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 and we’re shooting for about 45 minutes of Q&A with our experts Lew 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. Alright, with 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 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 meters hydrology instrumentation, including the SATURO, high prop 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.

CHRIS CHAMBERS 1:26
Hi happy to be here.

BRAD NEWBOLD 1:29
Alright, let’s get started taking questions. Our first question here is for the TEROS 21 sensor, what is the measurement range that gives better precision? Does it require calibration or not?

LEO RIVERA 1:46
Yeah, do you want me to take that one first?

CHRIS CHAMBERS 1:47
Yeah, go ahead.

LEO RIVERA 1:47
Okay, so the the TEROS 21. Is, is a calibrated sensor in the range of about -5 kPa to -100 kPa. within that range, we have really well defined accuracy specs and understand the behavior of that center really well. And having said that, though, it does measure beyond that down to about -100,000 kPa. And that range of a measurement, we’re continuing to work more on to better understand the actual performance of that the measurement in that range.

CHRIS CHAMBERS 2:20
Not necessarily 200,000 kPa.

LEO RIVERA 2:22
Not necessarily correct, yeah. Ideally, down to about minus at least down to a permanent wilting point and down in some applications down to about -4,000, -5,000 kPa, we want to really have that pretty well characterized, so that way, we can better inform that. But it isn’t calibrated, because it’s nearly impossible to calibrate a sensor in that range without making a sensor that costs you know, 1,000’s of dollars versus a couple $100. But we are learning more about it. And in in diving into it. And we’re seeing more publications come out to about the behavior of of the sensors and other sensors in this range. And yeah, so I think there’s more work out there that we’re seeing that, but it’s still beyond the calibrator range, it’s more of a qualitative measurement.

CHRIS CHAMBERS 3:09
And that’s the trick is when you get to that dry range, it’s hard to know what the real matric potential is.

LEO RIVERA 3:14
Yep, yep. So, but no, it does not require calibration because it is calibrated. So we take care of that for you.

BRAD NEWBOLD 3:24
Alright, next question. If the soil is dry, does this affect the suction measurements by the TEROS 21?

CHRIS CHAMBERS 3:33
Not beyond just the actual physics of how water moves in really dry soils. So it will the sensor is in equilibrium with the surrounding soil and water will get pulled in and out of the sensor based on the suction of the surrounding soils.

LEO RIVERA 3:54
Yep, now that’s absolutely right it really just depends on on water movement, the only situation where you might run into issues is in expansive soils where you could get shrinkage away from the center, but then all bets are off. That’s with any sensor though. Once you lose contact with water potential measurements of especially with field sensors is really highly dependent on good soil the sensor contact and expansive soils cause issues with so many different measurements and this is This one’s also susceptible to the same issues, unfortunately.

BRAD NEWBOLD 4:31
All right. Next one is asking Do you have any experience in measuring water potential in soilless media? What are the challenges? What is best Tensiometers, TEROS 21, etc.

CHRIS CHAMBERS 4:44
Yeah, and you know, I have my own my own personal experience with water potential in in soilless media has was heartbreaking. Because you know, I was thinking I want to mold, matric potentials definitely the best way to go if you want to know the status of water status of a plant, and I took my eyes off of the reading for like 10 minutes, and then the plant had gone flat and then the plant had wilted, 10 minutes is a bit of an exaggeration. But there’s a couple things working against you here. There’s your soil to center contact or your soilless to center contact. Yeah. And there’s the shape of your soil moisture retention curve. And that’s the nature of, of a lot of soilless media is that they hold water and then they hold water, but they can drain really quickly. So you’re it’s like you’re driving over a cliff. If you’re going to use matric potential, you better know exactly where your refill points are.

LEO RIVERA 5:44
Yeah, no, that’s absolutely right. And really, when you think about most soilless media, the bulk of the water is held in the really wet range. And once you go beyond that, it’s things change really quickly, really. And so when you think of your traditional ranges in soil, that you’re going to try to stay within is completely different in soilless media. You’re you’re you’re typically not going to try to push soilless media past -30 kPa. Really, because once you get to that point, then it really like, like Chris said, it drops off really fast. And there are other factors at play that actually impact the availability of water like hydraulic conductivity. And we’ve seen this and other in other research where it points where you would expect plants to not be stressing, you’re seeing stress. Say, for example, -10 kPa, we’ve seen this in some materials. So so your water potential ranges differ. And because of that, you’re you’re typically better served with something like a Tensiometer. In soilless media because they they give you more precision in and so it helps you understand those characteristics of the soilless media better. And especially because you’re not going to want to try to push those media’s very far.

