple sensors.

LEO RIVERA 17:21
Yeah, yeah. And it’s always important when you’re doing these types of measurements, is that there is preferential flow, and so there is potential where you might miss movement of water in the soil with a point sensor. So it’s not a perfect check for water fluxes in soil because of that preferential flow, especially in well structured soils, or soils with a lot of warm activity, things like that, you can you can definitely see movement of water deeper into the soil that might not get picked up on by a soil moisture sensor, just because of of some of that yeah that preferential flow so.

BRAD NEWBOLD 17:56
Should you select the depth for locating water content sensors based on the type of soil or the type of crop, and the development of the root system, or a combination of both.

SHAUN WELDON 18:06
I think it is going to be a combination of both. But I do think that in most cases, your type of crop is going to be a little bit more important than your soil type, because that rooting depth is going to change based on your crop type. So that means the water is going to be drawn from different depths. So knowing where those those points are will really help you get a better setup for that.

LEO RIVERA 18:30
Yep, yeah, I agree. I mean, this is, this is definitely an application dependent question, yeah. For example, people that aren’t even paying attention to crops might just want to measure at the different layers and integrate that, especially if you have big changes in texture in those layers, because then you’re going to have very different ways that the water is retained in those layers. And for example, with a lot of the soil moisture networks and the weather networks, they just have very specific depths that they measure at right. And so it always comes down to the application, but I agree with Shaun that really probably the more important piece is the rooting death, because ideally, you want to know what’s in the root zone, and then you want to know what’s moving past the root zone. And so making sure you’re getting a combination of both is critical there.

BRAD NEWBOLD 19:16
Next question here, can you comment on salinity effects, on water content values. At what point would the salinity really start to change the interpreted water content?

LEO RIVERA 19:27
Yeah, I mean, that’s definitely a technology dependent question. Some technologies are more sensitive to salinity, and some actually eventually start to they just stop reading at a point. You see some issues with TDR, where the waveform collapses as you get the higher salinities, and so they won’t measure. But it also depends on the frequency of operation of the sensor, yeah, and, and…

SHAUN WELDON 19:57
That’s really one of the advantages of the capacitance sensors is that you can change that, yeah, frequency. You know, the frequencies that that our sensors operate at you’re, you’re good for pretty salty, yeah, soils up to about 10 decisiemens per meter, yeah, which is, which is pretty salty for those soils. And above that, they’ll still read there just might be a little bit more error, yeah, than what you would see below that.

LEO RIVERA 20:23
Yeah and one way to correct for that is to do a soil specific calibration in those situations, which we see people do now, hopefully in the future, we’ll continue to work on making better technology that is less dependent on this. But as like I said, there’s, you know, every sensor has their their issues and their weaknesses and and it’s hard to build a perfect sensor that covers all situations.

LEO RIVERA 20:47
Yeah, absolutely.

BRAD NEWBOLD 20:48
This next person is asking, what is the best way to measure water content and water potential? Is it a good alternative to install the combination of both types of sensors, or is it better to use soil retention curves?

LEO RIVERA 21:02
Yeah, that’s a really good question. I mean, there’s a lot of options, right Shaun?

SHAUN WELDON 21:05
Yeah, yeah, absolutely. And this is probably won’t be a very helpful answer, but I’d say yes to both. When you’re when you’re doing the soil moisture there the soil retention curves, you’re going to be getting a really good indication of what that soil is doing. However, when you’re using the point sensors, and you have both of them in there, you’re going to be seeing not just what the soil itself does, but what the whole structure and the whole picture is doing in the field. Because that’s going to be different. When we measure it in the in a lab setting, we’re doing a very clean, yeah, you know, structured, kind of controlled measurement, which is almost impossible to get in any field, yeah, so I think it’s good to do both.

