In this episode, METER application specialists Leo Rivera and Shaun Weldon discuss questions asked by product users when generating soil moisture release curves (SMRC) and measuring hydraulic conductivity. Join Leo and Shaun as they discuss:

• The role does sample prep play in the accuracy of SMRCs
• How is air entry identified on a SMRC
• If the SMRC of higher depths be used to extrapolate the curve of lower depths
• How the Kfs error estimation is calculated by the HYPROP
• How the SATURO takes into account cracks in the soil
• And more

Presenters

Leo Rivera is a research scientist and Director of Science Engagement at METER Group. He earned his Bachelor’s and Master’s degrees in Soil Science at Texas A&M University. There he helped develop an infiltration system for measuring hydraulic conductivity used by the NRCS in Texas. Leo is the force behind application development in METER’s hydrology instrumentation, including the SATURO, HYPROP, and WP4C. He also works in R&D to explore new instrumentation for field measurements of water content, water potential, and hydraulic properties of soil.

Shaun Weldon operates as the SATURO, LABROS, TEMPOS and tensiometer product manager at METER Group. He earned his Bachelors degree in environmental science from Western Washington University, and his Masters degree from Washington State University. He has over a decade of experience helping researchers measure the soil-plant-atmosphere continuum.

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Transcript:

BRAD NEWBOLD 0:00
Hello everyone, and welcome to Office Hours with the METER Environment Team. Today’s session will focus on measuring hydraulic conductivity and soil moisture release curves, and we’re shooting for about 30 to 40 minutes of Q and A with our experts, Leo Rivera and Shaun Weldon, whom I will introduce in just a moment. But before we start one housekeeping item, if you’re watching this video and you think you have a question you’d like to ask our science experts, we encourage you to submit your question on our website, at metergroup.com, and then someone from our science and support team will get back to you with an answer via email. All right, with that out of the way, let’s get started. Today our panelists are application specialists Leo Rivera and Shaun Weldon. Leo is a research scientist and director of science engagement at METER Group. He earned his bachelor’s and master’s degrees in soil science at Texas A&M University, where he helped to develop an infiltration system for measuring hydraulic conductivity used by the NRCS in Texas Leo’s the force behind application development in METER’s hydrology instrumentation, including the SATURO, HYPROP and WP4C. He also works in R and D to explore new instrumentation for field measurements of water content, water potential and hydraulic properties of soil. And Shaun Weldon operates as the SATURO, LABROS and tensiometer Product Manager at meter group. He earned his bachelor’s degree in Environmental Science at Western Washington University and his master’s at Washington State University, and he has over 13 years of experience helping researchers measure the soil, plant, atmospheric continuum. So thanks for joining us, guys.

LEO RIVERA 1:30
All right, glad to be here.

BRAD NEWBOLD 1:33
All right, let’s get started taking some questions. And our first question here today is, how do you locate air entry on a soil moisture release curve?

LEO RIVERA 1:44
Yeah. Do you want me to start with that one?

SHAUN WELDON 1:47
Sure, yeah.

LEO RIVERA 1:48
Well, it’s actually pretty simple thing to look at, and if you ever look at a soil moisture release curve, you’ll notice that it always starts off pretty flat, where you see little to no change in water potential, even though the water content is starting to decrease, and then all of a sudden, you’re going to start to see a drop off in that curve. Sometimes that drop off might be really steep, or it might be kind of a gradual drop off, but that point at which that drop off occurs is what we call the air entry point for soil, and that’s what when that air entry occurs, that’s what actually allows for the water potential to start to actually change as the water content is changing. So that’s kind of the basic, basic context of how you find that. I don’t know additional thoughts on that Shaun?

SHAUN WELDON 2:36
Well, I think one of the nice things about instruments like the HYPROP is you can actually see that really easily as well, in both the measurements and in the data that once it’s once it’s applied to the models, you can see clearly when that air entry hits those tensiometers, as those suction values drop off really, really sharply.

LEO RIVERA 2:54
Yeah, yeah. It’s really true. I mean, just the resolution that we get in the data with newer tools really helps us identify those things, and even helps us identify when we have bimodal soils that have multiple air entry points, which are pretty interesting to look at as well.

BRAD NEWBOLD 3:09
Next question, how can soil moisture release curve data be used to interpret the changes in soil moisture behavior in forest soils affected by wildfires?

