Office Hours 2: Soil hydraulic properties

Office Hours 2: Soil hydraulic properties

Learn soil hydraulic conductivity measurement methods, best practices, applications, and more in this Q&A session with our science and product experts.

Hydraulic conductivity is critical to understanding the complete water balance

Hydraulic conductivity impacts almost every soil application: crop production, irrigation, drainage, hydrology in both urban and native lands, landfill performance, stormwater system design, aquifer recharge, runoff during flooding, soil erosion, climate models, and even soil health. In this live Q&A session, METER research scientists, Dr. Colin Campbell and Leo Rivera answer questions about understanding water movement through soil.


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

Leo Rivera operates as a research scientist and Hydrology Product Manager 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 HYPROP and WP4C. He also works in R&D to explore new instrumentation for water and nutrient movement in soil.

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Hello, everyone and welcome to Office Hours live with the METER Environment Team. Today’s session will focus on soil hydraulic properties. And we’re shooting for about an hour of live Q&A with our experts, Dr. Colin Campbell and Leo Rivera, who I’ll introduce in just a moment. But before we start, we’ve got a couple of housekeeping items. First, we want this session to be interactive. So we encourage you to submit any and all questions in the Questions pane. And we’ll try to get to all of your questions. And if we don’t, which more than likely we won’t, someone from our science and support team will get back to you with an answer via the email that you registered with. Second, if you want us to go back or repeat something you missed, don’t worry, we will be emailing you a recording of the session within the next three to five business days or so. Alright, with all of that out of the way, let’s get started. Today our panelists are METER research scientists, Colin Campbell and Leo Rivera. Dr. Campbell has been a research scientist at METER for over 20 years following his PhD at Texas A&M University in soil physics. He is currently serving as Vice President of METER environment. And he’s also adjunct faculty with the Department of Crop and Soil Sciences at Washington State University, where he co teaches environmental biophysics a class he took over from his father Gaylon nearly 20 years ago.

Leo Rivera operates as a research scientist and director of client success at METER Group. He earned his undergraduate degree in agricultural 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. Leo has over 10 years of experience helping researchers measure the soil plant atmosphere continuum. So thanks for joining us you guys.This first question here is one that we’ve gotten quite a bit. And so we figured we’d open with this one, but basically, it’s on the topic of soil health. And the basic gist of the question is that there’s a lot of interest in the topic of soil health recently. So how do soil health and infiltration tie together?

Yeah, that’s a really good question.You know, the soil health institute has identified, infiltration and hydraulic conductivity as one of those tier one measurement parameters to indicate whether or not we’re trending in the right direction. Now, it is something you have to be careful with when you make these measurements. Because there are many factors that impact infiltration and hydraulic conductivity. It’s not just the healthiness of the soil, are we increasing the microbes, are we increasing, all those things that help bind the soil together improve structure. So that’s where infiltration in soil health tie in is if we look at the same soil type under different conditions, we can then look at impacts that soil health practices are having on this value, because in theory, if we’re improving our soil health, we’re improving those things that improve soil structure, and we’re generating more macropores do more activity of worms and things like that. And fungus is going to help generate those glues that bind the soil together. So it’s going to make a more stable soil that helps improve infiltration, which is a great thing, because we’d much rather infiltrate water than run it off. So that’s kind of where it ties together along with many of the other measurements that go into soil health.

When I think of that I mean, I think of kind of extreme cases, right? That so as probably many people who are listening know, he’s done a lot of work over him in Fukushima prefecture in Japan, trying to rehabilitate the soils and the environment from that nuclear disaster. And they actually went out and put down a material, they scraped off the top 10 centimeters of the native soil and put down a crushed granite material, that actually took out of a mountain, crushed it and put it down. In there, we went out and looked at infiltration. Right. You know, I remember when we went out did this I emailed you and said, Hey, Leo, can you just check our numbers? Yeah, here’s the numbers for for infiltration. And then you wrote back, I think, hey, those are akin to kind of concrete. Yeah. Really refinements maybe in some soil. We can’t say, oh, you’ve got a number here. And it’s either good or bad. That’s kind of what you’re saying is we can’t just say that’s good or that’s bad unless we’ve got crushed granite, that’s, applied with a roller. Yeah. And I just sent you pictures of that. And there are literally street rollers, right, rolling back and forth on that, that didn’t help. No. So there’s a challenge there the soil health, you know, there’s a bare minimum, can’t roll the soil compaction is an issue, all of these things, and then we can kind of work up from there. That’s what I heard, you saying, Okay, we need to, you know, microbes need to be active, we’ve got these funguses that will create these pets, right, that once we have pet formation, actually have this macro portfolio, we get better infiltration. So there’s a lot of information that would correlate these two things, it wouldn’t necessarily cause it for sure. But there’s some really good indications. And we can start with by saying, you know, a good soil has a lot of these problems. Exactly. Yeah. Yeah. So it all ties together. But you really, they need that background information. But it is one of those things that if you monitor over time, you can see the improvements and see what’s happening. So when you were I mean, one of the things I like to do is hear some of the stuff you did in Texas, for your master’s degree, because it was a really interesting project, were you able to identify or really, you know, I mean, either anecdotally, or through experiments, some of these things?