LEO RIVERA 21:50
Yeah, yeah. I agree. I mean, you know, sometimes it’s not always an option to put water potential sensors out in the field for various reasons, either due to maintenance or budget, a retention curve is a good alternative, if that’s all you can do to get that relationship. But like Shaun said, there’s other variables at play in the field. Hysteresis is a big one, that in the lab we’re doing a wet drying curve, and so we’re always we’re on that one drying curve and in the field, because, you know, we know that water potential acts on a scanning curve, that you will see some variability in the field because of that. And so if you have that direct measurement of water potential, and there are quite a few different tools out there for using this, there’s tensiometers, if you know you’re going to be in the wet end, they’re the most accurate tool out there for doing that measurement. If you’re staying within that range of the tensiometer, if you’re going to be outside of that range, a solid matric sensor like the TEROS 21 or 22 is a good option as well for getting those measurements. So yeah, but yeah, both are, both are good approaches, but I definitely encourage making sure you’re getting water content and water potential together in one way, shape or form, whether that’s through the retention curve or through the actual direct measurements of water potential, because that just tells you so much more about the actual status of the water than just measuring water content on its own.

BRAD NEWBOLD 23:12
This one is asking, how much is the soil water content reading affected by soil texture? Is there a standard method to calibrate that reading for different soil textures?

LEO RIVERA 23:23
Again, we’re on a technology dependent question right?

LEO RIVERA 23:25
Yeah, yeah. So, yeah, I don’t know it. Again, it goes back to technology, the frequency that the sensor is operating at ultimately, we’re trying to minimize this impact, right Shaun?

SHAUN WELDON 23:36
Yeah, yeah. Well, I mean, that’s, that’s one of the things for our calibration of our sensors is we try to include a wide range of textures, because they do behave differently. They’re, you know, they have different ranges as far as, as far as, water holding capacity and things like that, that will affect it. Standard method, I guess, for calibration would your best bet would just be to do a soil specific calibration to get that that water content for that specific soil, our calibrations do a really good job of being accurate across a wide range of those but you’re always going to increase that accuracy if you do one specific for your soil.

LEO RIVERA 24:15
Yeah, no, for sure, you can definitely get better accuracy with the soil specific calibration. But for the most part, the general calibrations do a pretty good job. So if you don’t need you know half a percent accuracy, which most people don’t, then you can kind of work with that standard calibration in most soil types, there are some technologies that are more sensitive to soil texture because of either the frequency they operate at or just various aspects of the technology that can impact that. But, you know, this is something we spend a lot of time trying to optimize and minimize that impact of soil texture on the readings.

BRAD NEWBOLD 24:57
All right, we are coming up to the end of our time, so this will be. Our last question, and it’s a good one, similar to a previous one, or at least related to a previous one. And here they’re asking, can you build and trust water retention curves if you bury water content and water potential sensors at the same location and depth, and will that compare well with what you get from HYPROP?

LEO RIVERA 25:18
Yeah. I mean, for sure, I think you can build and trust water but since build moisture release curves from an in situ water content water potential sensor, but I think it’s important that you have a good water potential sensor that can cover a broad range, especially for the range that you’re trying to measure. Now, if you’re focused specifically on the very wet range, then I would probably do a water content and tensiometer, yeah. But if you’re trying to cover that broad range, which we can see in the field, then something like the newer generation of soil matric sensors that cover that range better is a is a good way to go. And we’ve seen a lot of actually comparisons with the HYPROP and with the field generated curves, there are some differences. And again, it goes back to hysteresis and the wetting and drying curves, the scanning curves, and what they look like, but they can compare well, yeah.

SHAUN WELDON 26:06
And I would kind of also say that if you’ve got a good installation of both of your sensors, that it often gives you a better representation of what’s actually happening in the field than what the HYPROP will, the HYPROP is a fantastic tool, and it gives you great data, yeah, but again, it is a very controlled environment, so the sensors will actually show you exactly what’s happening in real time with those, with those, yeah, soils.

LEO RIVERA 26:33
Yeah, yeah. So, no, I definitely think that can be done. I mean, we’ve done it ourselves and done several comparisons, and we’ve seen other researchers do this as well, but yeah, it always comes down to choosing the sensors that match the ranges of what you’re trying to cover, and making sure you’re making good selections there.

BRAD NEWBOLD 26:49
Yeah, all right, that’s gonna wrap it up for us today. Thank you everybody for joining us. We hope that you enjoyed the discussion. Thanks again for all the great questions. And also, if you have any questions that 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.