LEO RIVERA 3:18
Wildfires?

SHAUN WELDON 3:21
A lot of questions about this in our in our support and science group and so, so a lot of the effect of wildfires is pretty superficial. It’s really right at the surface there. And you start to get, in some cases, in severe wildfires, you’ll start to get some hydrophobic behavior on the surface’s there, but the soil moisture behavior itself below that surface doesn’t change a whole lot. It’s just that that organic material on the surface that really gets changed in these wildfires. So I think one of the main things that you’ll notice is a difference in that infiltration, that initial infiltration, but the deeper soil behavior tends to stay fairly similar.

LEO RIVERA 4:05
Yeah. Yeah, I mean, it really depends on the intensity of the burn, of course. I mean, in some cases, you can get permanent changes to the actual soil makeup, or clay minerals get bound, and things like that, just due to the high heat that occurs, which, in that case, that could affect what the retention properties look like. But even then, it’s still very much so at the surface, I think tools that are even more useful when trying to look at soils affected by wildfires, is actually looking at how they infiltrometers, whether they become hydrophobic and things like that. And that’s where using tools like the mini disc infiltrometer and the SATURO are helpful. Because really one of the biggest risks after a wildfire is is, is high runoff events, or that can cause mud and debris flows. Because oftentimes what happens is, sometimes you get a wildfire that comes through, and then you have some intense rainstorm event that comes shortly after. And in we see, we’ve seen this time and time again, where you get these, these severe runoff events that occur, that cause massive debris flows and things that have affected towns and people. So that’s really the, I think, the more important measurement to look at. Yeah.

BRAD NEWBOLD 5:12
All right. Next question, can we extrapolate the curves of the SMRC for deeper depths, for example, 10 or 20 foot depths?

LEO RIVERA 5:21
Yeah, this is an interesting question. I don’t know. I think it depends on what your goal is. If your soil is consistent down to those depths, which that would be pretty consistent, then sure you could do some extrapolation. But it’s not something I would typically recommend doing, because as you go to deeper depths, your soil is likely to change. And so really, what you need to do is characterize the properties for that soil at those depths.

SHAUN WELDON 5:44
Yeah. And I think it even if the soil type remains the same, you’re going to have different effects, like compaction. You’re going to have lack of pores that you might have at the surface, yeah. So it is going to behave differently, so that, that’s something that you’ll need to consider as you’re looking at this as well.

LEO RIVERA 5:59
Yeah. So yeah, so, yeah, I would definitely avoid trying to extrapolate down to deeper depths. But characterizing those samples is what you need to do, because, yeah, you need to understand the whole profile and what that looks like, yeah.

BRAD NEWBOLD 6:11
All right, this next individual is asking, how high is the impact of evaporation rate in the evaporation method?

LEO RIVERA 6:20
Yeah, well, I mean, really, the big impact, right, Shaun is, is that it speeds up the measurement, yeah. But I think one of the things you want to avoid is, is having an evaporation rate that is so high that it violates some of the assumptions in the measurement, and the biggest one being that you’re in phase one evaporation, which means you have a constant evaporation rate, from the sample throughout the time of the measurement, because once you go beyond that, then some of the principles behind the measurement are no longer valid.

SHAUN WELDON 6:49
Yeah, and that’s a that’s a topic that we discuss a lot with customers when they call in asking about specifically HYPROP, is ways that they can speed up that measurement, because it does take a while, but keeping, keeping that of that constant rate is really, really important, and also making sure that you’re not introducing other sources of error by trying to speed that up, things like vibration or heat, can really, can really change the way that that measurement plays out, and can cause some bad data.

LEO RIVERA 7:21
Yeah, exactly, yeah, the thing I always you know, if you can bring down the humidity around the sample, that’ll help increase that badger layer conductance in a rate that is still within the assumptions of the measurement, yeah. And in many cases, you can shorten your measurements within, I mean, in most sample, in most samples, if you really can draw that evaporation, draw that moisture down, you can get it down to two or three day measurement. Yeah. So. Yeah.

BRAD NEWBOLD 7:50
This next person says that they work with soil surfactants, and is asking if it’s better to use water potential or water content to test the effect of their surfactants.

LEO RIVERA 8:01
Yeah. I mean, I think both are important.