Yeah. So that’s actually, it’s funny, because a lot of the work I did when I was putting in it was before, soil health was really a thing that we talking about. It was a thing for us. But we were looking at differences in land use, and how that impacted soil hydraulic properties. And we had the luxury of working in the sites that had been in the same land use for 50 plus years. And maintain that way for a reason because it’s a ARS research facility. And when you went from a conventional tillage to an improved pasture to a native prairie, you saw big changes. And especially when you went from the improved pasture to the native prairie site, you saw the impact that all those native grasses had, the fact that the soil is not getting compacted by heavy animal traffic in the grasses were healthier than the organic carbon was higher in the soil. And because of that, there was better soil structure and there was better infiltration, higher infiltration, higher hydraulic conductivity in that land use versus

So that’s super consistent. I mean, were you able to observe that just, I mean, I know you drag out these monster containers of water, because that’s how you had to run this double ring. Could you see that? Like, right off the bat?

Absolutely. Yeah. Especially the one indicator was how many times I had to refill my trailer.

So on these native praries you literally were refilling your trailer much more.

Yeah, we usually would go through 1000 gallons of water on a native prairie site without doing a full day of measurements. And on like the improved pasture, we could get through with half that and usually only had to fill up once during the day. So just anecdotal stuff like that. But then when you actually look at the data, you can see those consistencies.

So I mean, it’s important to know that there are both of these things at play, right? I mean, from your answer, initially, I’m like, oh, we can’t really go. You know, it’s gonna, you know, let’s, if we made a change in our management practices to system, we may never know, you know, whether that effected I think he gave a great scientific answer. But, but the other side is, you know, from a practice idea, you actually can see that. They’re, like, all things being equal. You went out there, and you could see it right away. Yeah. That was a really fun place to get to do research. Except for the weather, except for the weather.

I’m glad I live here.

Yeah, exactly.

Okay, shall we try another one? Yeah. Okay. All right. Okay.

This next one is a pretty big compound question. So we’re gonna cut it into three segments here. They’re asking about the relationship between hydraulic conductivity, infiltration, irrigation, decision making, plant soil interactions, all that kind of stuff. So the first part is, they’re asking basically, what is the difference between soil hydraulic conductivity and soil infiltration?

Yeah, that’s a great question.

That is a great question. And surprisingly, one that we deal with all the time and see. So oftentimes, confusion between the two. Hydraulic conductivity is, if we were to take an infiltration measurement which an infiltration measurement is just, let’s go out. Let’s apply some water to the soil and see how fast that water moves into the soil. But there are so many factors that actually affect how fast that water infiltrates, it’s not just the capability of the soil, but it’s, you know, how much is your ponding head height? How big is your area? How deep did you insert the rings? Infiltration doesn’t really take any of that into account. It’s just infiltration value. So it’s just how much water you’re actually applying to, it’s actually being absorbed by the soil at that time.

So if I were teaching my class about this, I would say that this infiltration relates to the flux idea. Yeah, how much water goes from point A to point B? Just really simple.

Exactly. That’s exactly right. Yeah, that’s a good way to compare it. And then when you switch over to hydraulic conductivity, hydraulic conductivity is that factor in the soil that is actually controlling its max rate. So when we think of saturated hydraulic conductivity, we’re thinking we’re taking a segment of soil, and we’re saying, given all of the other factors being negated no pressure, no pressure head, this is the limiting factor that controls how fast the water can move through the soil, given these conditions. And then that’s what we would take and apply it to our models that then take into account all of those other factors. So it’s trying to think of something else that’s akin to.

Well, I’ve been thinking about this, because you know, when we teach people about movement of mass or energy in the soil plant atmosphere continuum and I noticed, both of us are studying that, Leo. So when we talk about this, we would put up a simple equation that says flux is equal to ability to move the conductivity times a concentration difference. And I think this goes back to your original point, that the hydraulic conductivity just talks about something’s ability to move, and it has its ability to move whatever we’re interested in mass, or an energy from point A to point B. And then, to get to the flux, we have to multiply it by this other thing called the concentration difference. Is there a difference between point A and point B? And you talked about like a pressure head? Yeah, that kind of thing. In fact, we were talking just yesterday about the antecedent moisture Yep, in the soil, that determines whether it will get from from point A to point B, right? And so if it’s really wet there, versus if it’s really dried, that’ll affect this flux, this infiltration, that the F value, but it doesn’t affect its ability to move. The ability to move, is it kind of a native, permanent thing associated with that soil assuming other conditions change? We talked about roots, talked about other things, assuming that doesn’t change, that will be something static, this infiltration could be different. Like, you pointed out some things. Yeah. And I’m thinking about just hey, if it’s wet soil or dry soil? Yep.

Yep. If it’s wet soil or dry soil and, you know, what are our matric potential gradients? Like how much can we actually pull water throughout water? There’s a lot of limiting factors that can affect that.

So I got a little question as we’re doing this. So I’m over there in the Tatay, measuring this with the SATURO, yeah. And I’m sending you back emails, hey, here’s the number and you’re like, Oh, that’s terrible. Yeah, um, you know, should I have been worried with the SATURO about antecedentwater in the soil?

No, you shouldn’t, because at the end of the day, what we’re doing with the SATURO, is we’re pushing it to saturation, and then using those final values to get the saturated hydraulic conductivity. Now, having said that, there is literature that talks about the impact that antecedent soil moisture can have on that final value, and it does have an impact, but it’s more so in expansive soils and other areas, much in compacted soils.

Crushed granite put in place that will never grow plants again. But I think that’s important to say, I wanted to go through there because initially when I was making these measurements, that’s exactly what I assumed. But, when maybe listening to us, other people are like, Oh, does that mean, I can’t go out and make these measurements with something like METER Group’s SATURO. What about a mini disc infiltrometer?