CHRIS CHAMBERS 7:02
And where you’re used to watching matric potential in, say, a mineral soil. You can watch it creep, and when it starts to dry down, you can kind of watch the projection, projection, you know, and look forward. And it’s like, well, a couple hours, this is going to hit my my refill point. And so I can just chill out for a little bit, come back and check on the data later. Be careful doing that if you’re in soilless media, yeah.

BRAD NEWBOLD 7:29
Okay, this next individual is asking, can you talk about conditions where you would see positive pressures in conditions that are less than saturated?

LEO RIVERA 7:38
Yeah, that’s a really good question. So there are a couple of instances where you’re going to see positive pore pressures. And it also is dependent on having the right tool to actually be able to make that measurement. Because if you’re using something like the TEROS, a solid matrix sensor, like the TEROS, 21, where you’re using another relationship measurement to relate and get that water potential, you’re actually not going to see that so you need something that’s more of a direct measurement, like what you get with a Tensiometer, where you can actually measure those positive pressures. And, of course, for sure, you’re gonna see positive pore pressures in saturated conditions. But you can see that occur in less than saturated conditions if you haven’t trapped air, or if there’s something physically imposing a positive pressure on the soil. So in many geotechnical applications, we see the other things that cause positive pore pressures, even though the soil isn’t completely saturated. But you know, there’s a lot you have to understand about that. But the, the measurement of positive pore pressure is really important, especially in applications like slope stability, and understanding well the risk of slope failure.

CHRIS CHAMBERS 8:46
Where it’s going to be much more likely to slide if you’ve got that positive.

LEO RIVERA 8:49
Right, right pressure, Yeah, cuz the beautiful thing about negative or dry conditions when the soil is is has an, you know, a lower water potential. Now water potential acess actually is something that helps increase the soil strength, because it’s actually helping hold the soil particles together. But when positive conditions, approach positive pore pressures and then become positive, then the those equations completely change and…

CHRIS CHAMBERS 9:17
And that’s where your Tensiometer is going to be great because that pressure transducer, you know, the water column pulles on the transducer, when it’s really dry, and you get tension, but it’s just you flip the sign in positive pore pressure. So you’re exerting pressure on that, on that pressure transducer instead of pulling on it, yeah.

BRAD NEWBOLD 9:41
This question is, how does or can organic matter content affect water potential? Is it something that could be calibrated to an existing water release curve?

LEO RIVERA 9:52
Well, it definitely impacts it.

CHRIS CHAMBERS 9:54
Definitely impacts it.

LEO RIVERA 9:55
Yeah. I mean, we know that typically as you increase organic matter in soil, it increases the water holding capacity of the soil. And in turn, that means it’s going to impact the soil moisture release curve. Now the challenge there is and I think what this question is trying to, to pose is, can we determine what that change is going to be maybe just based on the percent of organic matter increase?

CHRIS CHAMBERS 10:20
Well the nice thing is that the matric potential just by its nature, it doesn’t doesn’t matter. Absolutely. Yeah. 100 kPa is at -100 kPa when you have a high organic matter content or not. Yeah. So that’s working in your favor. But as far as calibrating it, that’s right. You were you were about to mention how you could use the soil water retention curve to look at a shift in the curve with organic matter.

LEO RIVERA 10:51
Yeah, I think it’s really hard to estimate exactly how it’s going to affect it. Because there are other factors at play, of course structure and other things. There are pedo transfer functions out there that people use to try and estimate these curves, and what that would look like at different organic matter contents. But they’re not, you know, they’re not 100% accurate. So if it’s something you’re really interested in understanding, the best thing to do is measure and take samples and measure those changes over time. Or even better, have In-situ measurements of both water content and water potential. And then you can actually monitor how that relationship is changing over time in the field.

CHRIS CHAMBERS 11:31
Or if you’re making a comparison to the effects of it, then sample, bring it back, put it on the HYPROP. Yep, exactly. Yep.

BRAD NEWBOLD 11:38
A client question for Chris Chambers. What is the relation between water potential and turgor of a plant?