SHAUN WELDON 8:03
Yeah, I was gonna say that. I think just like with any any soil sample, any measurement you’re taking, and this, using both is going to be important, I think that characterizing that soil first before with both of these and then characterizing it after with both measurements is going to give you the greatest impact and the greatest measurement of how those surfactants are impacting the soil that you’re measuring.

LEO RIVERA 8:26
Yeah, exactly. I mean. So it’s changing the wetting angle and how that how water penetrates and is bound on the surface of the minerals. And so it’s definitely going to impact the moisture, soil moisture release curve and and so you want to understand that impact. I think Shaun, Shaun is exactly right. You need to characterize the before and the after to really understand that. But also, you know, I don’t know how long many surfactants last in the soil, but I’m sure they will degrade over time, and they’re effective, will degrade over time.

BRAD NEWBOLD 8:56
All right. Is it possible to get a curve using the TEROS 12 and TEROS 21?

SHAUN WELDON 9:02
Um, yeah, this is a great question. Yes, you can, but there are some caveats to this. So if you’re trying to do this in a natural system, if you just plug these in out in the out in the field, you’re going to have to remember that you’re not going to get that full range of wetting and drying in there, very rarely is your soil going to get to that dry point that you can get in a lab setting, right? And the saturation also probably isn’t going to be super common unless you get very heavy or frequent rain events, yeah. So in a natural system, you’ll have to keep that in mind. If you’re doing this in a lab setting, you can definitely get more control over it and get a better release curve, but you will also need to make sure that you’re taking into account the volume of soil needed to get good readings with the TEROS 12 in this case. The TEROS 12 has a very large measurement volume, and so you’re gonna make need to make sure that your sample is representative of that, yeah, so that you’re not artificially depressing those values from the TEROS 12.

LEO RIVERA 10:07
Yeah, yeah and when using the two together, if you are, the goal is to get some in-situ soil moisture release curves. There are some keys to getting good measurements that way. Of course, co-location is critical, having the sensors installed near each other at the same depth. But the other, I think, more important part, is actually how you use the data to get your your your retention curve, because, like Shaun said, things are very dynamic in the field, and so you need to understand that. So a good example of this is, is we’ve done measurements in our fields where we have co-located sensors, and we’ve actually used those to generate soil moisture release curves, but the only data we use to generate that soil moisture release curve is a prolonged drying period where we start close to saturation or wet enough and let it and we’ve had enough of a drying period where there’s no new moisture coming into the system, because when you get that, things are much more dynamic so it’s hard to get hard to get a good retention curve that way, but if you do get those periods, you can get actually really nice retention curve, that way from in-situ measurements. But like Shaun said, you know, there’s definitely differences between lab and field measurements, but I think it’s a great tool to use and and what’s nice is you can look at changes over time, if you look at the data right and aren’t trying to use wetting data, where there are some slight differences between the rates and the sensors to try and do your soil moisture release curves. But they can be really powerful tools when combined together.

BRAD NEWBOLD 11:34
Okay, and speaking of lab work, and as a follow up to that, how can the HYPROP be combined with TEROS sensors to extend the soil moisture release curve into the dryer ranges?

LEO RIVERA 11:46
Hmm, yeah. I mean, there’s a couple ways I look at that and and like to get Shaun’s take on this, but you know, like Shaun said, we don’t always get the full range in the field, but what we can do is use HYPROP, and probably HYPROP combined with WP4C to generate that full soil moisture release curve, and then we can kind of use that to interpolate some of the field data and understand what’s going on there and look at that. So I think one of the other advantages is it’s a lot easier to get that full saturation point in the lab and and then use that to really see, you know, to look at that which one thing to be careful we never, may never, actually achieve that full saturation in the field, right.

SHAUN WELDON 12:28
Yeah. And I think that’s one of the key strengths of the HYPROP in using that is that the most sensitivity for the HYPROP is in that wet range, because we’re using tensiometers. So the accuracy up there in that wet range is just incredible compared to what you might see in the in the field setting. And I think that the HYPROP, or LABROS software, which is what runs the HYPROP has, is just a really powerful tool for getting that drier range as well, particularly, like Leo said, if you put that in with WP4C data, which is all really easy to integrate in that software, you can get some really solid data from that dry range.