So the mini disc is a completely different beast. It’s a totally different measurement. So because why? Because it’s applying a suction and resisting the water movement out of it. So it’s a tension infiltrometer. And people often try to use it to measure saturated hydraulic conductivity, but you can’t because to measure saturated hydraulic conductivity, you have to have free water. And you can’t do that with the mini disc. There’s a sintered stainless steel plate that you put in contact with the soil, and there’s a Marriott bubbler that controls that suction that’s being applied by the infiltrometer. And there’s no real way to go to zero suction and measure that saturated hydraulic conductivity. It’s a different measurement, but it’s a great tool for doing this other measurement.

And it’s not a bad thing that it’s a different measure. Yeah. I mean, I’m just making sure as we talk about it, we know that mini disc infiltrometer is not just a simple version of a SATURO.

It’s not, it’s a completely different thing. Yeah, it’s a great tool. And it’s a lot of people love using it. But yeah, you’re looking at totally different measurements.

So that actually leads us into our second question there. Yeah. Brad do you want to?

Yeah, sure. All right. Yeah. So the second part of this question is, then, if we do understand hydraulic conductivity, how can we use field measurements of water contents to improve irrigation decisions?

Yeah. So I think there’s a couple of things that we can think about. One, if we understand what’s happening in the soil, especially if we know the hydraulic conductivity. We can make better decisions about when and how fast to apply our irrigation rates, especially in understanding how deep that’s going to move through the soil profile, especially if we need depending on how deep our crops are rooting, and all of those things, and it’s something that we need to understand, especially as water becomes a more limited resource, we need to understand this.

So I’ve got some practical questions for you, Leo, about some of the work we’re doing right now. So some people here listening may be familiar with our turf grass efforts. We’ve been irrigating a lot of different turf grass, trying to do just what you say. Yeah, there’s a significant amount of overuse in water and turf grass. Yeah, and I can’t remember the number but it’s, you know, it’s our most irrigated crop here in the US, you know, even though we’re not eating it, my dog eats it sometimes. A dollar point of view is our golf courses, our sports fields and things like that, you know, a lot of those have you know, sand base, they have a standard golf standard, for example, that they adhere to. Now, my lawn, probably like your lawn has grown on a silt, loam, soil, much, much finer texture, what does that do with respect to this question? Hydraulic conductivity, how can we use these measurements? You know, if I took a SATURO out and measured on a golf terrain, and then went and measured on your new lawn? Yeah, that’s grown and so long, what we learn about it? I mean, I have my theories, but I’m curious what you say?

Well, you know when I think about some of the stuff, you know, obviously, with most of these we’re irrigating from the surface with sprinklers. And there’s a whole nother area of drip irrigation, which is a completely different, I probably have to put that on the shelf for this question, butwhat that really means is for example, when I irrigate my yard, I actually do it in shorter intervals more frequently, because I do it at night, because I don’t want water to run off my yard. Okay, I know that the infiltration and the hydraulic conductivity in my yard is slower. It has different factors because if you over irrigate too fast, that water can actually drain through the soil and be wasted. And so I think just understanding how the water is going to move through the soil, especially in a lot of fields that have those drainage layers, the way it retains water is completely different than the way my yard retains water that has a nice long soil profile that has in contact. So there’s a lot of factors to consider there. But usually, it’s I just, you know, my goal is to put enough water to cover the root zone and not push it past.

Okay. So if the infiltration was slower, you brought a good point that how can we understand you know, this field measured water content? If the, you know, if we’ve got a silt loam, there’s a potential that we could get runoff. Yep. Now I was actually thinking about it the other way. Yeah. So I was thinking about little short bursts for the sand to try to just keep that water. Yeah, in the root zone. It’s only six centimeters, and 10 centimeters deep. I was thinking about my lawn, and I was like, you know, if I wanted to approach this, to try to keep water in the root zone, maybe what I would do based on it, you know, presumably, soil infiltration, you know, that would tell us like with a SATURO measurement or something like that, that I would actually delay, you know, maybe irrigate every second day but a little bit longer. What would that do to the soil?

Yeah, well, I mean that’s the beautiful thing about silt loam is it retains so much more water in that plant available range. And so we can get away with that and that actually helped push those roots a little bit deeper by getting a little bit deeper irrigation, whereas the sandy soils you’re gonna just this comes back to morsel understanding the hydraulic not just the hydraulic conductivity, but also the moisture characteristic curve of that material. Knowing that with sandy substrates, we go very quickly from being in a very happy range for the grass to being in a very unhappy range. And we live on a knife’s edge for sure. That’s one of the things that we’re experiencing as we do these, especially with the sports fields. Yeah, yeah, they’ll dry out in a heartbeat. You know, we will probably have two days on that grass. Yeah. Or that sand? And then they’ll dry out. Yeah, yeah. So it’s really just understanding these factors that we can keep water in the right area. And this is where if you have sensors too, in the profile, you can help understand that and monitor to make sure you’re not pushing past the root zone and wasting that water which not only is wasting water, but it’s also wasting nutrients, which can be problematic, and many reasons are for in many ways. Yeah. And so we’re, you know, we’re actually doing a lot of that related this question measurements and water content for irrigation decisions. We do measure water content, especially below the profile to make sure we’re not washing water through. But honestly, in the profile we’re mostly using water potential. Yep. To make sure that there’s water available to these plants, especially when that falls off so quickly. Yeah, yep. Okay, Brad, we’re having too much fun. No, that’s great. I think you talked about different factors that affect an infiltration rate and other things like that. And some of those factors will be also you know, dying around diurnal nocturnal cycles. And so this last part of this question gets into, so how do plant and soil interactions differ, day versus night? Yeah, I got actually defer to call him for this. He’s more the plant person than I am.