CHRIS CHAMBERS 11:47
Oh! Fun! We’re really getting into kind of the physiology here on this, one. So in this is going to depend a lot on plants, basically, the turgor pressure is what gives a leaf or a non woody plant, it’s its structure, right? It’s what it allows it to stand up, change the leaf angle to track the sun or whatever. And that’s going to be directly related to the water potential of, of the leaf. Now this strays a little bit from our soil water potential, because mostly we’re looking at stem or leaf water potential for things like this, which, of course are going to be dependent on soil potential to some extent. But basically, the point at which the potential at which you lose turgor is a pretty important physiological trait in a plant. And that’s going to vary by species and species and it’s going to be affected by environmental conditions. But basic- basically, the turgor loss point is going to be where plants wilt and then lose a lot of function hydraulically.

LEO RIVERA 13:08
Yeah. And so and there’s no real I mean, and I’m gonna ask this question, because I’m the soils guy, not the plant guy. But, you know, when we measure water potential in the soil, it is not a direct indicator of your plant hurt, turgor pressure and what that’s going to be experiencing, right? Because there are other factors at play?

CHRIS CHAMBERS 13:29
Exactly, there’s a lot of there’s a lot of different factors at play. And then when you get into plants with more sclerified tissue, then it gets harder to observe and relate that turgor loss point to, to, to the water potential.

BRAD NEWBOLD 13:50
Okay, here’s a big one. How can we relate water potential with water content?

LEO RIVERA 13:58
The soil moisture release curve. Yep. So, I mean, this is, this is a pretty common question. And an important question to ask is how can we make that relationship and of course, we have this wonderful thing called the soil moisture release curve. I think the more important part is, how are we generating that relationship? And how are we using it especially using Yeah, and especially because, you know, we can make this relationship. But one of the challenges with doing then trying to relate that in the field is, we have hysteresis. And we have other things that play that actually cause different soil be on different scanning curves and things like that, that we don’t have a lot of time to dive into. But but a common tool is to make soil moisture release curves in the lab, and then use that relationship to apply that to field and data in the field. Or people try and go the route where they they model these relationships using the pedo-transfer functions and which isn’t my favorite way to do it because of it pedo-transfer functions don’t take into account all of the things that impact the soil moisture release curve in that relationship. But it’s, it’s a way to go.

CHRIS CHAMBERS 15:10
And it has a lot of value beyond just being able to transfer back and forth between water content and water potential. Yep. Because it’s really, maybe fingerprint for your soil isn’t quite the right term. But if you’re characterizing soils, and the soil is what is important to you, and how water moves through your system, then that soil water retention curve is going to be giving you a lot of insight. Yeah. between different sites and in different growing conditions.

LEO RIVERA 15:40
Yeah, absolutely. I mean, it’s a soil moisture release curves are commonly used in in, in Vadose zone hydrology modeling, to understand how water is going to move in soil and those types of things. So it’s a really important curved understanding. We’re all familiar with the term Van genuchten parameters, which is just one of the equations that’s used to to fit this relationship. There are other tools that are also used for that. But yeah, it’s, it’s a powerful thing to understand in many areas.

BRAD NEWBOLD 16:11
Okay, and a follow up to that, or a secondary follow along question. If you are co locating water content, water potential sensors, how close can or should they be to each other?

LEO RIVERA 16:23
Oo, that’s a great question!

CHRIS CHAMBERS 16:24
Close enough that you’re reading basically the same hydraulic conditions. Yep. In soil. And everything soil structure, right? Because they’ve got to be close enough to, to be measuring the same forces. Yeah. far enough apart to not interfere with each other. Yep, exactly. So if you set it a few centimeters, basically, right, yeah, it’s gonna be ideal.

LEO RIVERA 16:46
Yeah and we know, I mean, you want them as close as possible, we know that there’s always a chance of some variability just due to finger flow of water and soil, and just different things about spatial variability within the soil. But the closer the better. And for the most part, that little bit of error that you’re gonna get from their slight difference in proximity to each other, is going to be fine. Now, if you’re dealing with transition horizons, where you might have an irregular boundary, then you might want to be careful, this is something if you’re if you’re not actually characterizing the soil, you’re not going to know this. But you you run the risk in some of those transitional horizons, where you could wind up textural discontinuities between the two spots. And so you do have to be a little careful. But this is not, I mean, it’s not that common, where you’re going to run into a site where you’re gonna see that big of a difference. But if you have your sights characterize well enough, you’ll know that you’ll be running into you could run into something like that.