BRAD NEWBOLD 13:08
Okay, how do you determine when a measurement needs to be repeated due to poor data quality?

BRAD NEWBOLD 13:14
Yeah, I think it’s a really good question. I mean, I think it depends, of course, for talking about HYPROP versus WP4C or other some of these other instruments. But I think with the HYPROP, it always starts with looking at the tensiometer data to make sure it looks clean, right.

SHAUN WELDON 13:15
Yeah, that’s one of the beauties of it is you actually get to see that raw data, and not just the interpreted model. So you can look for anomalies in that data, looking to see if it reaches that full range before we would expect to see cavitation, looking for anomalies in weight or fluctuations in that suction data as well. Those can both give you kind of a hint of something that might be going wrong with it. I think Additionally, the software also gives you some nice data, some analytics at the end that shows you like your error on the the actual run that you did. So these are all great ways to kind of determine what your how the quality of your data is.

LEO RIVERA 14:12
Yeah, yeah. Now, always, I mean, it’s always good to do an initial review on those, on those and also, one good thing to another good thing to do is do replicates, because you’ll catch things if your replicates, if one is way off, then there’s a good chance that that was a bad measurement. It’s always good to replicate your measurements if you can, just to ensure you are getting a good representation of what you’re trying to measure.

BRAD NEWBOLD 14:36
What role does sample prep, for example, sieving and packing play in the accuracy of soil moisture release curves?

LEO RIVERA 14:45
Plays a really big role. I mean, it’s it’s everything, because what we’re trying to represent is the soil and its native structure and everything. And we know that structure, density, all of these things that we’re talking about here play a significant role.

SHAUN WELDON 15:00
Yeah. And I think that’s kind of where you make or break your your data is your preparation and your setup, yeah. So even even beyond sample preparation is degassing your water, is making sure that your sensor has good contact with your sample, these things are all going to really make or break that data that you’re that you’re that you’re getting from your, our instruments.

LEO RIVERA 15:23
Yeah, no, I always recommend intact sample samples if possible, because then you’ll you’ll have the native structure from the field. And if you can’t do that, then try to approximate the bulk density as closely as possible. Try not to remove any organics, things like that, because those all play a role. So the better you can represent the field conditions, the better your measurements going to be. Yeah.

BRAD NEWBOLD 15:48
All right. Next question, what insights can be gained by combining SMRC data with continuous field measurements from soil moisture sensors?

LEO RIVERA 15:56
Yeah. I mean, I think this is, you know, where soil moisture release curves are. Are such a powerful tool, especially if you don’t have in situ water potential data is we can use that soil moisture release curve to interpret what the water potential is from that water content data.

SHAUN WELDON 16:14
Yeah, and I think it’s, it’s really important, because this is kind of where that laboratory versus field kind of meets each other. We can, we can have this nice, controlled data from a lab setting, and then we can see how that’s really playing out in a larger field, where there are conditions that are not as ideal to taking measurements as there are as like a lab setting. So we can really kind of see how well that matches up with what we’re seeing in the field. Yep.

BRAD NEWBOLD 16:40
All right, moving on to the next one. How do dual porosity soils, for example, aggregated clays affect the interpretation of soil moisture release curves?

LEO RIVERA 16:52
Well, the way I look at them, it’s not necessarily the interpretation of the curve, but it’s just how they affect the curve in general. And really it’s probably actually how they affect what model you use to fit that curve, because typically with dual porosity soils, we wind up with what’s called a bimodal soil moisture release curve. And like I hit on earlier, sometimes you’ll see on the soil moisture release curve where you have a second air entry point that’s due to these dual porosity soils, it really what it affects is how we try to fit that because certain models can’t fit bimodal soils very well, and so you need to use a proper model for that.

SHAUN WELDON 17:25
Yeah. And luckily, these are fairly easy to see. Like you said, you can see that second air entry point, and when you’re fitting it to models and things like the LABROS soil view software, you can see that it doesn’t have as good a fit, and it’s really easy to then change it from a standard model to a bimodal model right in there.