To answer something. So the biggest thing that I think about day and night in terms of plants is the transpiration stream. So during the day, we have a heavy demand on the plant to remove water out of the leaves, they are gaining co2, they open their stalemates to gain that co2 and in by default, they lose water vapor. And so we can just assume that there’s a large pull on the water during the day, at night, everything I mean, if you thought about this as an elastic, so water has adhesive and cohesive properties, and during the day, you know this cohesive is pulling the water through the plant, we get this along a water potential gradient from the soil that is the highest water potential to the air, which is the extreme end of the low energy, the low water potential, and that just that pulls on that water column constantly through the day. And there’s a gradient, a strong gradient that’s pulling water out of the soil into the roots, xylem, etc. Now, there’s research going on on this all the time. And we understand some of these things a little bit differently than this classic analysis that I’m giving so understand, there are caveats to everything I’m saying here, but during the night, all of this relaxes, there’s no reason for plants to open, they’re stomates generally, although of course, this is not always true. But generally, the closer stomates there’s no, the conductance, we talked about hydraulic conductivity and soil that the leaves have it a conductance to water vapor too, and that essentially goes down by a couple of orders of magnitude. Yeah, at least one, maybe two. And so essentially, we have no pull on that water during the night. And that should all come to kind of an equilibrium. So during the night, the plant soil reactions, that it’s all kind of sitting, all quiet. Yeah. In terms of what’s going on until we have the sun come up, we have a need for co2 photosynthesis, and then we start the cycle again. And maybe there’s more to this question in terms of the interaction, but that’s what I think about that’s the main diurnal driver.

One of my favorite things or sets of data to look at is riparian zone data, where you are monitoring the water level in contact with the streams in these areas that are heavily transpiring during the day. And then at night, everything relaxes, and you look at the water table levels. And you can see that relaxation in the water table as that demand in that pool relaxes and it’s so cool to see kind of that offset in it. It’s just slightly offset with when the sun starts to come down and they’ll start the plants start closing shop for the night. And just, you know, there’s not as high of an evaporative demand. And just monitoring that. And you not only do you see that in water, the water table data, but you can see that in water potential data throughout the profile. And it’s really a cool dataset to see in. And I think as we get better and better instrumentation, to be able to monitor this stuff, you’re gonna see even more interesting patterns and some of that stuff. And I think that will help us better understand some of these, especially as we’re looking at some of these plants that might be transpiring at night and helping us better understand some of those interactions.

I was just actually thinking about this. Because when I started into the science, you know, let’s say back in the late 1990s, we could not see that during the day, it’s almost a, you know, this diurnal experience, in fact, I remember going to a meeting talking about Hey, does water move in soil? You know, do we get this hydraulic lift? Yeah, and there was some conversation about plants preferentially pulling water from lower regions, these trees and kind of giving it up to the upper reaches in the soil through their roots. And and then these annual plants could gather that water, and they could go through some kind of symbiotic relationship. But I remember just sitting in these discussions and having kind of, like, people really get animated. No, that’s not happening, or yes, I can prove that’s happening. The good thing about today is we actually have measurements, you know, over the years, so we can show exactly what’s happening. And I don’t think there’s any altruistic you know, trees, you know, out there, like, Hey, how about I hand back water to these poor little plants into my branches? Right, kind of a kid story? Yeah. But I do think, you know, we’ve observed, for example, in wheat and other things now, with the quality of measurements we have, with water potential or water content this beautiful, dry down, shall we say during the night, you can see a very clear drop, and then a flat during the day, and then a tabletop. Yep. Yep. And, and in some cases, especially where we have active roots in this zone, we also do see, you see this downward during the day, and then we actually do see somewhat of a rise in terms of water content. So what is that? Well, we’ve got a relaxation of that transpiration stream, and then the water could if there’s a water potential gradient leak back out into the soil, and so no one’s kindly sharing with their neighbors. But if they want to, if the gradient goes the other way, yeah, we are going to, you know, water is gonna go out, water is gonna equalize in the system. And we have instruments today, which we never had, you know, that it’s, 25 years ago, I don’t like to, you know, even imagine it somehow. And, now, we do see that, yeah. Okay. I’ll get off that question, since I’ll talk about plants all day. Brad? Well,

I was gonna say this is another question that kind of follows along talking about different factors. And this individual is asking, how do seasonal changes affect soil infiltration?

Yeah. So that’s another really good question. And there are absolutely seasonal impacts on not just infiltration, but the hydraulic properties of soil. And there are quite a few things. But the main ones that I think about antecedent soil moisture, we know makes an impact, especially in highly soils that shrink and swell, because the soils will open large cracks. And then as you hit the wetter season, those cracks will start close up. And so your hydraulic conductivity can change significantly due to that. But there are other factors along with the plants that are growing over time, you know, you have active root channels coming in. And then as those channels start to decay, you get more open pores. So there’s a lot of those factors. And then also just worm activity that increases or can increase in decrease seasonally. Yeah, so there’s a lot of those things that can change the hydraulic conductivity of the soil. So we often recommend if you’re trying to compare a site, that you’re wanting to look across a whole site trying to get those measurements all within the same season. Because if you don’t do it, if you do a measurement on one site, maybe in the fall, and then we go back and do it in the spring, when things are starting to change, you might not only be measuring the site differences, but you’re gonna be measuring some of those seasonal differences as well. And so it’s something that we have to take into account and understand and soil temperature too, soil temperature, as temperature increases the viscosity of the water changes. So that’s another factor that can impact that. Now. There’s ways to correct for that if you monitor the water temperature in the soil temperature. But that is another thing that can impact it.