CHRIS CHAMBERS 17:50
So basically, find the measurement value of your water content sensor, and give a little bit of a healthy boundary beyond the most conservative limits of that and you’ll be in good shape.

BRAD NEWBOLD 18:03
Next question, how do you determine the volume that you are measuring?

CHRIS CHAMBERS 18:06
You know, for water content sensor? That’s really easy. We’re emitting an electromagnetic field into the soil. But for matric potential sensor, all of which, to my knowledge require equilibrium with a surrounding sub- substrate, right. Yeah, then so how, how big how representative? Is that? Yeah, how far away from your sensor would you expect the matric potential to differ?

LEO RIVERA 18:34
Yeah, I think the nice thing about water potential I mean, it is it’s a very point measurement. Like it’s whatever is in direct contact with the sensor is what you’re measuring. But the beautiful thing about water potential is it’s it’s an it’s an energy state. And so it’s always trying to come into equilibrium. And so it typically will represent a decent area around it. Now, as you go through, like big changes in water potential or rapid changes in water potential, you might see some differences in areas that are further from the sensor, or as closer to the sensor then what you would be would see in maybe drying conditions where it’s not transitioning as quickly. But it’s not like water content, where I think you can see bigger variability in water content than you would see in water potential because of the fact that it’s an energy state. But it really depends. I think there really depends on the soil that you’re working into.

CHRIS CHAMBERS 19:25
Number one variable that’s going to affect that.

LEO RIVERA 19:30
Number one variable that’s going to affect that I think is going to be…

CHRIS CHAMBERS 19:33
Hydraulic conductivity, right?

LEO RIVERA 19:34
Yeah. Hydraulic conductivity, yeah. And hydraulic conductivity and flashiness of the system. So like, if you have a really rapid rainfall event, or if your soil is more prone to like finger flow, and things like that, then or if you have big macro pores, then I think you’ll see bigger differences in smaller areas.

CHRIS CHAMBERS 19:56
So really complex question in that, you know, we don’t have all the answers to, but it’s fun to delve into it. Yeah. And you know, as you get kind of related to finger flow, as you get drier, then you know, the the path that it takes for water to move through the soil gets longer. Yep. So it’s going to vary by condition and, you know, fun question, but really, really tricky to fully get to the bottom of.

BRAD NEWBOLD 20:27
How can we calibrate or validate the Tensiometer and make sure the sensor is working?

LEO RIVERA 20:34
Yeah, well, I mean, it’s really the bigger question is how can we validate the performance of the sensor in the field.

CHRIS CHAMBERS 20:43
They don’t drift very often, right?

LEO RIVERA 20:45
It they don’t, depending on the pressure transducer that’s being used, and the quality of the pressure sensor. So it also depends on on where you’re getting your Tensiometer from and, and the quality of the transducer that they’re using. For the measurement. Most sensors, we don’t see a lot of drift. And so how do you validate that it’s behaving in the field? And is accurate? It’s actually kind of hard to do in the field, because you need something that you can apply a new inspection.

CHRIS CHAMBERS 21:13
That’s right, you’d have to excavate it. Yeah, for one. But, you know, it’s fairly easy to a hanging water column with the length of your shaft, and a zero point for pure water. So it’s not, it’s not an insurmountable obstacle to validate it. And then a good test more frequently, we get the question of is my Tensiometer working is not changing what’s going on? Has it cavitated? Has it got air entry? And so in that case, saturating it, you know, saturate the tip and then let it dry out? If it’s just drying in the air, especially on a hot day? It should it should respond really quickly. Yep.

BRAD NEWBOLD 21:56
Okay, here’s a combo question here for a solid matrix sensor. What happens if the calibration is wrong? It’s calibrated wrong. For instance, if the sensor gets changed from one soil to another, what do we need to do? Also, who calibrates the sensor? Is it METER Group? Or can the user change the calibration based on needing to move the sensor?

CHRIS CHAMBERS 22:19
Yeah, all of our matric potential sensors are calibrated, right?