LEO RIVERA 17:44
Yeah, yeah. But I think it’s super this stuff is super cool because, you know, we some of the things we don’t think about with bimodal soils is actually how it impacts water movement through the soil. One of the good example of this is, is a research project we worked on a while back where we were looking at some soilless media that that the plants were not performing as well in and what we found with that is it was because the soil had was dual porosity, or had this bimodal relationship, and it was actually limiting how water was moving back towards the roots, which was causing the plant distress at water potentials that you typically wouldn’t expect to see. So, so, and we wouldn’t have seen that with traditional methods, yeah. And so this is the things that make me excited for the tools that we have now, is we can really dive into detail of how this soil is behaving.

BRAD NEWBOLD 18:32
All right. Next question, can HYPROP measurements be reliably scaled up from lab cores to field conditions, and if so, how?

SHAUN WELDON 18:40
I’m going to give my favorite answer here and say it kind of depends which nobody likes that answer, but it does kind of depend, and it depends on a couple of things. It depends on how uniform your soil is, so how good of a representation that lab sample is of the surrounding soils. And you’re also going to need to kind of remember that when we’re doing it in the lab, it’s a very constrained, small sample, and when you’re doing it out in the field, there’s other interactions with the lower soil layers, with neighboring soil layers, that’s going to affect the way that that that behaves in a field rather than in a lab sample. So it’s going to give you a great understanding of the basic behavior of that soil but there’s definitely going to be interactions in a field condition that’s going to change how that’s going to behave overall.

LEO RIVERA 19:34
Yeah. And I think the biggest thing is, are you accounting for that heterogeneity, which we know exists in soil science, or in many sciences, there’s a term called representative elementary volume, or REV, and the smaller your sample, the smaller your representative elementary volume. And so what that means is you need more samples to represent that variability that you see in the field. And so it really depends, and like Shaun said on how variable your site conditions are, but and when you see those situations, you want to make sure that you have enough samples collected to try and characterize that variability. Yeah.

BRAD NEWBOLD 20:16
All right. Next question, what quality control checks does METER recommend to ensure the accuracy of HYPROP curve fitting.

LEO RIVERA 20:23
I’ll let Shaun take this one.

SHAUN WELDON 20:25
Yeah, this is a great question. These are things we talk with our customers about all the time. I think the first thing is going to be making sure that you are following all the steps for sample preparation, including that degassing. A good degassing can change your data dramatically. There’s, there’s often, it’s often, really easy to point out ones that have a good degassing and one that have an incomplete degassing. So that’s going to be the first thing. I think the second thing is going to be replicates, like we said earlier. That’s going to be something that’s really going to ensure that accuracy of your of your data. And then I think the the third big thing is just making sure that you’re being really careful looking at if there are volume changes that you’re recording, that you’re putting that in the software, making sure that you’re you’re really accurately putting in the weights and all those things that need to be put in there in that first section, because that will really change how that data looks for you in the future.

LEO RIVERA 21:28
Yeah, yeah, no. And then, you know, once you’ve taken all of those steps to ensure that you have collected good data, I think the last, the last piece, is selecting the right model to fit your data. And you know, oftentimes we’re constrained, oh no, our the model that we’re using uses the traditional Van Genuchten equation or something like that, but we know that that equation doesn’t fit all soils as well. And you can look at the statistics. So in the HYPROP software there is that it gives you all the statistics on how well the curve fits your data. So you can look at, at, at the RMSE values and different things and see how well statistically it fits. But if you have more flexibility, you should try the different models that exist within the software to get better fitting to represent the shape and behavior of the soil, because it does vary soil the soil shape looks like, and not all models can handle that well.

BRAD NEWBOLD 22:26
All right. We’re going to shift our questions from those related to soil moisture release curves on to more hydraulic conductivity specific questions here. All right. This first one is asking, how is the kFS error estimation calculated by the SATURO?

LEO RIVERA 22:43
Yeah, so that one is actually pretty simple one to take. So what we’re doing is looking at the variability of the infiltration rates from the high and the low pressure heads in the SATURO, and looking at how much that variability might cause error in the measurements, we do an error calculation based on that variability. And and then from there, we say, okay, that we try to give that to you as a kind of a confidence value, like, okay, there wasn’t as much error. So our kFS error value is lower, and my kind of rule of thumb is, I always like it to be an order of magnitude smaller than what my hydraulic conductivity value is.

SHAUN WELDON 23:32
Yeah, yeah. And just like your your hydraulic conductivity value, this is only calculated for the last, the last pressure cycle, high and low pressure cycle, because it is we want it to reach that semi steady state. So if somebody is out there trying to, you know, calculate standard error of their of their readings that they get, that’s where we’re actually looking. Is just that last one where it’s reached that semi steady state. Yeah.