And so when you were out with your giant truck, and it was really, really hot,

yep. I was monitoring water temperature and soil temperature. Okay. So,

you did the right measurement. Yeah, okay. Yeah, I’ve got an example of this actually, because you know, when we were doing some of this turfgrass research I’ve been talking about, what I really wanted to show is a moisture release curve, in situ. So we use HYPROPs in the lab. And we love those things. But when I was thinking, you know, if we could even make, we can get some lab stuff done, and also make a combined water potential water content measurement in the field, we could do some of these things. So I actually had one of my technicians go out to the field and sample out there and run some of these in the lab. And I was really excited that you know, these things started to match up, we had kind of a finer texture soil out there. They called it a loam. It wasn’t a loam, or like sandy loam or even a loamy sand. Yeah, but they ran it. And those things actually matched up pretty well. But then we ran this, this golf, you know, this turf sand, this green sand. And I looked at it, and it was way off. And I’m like, Hey, to this technician, what’s going on? What did you do here? And it finally came out that on one he had sampled it and then gone and kind of wet it up in the HYPROP and run it pretty much quickly from the field in there, hadn’t let it dry out and the other, the one that I was having problems. he’d let it dry out all the roots, had tripled the you know, sat on the bench for a few weeks, and then he’d run it. Yeah. You know I think the roots had changed. Yeah. And the moisture of these curves was weighted. Yeah so it’s something to think about as you’re sampling, you know, those roots will impact maybe not so much if you’re using a clay. But if you’re in the sand you have to think about that, and all the other pieces,

and that makes me think of another thing that I think people often forget is sample storage, when you bring samples back to the lab

Wait undergraduate student workers don’t always do what you say? No. Okay.

So always remember, if you’re collecting a sample to do any type of hydraulic properties measurement, and you’re not going to run those measurements within a couple days store it at four degrees Celsius. Yeah, so that didn’t happen. Yeah. Because that limits microbial activity, it’s going to limit the decay of the roots. That way they stay more impact. Yeah, so yeah, sample storage people.

On these things in extreme, we talked about kind of normal cases here. And then we’ve kind of identified some extreme cases. Right. Yeah, we’re talking about, you know, this infiltration. Soil health go hand in hand, we said, Actually, generally, you can see it. Yeah, in this case, you know, if we’re working in this coarse sand, and we’ve got root growth, that’s actually forming some of this. Some of the soil structure, yeah. Extra special care over that, presumably? Oh, yeah,

absolutely. I mean, these, you know, this makes me think of when we were doing a comparison study with the double ring infiltrometer and the SATURO. And we were inserting into a little bit more frail soil. Yeah. And it takes so much more force to install a double ring, because you’re dealing with two larger rings. And what we were finding is the force that we had to exert to drive the rings into the ground was actually destroying the structure which is an important thing to understand. But I don’t want to understand it that way. I’d rather, understand that the soil is less stable with maybe looking at aggregate stability or something like that, because now I don’t have an accurate measurement of my hydraulic conductivity. Because I’ve destroyed the structure. And it’s the same with that type of stuff like we need to do or have the best care to try and maintain the soil in its actual state, which is one of the reasons I like field measurements over lab instruments myself, is we get a better representation of what’s actually happening in the field versus when we bring back things back to the labs, there’s a lot of things that can happen. We don’t take care of the sample well, roots can decay, there’s a lot of things that can happen. So there’s other things that you have to understand when we’re trying to compare, you know, look at impacts of all of these things in the field. And the other part is you know, the less we have to do to insert the rings, the better. So, like most sites use the shorter insertion ring if you can, for the SATURO, for example, because if I’m using a 10 centimeter insertion ring, that’s more than I’ve got to drive in and it’s more chance I’m gonna screw things up when I push it down. So

Well, I’m glad you’re bringing this up. Because would say, if I had one biggest fault as a researcher, I like to kind of run with it. So I like, what is there to know about about running a HYPROP sample right in the field. And so I maybe shouldn’t admit it here, but I don’t read the manual. And I just like, let’s get to the field and make these measurements, you know, let’s go collect the samples and get in. And I think that, you know, that cornice of that approach was, you know, apparent on this particular example, but also when you and I, you kind of took me through how to run a HYPROP. Yeah, you know, a few years ago, it’s my first introduction to it. And I realized that just winging it on some of these things like hydraulic conductivity. Yeah, infiltration isn’t a great way to actually get to the measurements you need.

No, sometimes there is some care that’s needed. And just basic understanding of the things that you’re doing and how it impacts the quality of your measurements. Yeah, I always, and I probably have my former adviser Christine Morgan to thank for that, because I was more like that until I was under her tutelage. And when I made mistakes, she was like well, did you read the literature? Did you read the manuals, you read the documentation? Like Well, no. And so she kind of just like, you gotta have better practices here.

She was also my student worker. Yeah. So I’ll come back to you. I’m just kidding. After she probably taught me all of that as a grad student. Okay, Brad, we’re having too much fun again.

No, that’s good. Because, again, you guys are great at segways. Speaking of correct HYPROP use, we did have a new publication come out that was shared on social media about modifying the HYPROP just from I think this is from the the abstract, but basically, the quote says “To expedite the method, here, we propose a modification of the HYPROP that facilitates consistent temperature and airflow around and over the soil sample ring, to ensure constant evaporation from the soil sample.” So I just wanted to get your thoughts. Maybe a little bit more information on hacking the HYPROP? And what you think about this new modification?