LEO RIVERA 22:23
They are all calibrated every single one every single one, we do not ship an uncalibrated we better not be shipping an uncalibrated [inaudible] calibrated an uncalibrated sensor. And the beautiful thing is, the calibration is based on the properties of the sensor itself, not moving from one soil type to another. So it’s not something you really should ever have to worry about changing.

CHRIS CHAMBERS 22:48
And if it’s functioning properly, it doesn’t drift. The TEROS 21 is based on our water content sensing technology. And then we use the water retention curve to infer matric potential from the known ceramic. And that that just doesn’t drift. So you don’t have to worry about the calibration. I do want to circle back. Our ZENTRA software will tell you if it’s not calibrated. So you can just not worry about that uses ZSC or ZL6. And if there’s a problem with the calibration, it will, it will let you know. And then we’ll give you a sheepish very embarrassed replacement as soon as we possibly can. It hardly ever happens. Now, if the calibration is wrong, that’s a little trickier.

LEO RIVERA 23:36
Yeah. And that’s one thing we work really hard to try and prevent. One of the things that we do when we calibrate our sensors, is we then run them through a verification process where we’re checking the input, the the calibration points that we’re input to the sensor to make sure that it’s reading within the spec that we we expect the sensor to read within. And then on top of that we do monthly verification of sensors, where we pull sub samples, and check the performance of those sensors to make sure that they’re also reading that they need to be within spec independent quality check. Yep, yeah. And so it’s not really something that we ever hoped that we ever expect to see. Now crazy things happen. But it’s not, especially with the solid metric sensors. It’s actually not something we’ve seen much of because the verification process does a pretty good job of checking sensors. And when they are out, they flag it and then we go through and recalibrate it or we don’t ship that sensor out it can’t meet the spec.

CHRIS CHAMBERS 24:34
Exactly.

BRAD NEWBOLD 24:35
Next question. The previous TEROS 21 Gen 1, 10 kPa lower limit generally corresponds with the ceramic head air entry value. How close does this limit correspond with fully saturated conditions? Is there a benefit to reducing that 10 kPa lower limit to zero kPa to better identify saturated conditions?

LEO RIVERA 24:57
That is a really good question and You know, when we were first, when we first developed the TEROS 21, or the MPs six or the MPs two, at the time, we we actually had that same assumption that the limit was due to the air entry of the ceramic. And as we started diving into refining the electronics for the Gen 2, we learned that the limit was actually not the ceramic it was the limit was the electronics did not have enough sensitivity to see changes in the ceramic in that range. Now we do know we have now that we’ve better refined that we’ve learned that the air entry of the ceramic is somewhere around three to five kPa, depending on ceramic, ceramic variability. And if you look at our specs, you’ll see that we spec from -5 to -100,000 kPa. And that’s because we know that we can rely on the sense the ceramic reaching air entry and responding at that point. But it really actually had to do with the electronics. And the really cool thing is when the sensor now goes on, that’s on the drying curve when you’re de-saturating. But when it’s wetting back up, it can actually detect changes all the way up to saturation. And so it might sit there for a little while after saturation before it reaches air entry, but we are able to actually better detect those ranges. And that’s really one of the really cool things about the Gen 2 for the TEROS 21 and now the TEROS 22 as well.

CHRIS CHAMBERS 26:24
But it can but it can also be why your your TEROS 21 might seem to linger at minus point 1 kPa for a while right on the drying curve. Yep. Because it really might not be able to distinguish between that really near zero matric potential and 3 or 5 kPa.

LEO RIVERA 26:42
Right, right, yeah. And then there are still definitely limits on what it can do in that range. But we’ve been able to improve that, that and better understand now the limitations there.

CHRIS CHAMBERS 26:52
And if you really need to know what’s happening in that range, you should be using a Tensiometer.

LEO RIVERA 26:57
Absolutely.

BRAD NEWBOLD 26:59
Okay, next question here, what factors affect soil water potential, and how do they interact?

LEO RIVERA 27:06
That is a really good question.

CHRIS CHAMBERS 27:07
That’s a great so this is where we need a board.

LEO RIVERA 27:09
Haha this is exactly where we need to have a board. One day Chris will have his portrait right.

CHRIS CHAMBERS 27:15
Just to bring it home how desperate we are for this. We’re going to airdrop we’re gonna have psi T over here, right? Yep. Okay, so you got a fact check me [inaudible] psi T is going to equal. Let’s see, psi P. We have your pressure potential. Yep. And then wait, I always get the sign around here minus psi M.