BRAD NEWBOLD 23:56
Can the data I receive from sensors and instruments link directly with storm water management calculations for drawdown?

LEO RIVERA 24:05
Yeah, well, I mean, you know, drawdown tells us how effective a storm water management system can infiltrate water to be ready for its next event. And it all comes down to your system design, and some of it is measuring the system. And so that’s where tools like the SATURO, tools like soil moisture sensors and even water potential sensors can help us understand what that’s going to look like. You know, I think of this, you know, first starts with design, designing the system to be able to infiltrate the water for the storm water events we expect to see, and then monitoring the system to see how well that drawdown is occurring and see how well that system is ready for its next next potential stormwater event.

SHAUN WELDON 24:50
Yeah, and I mean, in those calculations, the hydraulic conductivity is one of the key things that they’re looking at so this, this translates directly into those calculations that they’re doing.

LEO RIVERA 25:00
Yeah, yep. So no, I mean, monitor, measure, and build and design your system around that. There’s we’ve, we’ve seen a lot of work done by folks out of like, folks out of Villanova that are doing this stuff, that do really great work in this area. But, yeah, these measurements are super critical for understanding draw down for storm water management systems.

BRAD NEWBOLD 25:19
All right, here’s a fun one. How do the in situ measurements of the SATURO take into account pressure cracks and cracks in soil?

LEO RIVERA 25:28
Yeah, that’s a good question. I’m actually not familiar with what they mean by pressure cracks, but cracks in soil exist, yeah, and that’s just a part of the natural structure of the soil, and that’s what we’re trying to measure.

SHAUN WELDON 25:43
Yeah, I think maybe by pressure cracks, they mean the pores that will open up while it’s under pressure.

LEO RIVERA 25:47
That’s true.

SHAUN WELDON 25:48
Which you which you definitely see in the data of the SATURO. And yeah, that’s, that’s something that you can when you’re looking at both the flux data, yeah, and the pressure data, you can see that reflected really, really easily. So I would say to answer this question, that it takes it into account almost perfectly, because you see that behavior very easily with the different changes in pressure and water and flux rather, because you’ll see exactly as those cracks are filling and gaining and you’ll see as those other pressure cracks open and close as well.

LEO RIVERA 26:21
Yeah, that’s true. Actually, yeah, I’ve definitely seen data where it’s it’s not at a nice steady state. And all of a sudden the hydraulic conductivity, or the infiltration, increases, because if something opens up. Also, it’s cool, as you can see, when you hit limiting layers as well as as the wetting fronts moving through the soil, and then all of a sudden there’s a dramatic decrease in the infiltration, rate. So, yeah, there’s some really cool stuff you can see in that and, and, and kind of helps you try to understand, like, wow, this. There’s a lot about the whole profile interaction that’s going on there.

SHAUN WELDON 26:51
Yeah.

BRAD NEWBOLD 26:51
All right. Next question. Interesting one here too. Can we expect variation in hydraulic conductivity in peat soils?

SHAUN WELDON 27:00
Yes, absolutely. I think we can expect variation in all of our soils, but especially peat soils and heavy organic ones. We’re going to see a lot of variation depending on, you know, compaction, on pore sizes, on interactions with other other elements that are organic in there. Yeah. So we see a lot of variation in those and so for some of those peat soils, especially with things like the SATURO, we do recommend really extending that range to make sure that you get that that steady state that we’re trying to reach. And some of these organic or peat soils do require an extra pressure cycle sometimes just to really hit that steady state.

LEO RIVERA 27:39
Yeah, and PD soils are not any less susceptible to heterogeneity than other soils. So for sure, we can see variation across the site, especially depending on just how those soils form and all of these things. So yeah, it’s something that you have to measure and characterize and try to understand.

BRAD NEWBOLD 27:58
Next question, can you infer KFS, multiple soil horizons from a single run at the surface, if you run it long enough? Or is that a slippery interpretation slope?

LEO RIVERA 28:10
Wow, that was a nice, nice soil science pun, right there. Yes and no is kind of my answer there. You know, your measurements from the surface are always a measurement of how deep the wetting front moves and the most limiting layer. And so really, that’s what you’re characterizing, is the most limiting layer we have seen people in the past use tools like the SATURO to excavate down two different layers. But that’s why we’ve been working on new tools. And one of the things that is coming out soon is our borehole accessory for the SATURO.