Is it a hack? It is not a hack. I think I’ve seen this before. Yeah, it’s actually a really, you know, I had the joy of actually speaking with the author of this paper Yiannis a couple days ago. And so we talked about this, because I saw this post like, Oh, that’s really cool. I want to kind of learn more what you’re doing and talk about some of my experience. And it was really interesting to hear some of the challenges he had and why they wanted to do some of this stuff. And so they’re in Greece, or in one of the islands in Greece, and their lab isn’t super well temperature controlled. And so they were seeing diurnal changes in the evaporation rate. And so you can actually see it in their HYPROP measurements. So he’s like, Yeah, we needed to control the temperature better. And because at first I was like, Well, why don’t you just use desiccant? Because another way to do this to speed up the measurement, which is a common thing that many people want to do, because typically, it takes a while. It takes anywhere from three to seven. And on some soils, we

get back to my impatient. Yeah, sing along. Yeah.

But ultimately, the goal is, if we can increase the evaporation rate in a way that doesn’t violate the assumptions of the measurement, so we want to make sure we stay within the phase one evaporation rate, then let’s find a way to do it. And oftentimes, when I talk with people about this, and like, well put your HYPROP in a chamber with desiccant, increase that vapor pressure deficit, and that’s going to increase your evaporation rate. That’s a great approach. But he mentioned he’s like, Yeah, but one of our big issues is that our temperature changes so much. And what they actually did is wrapped the HYPROP core itself with a heating element, and they maintain the temperature, constant temperature around the core. And it’s like, oh, that actually makes a lot of sense.

But it’s not like it was on fire. It wasn’t on fire. Just they limited the diurnal temperatures. Exactly, pressumably it was like 25 degrees. Yeah,

I think they were doing 30 or 35, which is still fine. You don’t want to go too hot.

That’s what I’m getting at here, Leo. Yeah, I’d probably be like let’s do 100 Right. Get this thing rolling.

Again, we’re talking about violating the assumptions of the measurement, as long as you don’t increase the temperature too much. And you know, the temperature, the HYPROP is measuring the temperature of the soil anyways, yeah, you can correct from that. And so maintaining a constant temperature is fine. And the other thing they did is they put a little fan in the chamber and it’s just a small computer fan. And at first, I was a little worried about it when I saw the experiment, and I was like, oh, no, they’re using a fan. And because the fans are probably the worst things you can do.

So I wondered about that. Can you just talk about fans? Because it seems like I’ve seen fans in use with HYPROPs before? Yeah, well, what’s the problem with fans? Is it adjusting the weight? I mean, there’s, like

crazy issues with it. If you’re blowing air right over the scale, it affects the weight measurement and makes it really noisy.

Okay good. That was my concern.

But the other actually more critical factor, both are critical is because you’re increasing that boundary layer conductance so much by having air movement, as you know, wind speed has a really big impact on the boundary layer conductance. And that increases the evaporation rate to too high of a level where you’re actually violating the assumption in the measurement and so yeah, we have to be careful with that. So that’s like the desiccant, I think, is just enough, but what he was doing here is just a small computer fan, which was just pulling air out. And it’s not moving that much air, okay, it’s just enough to pull some of that moisture out of the chamber. So in a way, it’s kind of like the desiccant, where it’s just pulling some of the moisture out, and it’s not really impacting the measurement. So overall, I actually liked the method, especially the temperature control part. And if you’re working in an area where you have temperature control in the room, this is a good approach. But also just using a timber desiccant. If your goal is to just increase the evaporation, increase the speed of the measurement, a chamber with desiccant is probably one of the best ways to go.

But what does that look like? I mean, I’m imagining, you know, that little space, you know, flying saucer shaped thing? Yeah, there’s desiccant on the bottom. But how do you do the cords? What do you do overall?

Yeah. So you do need to have a way to seal the cords up when it comes out.

So you do run the cords out through your chamber, but you put the scale and everything inside. Yeah,

now there’s only two cords that have to come out the power cable for the scale and the USB cable that’s going to the computer. And I’ve seen a lot of really nice setups, I mean, if you’re wanting to get really fancy with it, you can have a USB hub in your chamber, that you can add everything to through there, and then that runs to your computer. That might be a little bit too high. But you do want to seal it up just so that way you’re really driving that relative humidity down with the desiccant. Okay, and only pulling moisture out of a HYPROP.

But yeah, now again, is this in all conditions? Because I mean, we run in the lab here without any desiccant and stuff like that, do I need to do that?

Probably not as big of a deal here, our relative humidities in our environment are a little bit lower. But if I’m working in a really high relative humidity environment, I’m gonna see a big difference in how fast the samples can dry down. And if sample throughput is a big challenge, we also don’t have to run through hundreds of samples all the time, people that are trying to run through, you know, 20 30, 40 samples and get the points if you can increase that.

So I mean, I think that’s your best point, or not best point, but a great point to make. You know, we do these M1s and 2s, you know, I do student projects, we have field sites that we’re bringing in, there are some people who are doing a massive amount of sampling. Yeah, and this is something that they may think about to really, you know, take a front effort, but eventually help you with throughput. Yeah, exactly. Yeah. Okay, Brad, that was fun. What do we got? All right,

So this next one, I’m going to try to rephrase this a little bit. But basically, they’re asking for best practices to properly prepare the soil surface and to calibrate infiltrometers to best perform field infiltration measurements.