LEO RIVERA 27:39
Yep. Your matric potential. It’s plus, yeah, less. So it’s all additive, because some of them are negative [inaudible].

CHRIS CHAMBERS 27:47
And then [inaudible]. And then there’s one more Yep, psi G.

LEO RIVERA 27:52
Yep. Your gravitational potential. So you got your four main components of water potential. And then you have the two that are primarily dominant in unsaturated conditions, which are psi M, your matric potential, and psi o, which is your osmotic potential. And so those are your factors that that impact total water potential water potential, but then if you dive even deeper, and just to the matric component of it, there are dozens of factors that impact matric potential, right? You have structure, you have obviously your particle size distribution, and you have bulk density, organic matter. Let’s see, what am I missing? There’s others. So and I’m gonna hit on pedo-transfer functions one more time. So if you look at a traditional pedo-transfer function there like what you see in Rosetta, there are a couple ways they’ll break it down to get your retention curve one is just purely based on the the not not the particle size distribution, but just on the texture. So you can say that you have a sand and Rosetta will generate a a soil moisture release curve for you. As you can imagine, it’s not super accurate.

CHRIS CHAMBERS 29:07
How does this tie in with Dr. Lee’s research with the VSA on specific specific surface area?

LEO RIVERA 29:15
That is an even deeper dive. So, I mean, I think I mean, what what what Dr. Ning Lu has been diving into is really all the components of the curve and like you deal with both capillary, the capillary portion of it, and then you have the surface the stuff that’s bounded the surfaces of the particles themselves. And that’s but that’s a that’s a deep one that I mean to today. But just I mean, those pedo-pedo-transfer functions can also break it down by particle size distribution or particle size distribution in bulk density. And as you add more factors, your pedo transfer functions get better, but that’s the thing is there are so many factors that impact your, your that relationship, and I’m probably like getting off way in the weeds here, but.

CHRIS CHAMBERS 30:02
But the point is that there’s various levels there are various levels of breaking down even your matric potential. But the dominant force is going to vary depending on the conditions like drier than -30ish kilopascals. The gravitational potential just kind of becomes insignificant in that right? And then your matric potential is your dominant force in water movement. Yep. In that soil. Yep. Yeah. So, tell me about asthmatic potential. I get confused on this one sometimes.

LEO RIVERA 30:34
Yeah, I think osmotic potential really only place. So it has it really plays a role when you’re dealing with semi-permeable membranes.

CHRIS CHAMBERS 30:41
So in biology in soil.

LEO RIVERA 30:44
Yep exactly. So for water movement, osmotic potential doesn’t really play a role. But for plant water uptake absolutely plays a role. And until you, you can see more stress induced, that’s one of the things that salt does is salt introduces stress, not just because it’s there, and it can damage bias, but it also more tightly bound to the water to the plant. Right. So yeah, and then pressure potentials only in saturated conditions, typically, right so.

CHRIS CHAMBERS 31:17
Yeah, we already covered that.

BRAD NEWBOLD 31:20
Alright. This next question is what are the implications of soil water potential for agricultural irrigation management.

CHRIS CHAMBERS 31:28
So I love matric potential in mineral soils for for growing crops. And when we’re talking with growers, part of this has been really good of a development over the past decade or so to get growers used to using water, you know, soil moisture as an indicator on how their plants are doing. But it’s also a little bit of a limitation because water content is much easier for people for people to understand. Right. I’m going to I’m going to talk to Leo in terms of kPa, if I know it if we’re talking about water status, but when I’m when I’m talking with someone on the street, it’s going to be percent water content, right. And it’s 35%, it’s 15%. It’s really easy for people to wrap their head around. But those numbers all mean something different in different soils. And almost every agricultural operation has this soil type variability to deal with. And you can just cut straight through that with matric potential. And just look at the water status for for that crop. And it doesn’t matter whether it’s a clay or sand, it’s -50 kilopascals is -50 kilopascals. So that’s where matric potential is really so much more powerful than water content. But it’s a lot more difficult to interpret. And to explain to someone how to interpret.

LEO RIVERA 33:03
Yeah, yeah, it’s true. I mean, unfortunately, water potential is one of the hardest subjects to teach in soil science. And you’re teaching that to soil scientists. So imagine trying to teach it to somebody who is not a soil scientist, it’s even harder. And so it’s really hard to to to explain that. Now. The really cool thing, though, is there are these, like, we have pretty well defined ranges that we know, many different types of plants are happy.