SHAUN WELDON 28:51
Yeah, which is, is really exciting for these kind of measurements, especially when you do have very different soil horizons. In some of the testing that we’re doing right now, even where our soil is very homogeneous here, we do see quite a difference as that compaction increases as you go deeper. So it’s neat to see this new tool be able to show us the different KFS values at these different layers, even in a very homogeneous soil.

LEO RIVERA 29:17
Yep, yep. And this is always something that I’ve wanted to see because we knew, we know that one of our goals is to also be able to characterize these different layers. And so that’s why we finally were excited to be able to push on getting a bore hole accessory built for the SATURO, and which will allow us to characterize those layers more easily.

BRAD NEWBOLD 29:36
This next person is asking if they can use the HYPROP for coarse grained green roof material to measure unsaturated hydraulic conductivity. How might the course material impact the results?

LEO RIVERA 29:48
Yeah, that’s a good question. It depends. We say that a lot. It depends. I think you can to an extent, it really depends. On how coarse that material is. I’ve we have run samples that such as crushed brick aggregate, before, and gotten pretty good results. But if the fragments are so large that you don’t have good hydraulic contact with the tensiometers, then you’re not going to get good measurements.

SHAUN WELDON 30:17
Yeah, that’s that’s the key any and it’s usually pretty easily, easy to see if you don’t have good contact, because those tensiometers are going to dry out very, very quickly. But yeah, that’s that contact is going to be the big key to getting a successful run.

LEO RIVERA 30:33
Yeah. And one other tricky part, especially when we’re talking about unsaturated hydraulic conductivity, is the HYPROP does have some limitations in the very wet end for doing unsaturated hydraulic conductivity, because you need enough of a deviation between the upper and lower tensiometer to do to use RC’s law to calculate hydraulic conductivity and coarse grained soils, much like sandy soils, those tensiometers tend to just track with each other. Sorry, yeah, coarse grained green roof material, much like sandy soils, tend to just those tensiometers tend to just track with each other until the very end of the measurement. So you might not have enough resolution there to actually do the unsaturated hydraulic conductivity measurement. So like we said, it depends.

BRAD NEWBOLD 31:16
Next question here, What method is appropriate to get both saturated and unsaturated hydraulic conductivity in a clay soil?

LEO RIVERA 31:25
Well, I mean, there isn’t one method that gets both.

SHAUN WELDON 31:29
Yeah, so and that’s irrespective, or that is regardless of soil type. There’s really not a way. So it’s not just limited to clay soil or it’s any soil. You can’t get both with a single instrument. If you’re looking for field measurements, then doing the SATURO for the the saturated and something like the mini disc infiltrometer for the unsaturated would be great, because you can run that mini disc while the SATURO is going automatically for you for things like laboratory instruments, the KSAT is fantastic for doing saturated hydraulic conductivity, and then running it on the HYPROP, you’ll get not only your soil moisture release curve, but also unsaturated hydraulic conductivity.

LEO RIVERA 32:17
Yep, that’s exactly right. Now, one assumption I often hear with clay soil is that they’re always going to have a really low hydraulic conductivity. But that’s not always true. Clay soils can have really strong structure and infiltrate just as much water as some other soils, depending on how their structure, how well structured they are. So so you know, there’s not necessarily a range limitation on some methods now and really compacted clay soils. It is possible that that some of these instruments, like the SATURO, might not actually you would have to run out measurements for 24 hours to get measurements in some of those soils. And it’s something say that hydraulic conductivity is slower than 10 to the minus seven centimeters per second might be a bit of a limitation there.

BRAD NEWBOLD 32:59
How does vegetation cover or root density influence the SATURO’s kFS measurements?

SHAUN WELDON 33:07
Yeah, this is a great question. It’s something to consider when you’re picking your site for your measurements, especially root density, is going to give you a lot of channels for preferential flow that can artificially inflate those kFS, move measurements as as the water just kind of channels, straight down those root channels. It can, it can make it look a little bit higher than than what the actual soil would be. However, that is going to actually be reflecting the behavior that’s happening with exactly so even though it might not be accurate to what the soil by itself would be. It’s going to be perfectly accurate of what’s happening in that soil system.