Oh, great question. So typically, when I’m going out and making measurements, I’m assuming this is referring to surface infiltration measurements, we’ll stay at something like the SATURO. I’m going to try and remove any large debris from the site, especially if it’s in the area that I’m going to insert the ring. So I don’t want to see like branches or rocks within that area. Because if you’re driving right on something big like that, it’s going to create a huge gouge when you’re pushing it. Other than that, I don’t want to touch the surface of the soil. I want to leave it in its original.

So I got a question for you. Yeah you know I would. So in our lab at Texas A&M, we were studying you know, macropore flow with the dye study, you know, something that we were doing quite a bit back in late 90s into that ships clay or whatever. Yeah. Houston black clay. I don’t remember what it was. But, you know, there were cracks in there visible from the surface. Okay, maybe it’s just the case we’ve been dealing with this a little bit, but how do you deal with that? You want to go out there, you want to make these measurements and you got cracks. Do you close your eyes, you know and throw a penny into the air and sit around the penny, what do you do?

Those, it’s not necessarily an edge case, because there’s a lot of highly expansive soils like that around the world. But they present a big challenge when it comes to making those measurements.

Those cracks, they’ll eat up a hole tank lot of water.

And so a good example of this is I was working on a site that I didn’t think was over a crack, okay, you know, and that’s one thing I do if I’m working in those shrink swell soils, I will inspect for any large cracks. We can’t measure over a large crack, it just takes on so much water. But at this site, I was like, Okay, it looks like I’m pretty clear. So I installed my rings, set up the measurement. And I was working with some of the folks at the NRCS, we were doing a pit study at the same time. So I was like, Okay, well, I’m gonna go over here and see what they’re doing. I left my stuff running because I set it up to be pretty much fully automated. And I come back and my trailer is empty, completely 500 gallons of water is gone.

Was there water in their NRCS pit?

Luckily, I was far enough away, because probably if I was close enough, there would have been 500 gallons of water in the pit. But I came back and I was just completely astonished about the fact that I’ve gone through five hundred gallons of water in maybe an hour and a half. And it turns out I was right over a crack. And just it’s the funny thing about cracking soils is if you just get a little bit of surface flooding the cracks can close at the surface, and then still be a huge void down below that. And then once you start infiltrating the cracked soils and water starts pouring through that crack. So when I was doing work on those shrink swell soils, I actually timed all my measurements right after the wet season. That way the risk of cracks was the least.

We only do SATURO measurements in the spring.

Yeah, in Texas. That’s really the only time you can do it.

Yeah, I think that helps. Yeah, yeah. So anyway, keep going with the site prepper. Yeah.

So other than that, obviously, if you’re dealing with tall vegetation that you need to clear from the site, we don’t want to have a bunch of grass pushing up into the sensor. So I will come out with some tremors and clip the grass down to at least, you know, four or five centimeters.

But there’s no need to clear the vegetation. No you don’t want to because you’re going to disturb the structure that’s there. So I’m glad you mentioned that. But I’m sorry to interrupt. No. But this is something that if you don’t read the manual, you may just assume that you need to just go to clear the surface because it would be different with something like the mini disc infiltrometer or a golf permium. Yeah. Something like that you do something different. So knowing what the site prep looks like, really important.

It is yeah, it’s really important. And we do see this mistake sometimes where people go out and start clearing stuff out. So I just dig a hole there. Yeah. I mean, it’s great if you want to get down to a deeper layer, and

can you do that? Yea so people ask us, hey, we want to not do it at the surface.

So you can excavate down to a lower layer. I mean, it’s no different than if you’re doing a borehole measurement, or anything, I mean, you’re going to disturb the soil a little bit anyways, so, but what I would do is, if I am going to try to get down, I’m going to excavate a little bit, and I’m gonna carefully prep the surface, maybe with a knife and just kind of picking away at it. That way you’re pulling the soil but not really smearing it. Yeah. Because once you start, especially if it’s wet, you start smearing the soil, you’re gonna start closing off those pores.

Yeah, that seems like a big concern. I mean, I’ve seen that done in the field where I’ve come up, and they’re like, right, and I’m like, How’s the water gonna get in there? Yeah you just made a butter paste, exactly.

Yeah. And that’s why like, when we did pit studies, for example, like when we’re trying to prep the surface of the pit to view, we would pick at it with a knife, we wouldn’t just smear the soil. Yeah, we’re trying to just pull the soil off and maintain some of those pores. And it’s the same thing if I’m trying to do an excavation to measure down lower. Now, if you’re trying to measure really deep, you know, it practically just becomes impossible. But if I’m just trying to get down to maybe, you know, 10, 15, 20 centimeters and just measure a little bit deeper, that’s totally fine. Just make sure you prep your surface. Yeah, right,

I think. Yeah. And I interrupted, you’re talking about clipping the grass. Yeah. Anything else I’m missing?

No, I think it’s mostly just trimming the vegetation. Make sure you get that down. Other than that, just make sure nothing is within your ring area that you’re going to push down. And then actually, once you get the ring inserted into the ground, obviously make sure when you’re inserting the ring, it’s a stable insertion you’re not going in a wobbly pattern. Yep, you want a nice straight down insertion. And then I often recommend coming in with a pencil eraser edge and just tamping the soil around the edge of the ring. So just going around the edge of the ring and tamping the soil just to kind of seal up and reduce the risk of preferential flow just down the

edge down. Yeah, and up or whatever. Yeah, exactly. These are the things that you don’t think about. Yes. I’m glad we have Leo around to correct all my poor experimental practice.

Wow. And it’s because I’ve made the same mistakes. So I’m learning from all of my mistakes.

I guess that’s why we do Office Hours. Right, exactly. Okay. Brad, did we nail that one? I think we got it.