CHRIS CHAMBERS 33:29
And you know, almost you can almost always find a starting point in the future, yeah, exactly.

LEO RIVERA 33:34
And so if you have that measurement a matric potential, you can always go back and say, okay, am I in the range the plants happy yes am I out of it? Okay, that’s bad now, we need to rectify that. But it again, it is hard. And also traditionally making the measurement has been hard. Yeah. There has not been really great tools for making these measurements. And that’s been one of the things that we’ve continued to strive [inaudible] TEROS 22 right. And, and so that’s the been the other reason that has been limitation is just the inaccessibility of good measurements. And, and so hopefully, I think we’re seeing this is that more people now that the measurements are becoming more accessible, are starting to adopt that as a way to do irrigation management and better understand what the plant is feeling rather than, you know, that. I don’t want to say guessing game because it’s an educated guess, of what your water content means in terms of.

CHRIS CHAMBERS 34:28
And water content is still really helpful. It still moves the entire effort forward. But accounting for that, for that, for that soil type is is always a challenge. Yeah.

Okay, we’re coming to the end of our time. So this is gonna be our last question. This one is asking how does soil water potential influence hydraulic conductivity and water flow dynamics within the Vadose zone?

LEO RIVERA 34:52
That is a great question. And we all know that water potential is the driving factor in water movement because As water is going to move from high water potentials to low water potentials, because it’s entered, it’s trying to come into equilibrium. And so it’s going to pull water into those areas. Now, having said that, water potential doesn’t really influence hydraulic conductivity, except for the fact that we know that unset unsaturated conditions, hydraulic conductivity, hydraulic conductivity changes, as the soil becomes, as the as water potential decreases, because there’s just that relationship. We know there’s an unsaturated hydraulic conductivity curve, and we can we can derive that relationship and we can measure that relationship between the two, but it’s not influencing hydraulic conductivity in in terms of like how the soil structure or the makeup of the soils actually.

CHRIS CHAMBERS 35:43
So the more we’re looking at the change in soil flux based off yes, so water flux based off of the gradient, right, of matric potential, right.

LEO RIVERA 35:51
But it’s absolutely like if somebody’s doing hydro vadose zone hydrology and trying to understand water movement in unsaturated conditions, they need to know water potential, and what that gradient is to really be able to quantify how water is going to move and how fast it’s going to move, because that relationship is what’s driving the movement. And so it is it is it’s it’s a those two go hand in hand, we need to know that unsaturated hydraulic conductivity at those given water potentials.

CHRIS CHAMBERS 36:25
Does it make a big difference that in saturated hydraulic conductivity, that that the gravitational potential is what’s basically driving it rather than retention.

LEO RIVERA 36:35
It I mean, and that’s the thing about when you go from unsaturated to saturated conditions as your your mechanisms that are driving movement, shift completely. And also, when you get to saturated conditions, you’re typically dealing with the entire amount of the soil to move water, whereas an unsaturated conditions, as it gets drier, it’s localized, the smaller pores are contributing more than the bigger pores, because the bigger part is that there’s no water left. So as you drain those pores, the amount of soil that’s actually contributing to the movement of water decreases. And so it is yeah, it’s a really interesting sort of process to understand.

CHRIS CHAMBERS 37:11
Is that the main difference between the saturated hydraulic conductivity, and the unsaturated is the pore sizes that are engaged in transport.

LEO RIVERA 37:17
Yeah, because and because those pore sizes that are engaged in transport shift, you move from where gravity’s driving it, to where it’s actually the gradient and major potential that’s, that’s driving it. So yeah, cause that at some point, gravity plays absolutely no role in the movement of water in non saturated conditions.

CHRIS CHAMBERS 37:37
And then your actual path gets longer and longer as your soil gets drier and drier.

LEO RIVERA 37:41
Longer and longer and more torturous. Because you get you’re dealing with less, and pores that aren’t connected and all these types of crazy things.

CHRIS CHAMBERS 37:48
Tortuosity yeah, time.

BRAD NEWBOLD 37:52
That wraps it up for us. Thank you again for joining us today. We hope you enjoyed this discussion. And thanks again for all the great questions. And also again, if you have any questions 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.