LEO RIVERA 33:48
Yeah, and that’s something. Our goal is to measure the system as a whole, not the soil by itself, which is also why I’m often my preference is field measurements for a lot of these things. Because, yeah, we want to understand how that system as a whole is behaving, and those are our key components. And I think another piece that plays a role there is also this seasonal variation that you get in some of that stuff. And so so that something that you may need to characterize that across different parts of the season to see how that is affecting that soil’s ability to infiltrate water.

BRAD NEWBOLD 34:23
Here’s a good general question maybe we should have started out with this one. How does the SATURO’s automated dual head infiltration method improve accuracy compared to the traditional single ring infiltrometers?

LEO RIVERA 34:36
Yeah. Well, I mean, it starts with the dual head method itself, and one of the reasons that we chose to go with that method is it helps limit the potential for human error in a measurement and and one of the biggest sources of error in that measurement is when you go from the infiltration rate to do your final hydraulic conductivity estimate, whether it’s from single ring or double ring infiltrometers, it doesn’t matter. One of our goals is we need to correct for that three dimensional flow that occurs from the infiltrometer, and a part of that is estimating your soil properties. We use the term alpha, this macroscopic capillary length factor that we have to typically estimate depending on the soil type. And the beauty of the dual head method is you don’t have to estimate that value. It you with the dual head method, we can estimate the sorptivity, which allows us to estimate the macroscopic capillary length factor, which allows us to just go through and do the corrections automatically, and so it just helps reduce that potential source of error.

SHAUN WELDON 35:34
Yeah, and that makes it much more reliable reading, and also much easier for us. That’s, that’s one of the nice reasons that you can just leave the SATURO to do its work and kind of walk away and do do other work that you have out there, which is great.

LEO RIVERA 35:48
Yeah, now, and the beauty is, you don’t have to be an expert to make this measurement and get good measurements. Yeah, that was the goal when we were developing this. Is that any practitioner can go out and just deploy this and get good measurements and and I learned the hard way in grad school, when working with double ring systems that you couldn’t just give the system to somebody who didn’t quite know what they were doing and get good measurements, because I definitely saw bad measurements come out of those those tools.

BRAD NEWBOLD 36:15
All right, we’re at the end of our time here. This is going to be our last question, and they are asking, can the SATURO be used effectively in highly permeable soils, like sandy soils, where infiltration rates are very high.

SHAUN WELDON 36:29
Absolutely, to a point, yes, it can. We’ve we’ve done measurements in some incredibly sandy soils when we were doing development, and also with some partners that we’ve worked with that are in very, very sandy soils, and the SATURO does a great job. The measurement range that we have it specked up to, is about 115 centimeters per hour, which is a pretty high infiltration rate. And at that point, it keeps up incredibly well. I will say that if you’re going to be doing measurements in soils like that, have a bucket of water on standby, because it is going to use a lot of water, and in cases like that, I actually prefer using two five gallon buckets as opposed to the collapsible water container, just because it’s easy to pour water into that bucket again, so you’re not having to try and fiddle with that container to try and get it in.

LEO RIVERA 37:23
Yeah, exactly. Yeah no, I think is very good application for tools like the SATURO. The, the one thing I also like with sandy soils is because sometimes they’re not quite as well structured, is to use the 10 centimeter insertion ring, because it just helps, kind of reduce the chance of some of that bypass flow that can occur when you get, we get kind of little blowouts that occur because the soil will actually disperse away from the measurement. So that really helps with that. I mean, and I’ve, I mean, to be honest, I’ve seen higher infiltration rates in in other soils with lots of macro pores than what we typically see in sandy soils, because sandy soils typically aren’t very well structured, yeah. So yeah, I think it’s a great use of that. But like Shaun said, make sure you have enough water for the measurement.

BRAD NEWBOLD 38:07
All right, that’s going to wrap it up for us today. Thank you again, everybody for joining us. We hope that you enjoyed this discussion, and thank you again for all the great questions. Also, if you have any questions we didn’t answer, please contact us via our website at metergroup.com and finally, subscribe to the METER Group YouTube channel and accept notifications to see previous episodes of Office Hours and to get notified when future videos are available. Thanks again. Stay safe and have a great day.