I think we’re good. So we’ve got five to 10 minutes left. Okay. So we’ll see, the way things are going, this might be our last question, but we’ll see.

And suddenly realize we just almost ate up an hour having fun here. Yeah. Sorry, guys.

it’s good. Okay, so this one, and I don’t want to turn this into a bash session. But this does get to broader, maybe some broader issues within scientific measurements, and other things like that. But the question is, basically, why is the double ring method still commonplace? If there are now newer methods like the dual head?

Yeah, no, that’s a really good question. And some of it is just, it takes time for people to trust new methods.

I think yea one retirement a time.

Yeah, exactly. Yeah, it takes time for people to trust new methods, they want to see the literature out there. The beautiful thing about the dualheader and the SATURO is that the literature has been out there for 30 plus years. So the methodology that it’s based on is been sound science for years, there just hasn’t been easy to use tools to make this approach.

So that’s, I mean, that’s absolutely true. But when you dig a little deeper, I think when you and I were talking about this, you always say, well think about the alpha value, like it’s not just that the dual head and the dual ring are the exact same measurement, and we put it in the box. And that’s not in a box. There are things that make the dual head more accurate and more robust to different soil types, right?

Yeah. And the alpha value that Collins is referring to are the macroscopic capillary length factor,

that’s way easier to say as alpha,

which is related, essentially, it’s just related to the sorptivity of the soil, it’s a factor that we use to correct from a ring infiltrometer. Because we know that water doesn’t just move vertically, it moves horizontally, and we want to correct that value to get that vertical flow value. With tools like the double ring, we traditionally have just estimated what that alpha value was. And that leaves a lot of room for error. Because you’re essentially having to make judgments on the soil type. But also, like, is it a well structured soil? Is it a poorly structured soil? What are some of these other things that can impact it? The beautiful thing about the dual head approach is it actually allows us to directly characterize that. Because when we’re infiltrating water at two different pressure heads, we can look at the rate of change between those two different pressure heads and use that to and mathematically determine what the sorptivity is and what the alpha value is.

So is it a case of maybe this is oversimplified but two equations, two unknowns, essentially, because we’ve got two different infiltration heads, and we can solve the equations if we’re doing two different things?

Exactly, that’s exactly what we’re doing. And there’s a great theory section right up in the manual on this, that I highly recommend reading. And it directs you to a couple of papers that we have used to base methodology off of. One is from the Methods of Soil Analysis, and the other is from John Nemo, where we kind of took the best of both worlds that we liked, and kind of combined that together. And some of the stuff that Galen did, it was really amazing along with the work that John Norman was doing there. And but to go back to why the double ring is still commonplace, is because people have double rings sitting around and also in the engineering world, which is really the more tricky one. In the science world. Once we start publishing and people start trusting literature, it’s pretty easy to start adopting new methods. On the engineering side of things, standards

no they save lives. They do you know, they’re there for a reason. But will there be a standard for something like that? Yeah. Yeah.

And there is and it’s been in the works for quite some time. And the challenge with standards is just takes a long time to get those standards through. And engineers have to rely on methods that are based on standards because it protects them from a liability standpoint. It’s like no, this was based on ASTM standard XYZ. And so that helps protect them. In the science side of things, we don’t really have to worry about that as much. But you know, it just takes more time to replace those methods. That’s why some of the tools like pressure plates and filter paper method, still are caught being used talking about water potential, but it’s the same challenge that just takes time. And also maybe goes back to what Colin said one retirement at a time. Yeah.

But honestly, I mean, I fall into this trap like everybody, I know the way I like to do things. And that’s probably why I make errors sometimes in the HYPROP, I have a methodology, it’s in my head to go follow the methodology. You know, it’s not necessarily the best way to get the numbering. Yep. We got time for one more.

Let’s see. We could do a quick, I don’t want to say lightning round, but okay. Wow. Okay, how about can you do a one minute answer on this one? Okay. So they’re basically asking you, How many measurements do you expect to make in a field to account for variability? And how do you navigate that issue?

Yes, one of my favorite questions, a couple of things that you want to look at, how does their soil type change across that field. So maybe use a soil map, go hop on to Web Soil Survey, or do an EM map of the field and look at the variability, that’s what I did for my research, to identify those zones of variability, and then start measuring those different zones. The other, area is hillslope position, do we have big changes in elevation? And what is our catena effect there, as we know, soil hillslope positions impact how soils form. What is the catena? The catena effect is when you’re looking at your hillslopes, you have your summit, you have your shoulder, you’ve got the actual sloping part, you have the foot, the foot slope, and the nutrients collect down at the lower portion. So that actually impacts the hydraulic properties. Just wanted

to make sure we were you know, Hey, man, what the heck is a catena? Yeah, something we talked about in Soil 201, but yeah,

it is a good thing to clarify. So look at those things and try and measure across those different areas. Soil type and landscape position are the biggest factors that I think make a difference.

I should have timed that.

Could I do that in a minute? Yeah.

Close enough. We’ll call it good. All right. I think that’s it. We’re up at the hour here. So that’s gonna wrap it up for us today. Again, thank you to everybody who’s joined us. We hope that you enjoyed this discussion. Thank you again for all the great questions. There are several questions that we didn’t get to. If we did not get to your question live, we do have folks on our environment team who will be able to get back to you via email to answer your question directly. Also, please consider answering the short survey that will appear after we finish here just to let us know what types of Q&A themes you’d like to see in the future. And for more information on what you’ve heard today, please visit us at Finally, look for the recording of today’s presentation in your email. And stay tuned for future METER Office Hours events. Thanks again. Stay safe, and have a great day.

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