Leo Rivera discusses the latest advances in lysimeter technology and teaches about the advantages of newer instrument design.

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Presenter

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.

Transcript

0:10
Thank you for attending today’s virtual seminar titled, Advances in Lysimeter Technology: Lighting up the Black Box, presented by Leo Rivera.

LEO RIVERA 0:18
Advances in Lysimeter Technology: Lighting up the Black Box. First off, what is the motivation behind lysimetery? Why do we even care? Why do we want to make these measurements? Why do we want to put all this effort in to these really complex measurements? Well, first off, lysimetery tells us a lot about the water balance. It really truly is one of the best ways to measure and estimate the water balance and what makes up the water balance. It’s understanding inputs from precipitation and irrigation, being able to measure evapotranspiration or estimate evapotranspiration, deep drainage, and then the change in storage of water in the soil. That’s what makes up the water balance. Now, being able to accurately measure all of those components together is not an easy task. Another motivation behind lysimetery is understanding the solute balance. So where are things like nitrates going in the soil, or pesticides, or other water soluble compounds? Lysimetery is a really powerful tool to help understand that as well. And so it’s really just, the motivation behind that is to get a well known water balance, and that allows us to have a good tool to also help estimate a solute transport investigation.

LEO RIVERA 1:51
So many of you’ve probably heard this before, but I always like to start out with the history of lysimeters. I think this is some really neat stories, and it’s cool to see where lysimeters have been used in the past, and how they’ve changed to now. One of the earliest known users of lysimeters is a scientist by the name of Philippe de La Hire. He was a mathematician and meteorologist for King Louis the 14th. And he’s commonly referred to as the instigator of the use of lysimeters. And his motivation behind using lysimeters is they wanted to understand where springs came from. And there was a variety of different theories behind this. And what’s funny is some of those theories were there were large crevasses in the ground that were recharged by either ocean water or condensation of moisture in the atmosphere. And so Philippe de La Hire wanted to better understand where these springs were actually coming from. And so in order to do this, he built three cylindrical lysimeters with different lengths to determine the weight that water moving through the soil or that drainage had on the amount of water flowing in the springs. And what he found, well, one of the really interesting facts that he found at the time was that soil without any vegetation on top of it had more water pass through it, than soil with vegetation on top of it. And of course, that was because of the impact that the vegetation had on the transpiration of water from the soil. And so just kind of an interesting fact that they found at the time, just kind of seems like common knowledge to us now, but at the time, it wasn’t.

LEO RIVERA 4:01
Another interesting fact about lysimeters. In 1875, a botanist and agronomist from Massachusetts, by the name of Edward Lewis Sturtevant built the first lysimeter in the United States. And then the first weighing lysimeter built in the US was built in 1937, in Coshocton, Ohio. And there’s an image of that lysimeter here. And this is really not very different from what we do now, besides the fact that we’ve improved the technology that we use to make these measurements. It’s the same principles that we use now in weighing lysimeters. And this weighing lysimeter was able to show us the influence that dew formation had on different crops and also what it had on the water balance and what the crops had on the water balance. And this is one of those first types of early tools that helped us develop crop coefficients that we use in a lot of our ET equations nowadays.

LEO RIVERA 5:05
So now let’s go ahead and go into some different lysimeters techniques, and then we’ll talk about the advances in the technology and where we’re at now and where we need to go. So one of the most basic measurements or basic tools available for measuring deep drainage, so the most basic lysimeter, is a zero tension or a pan lysimeter. It’s the most basic measurement of drainage. And the way you can think of it is a simple collection pan buried in the soil. You can either come in at the side and go below the native soil and push it up in contact with the soil, or many people will just excavate, install these, and then repack the soil above the lysimeter. Now, there are problems with zero tension lysimeters. One of those issues is, water flows from low tension to high tension or from high potential to low potential. So the lower boundary of a pan lysimeter is at zero tension. And typically, unless the soil is saturated, the water is going to be held in the soil at a higher tension than zero. And so water in saturated soil will just typically tend to want to flow around the lysimeter or diverge, and so we call this a divergence issue away from the lysimeter. And so a representative sample of drainage is only collected under saturated conditions. And so here’s just kind of a simple example of that. Here we have a pan lysimeter at 0 kPa. And say we have the soil at 30 kPa— or water being held in the soil at 30 kPa. Because of the issue with water wanting to flow only from low tension to high tension, it’s just going to go around the lysimeter. So like I said, there’s issues with pan lysimeters or zero tension lysimeters. You can mitigate the flow divergence problems somewhat. Some approaches people will take to mitigate this problem are a larger measurement footprint, and when I say larger, I’m talking about several meters squared area. And there’s a good picture of that here in the bottom left hand corner of the slide. And this is a large pan lysimeter that’s being used for measuring the water balance of an alternative cover for landfills. Another way that this can be mitigated is by installing vertical walls around the lysimeter that go all the way to the surface — preferably all the way to the surface — to essentially trap the water into the lysimeter area. So the only way it can go is down, and it can’t go around the lysimeter. But again, because we have this zero tension lower boundary, you actually change the way the soil is actually holding on to the water. So you actually change the water holding capacity of the soil. And so even with all of these mitigation techniques, collection efficiencies are still less than 10%, so not really the best tool for measuring deep drainage and trying to better understand the water balance.

LEO RIVERA 8:32
So the next step up from that, and the next, I’d say, advancement in the technology, would be the static tension lysimeter. And with the static tension lysimeter, you can either have a vacuum pump, or a wick used to create a static tension. And this has helped really a lot with the divergence issue. But because it’s a static tension, there is also the possibility of flow convergence now. So essentially, if the water is closer to say saturated conditions, and we have a static tension that’s maybe around 15 kPa, and the water in the soil is actually being held at 5 kPa, then we actually will get flow convergence into the lysimeter, and so we’re actually overestimating deep drainage. But here’s an example of a wick type lysimeter. And so they’ll choose the length of wick to kind of set that tension, and then the water will drain into some type of measuring device, whether it be a tipping bucket or if you’re collecting and storing the water and measuring the actual volume of water, that’s another way that the measurement can be made. And then of course it’s measured with a data logger at the surface.

LEO RIVERA 9:55
So here’s some examples talking about the tensions we apply and how they can affect the water flow in the soil. So here’s an optimal condition. Say the water is being held in the soil at the area of the lysimeter around 40 kPa. And the tension we apply to the lysimeter is 45 kPa. Again, we have apply a slightly higher tension to actually pull the water out from the soil or to make it move down. In this condition, we would probably have close to 100% collection efficiencies. And this would be the ideal situation. Now, again, if you go back to the same example, with the lysimeter having the static tension to 45 kPa, and the water in the soil, or the water conditions in the soil are that where the water is being held at the area of the lysimeter around 10 kPa, then we are now getting flow convergence into the lysimeter and we’re overestimating deep drainage. So that’s something that you have to consider when selecting the tensions that you apply to the lysimeter.

LEO RIVERA 11:01
So one of those tools that we commonly use now, we call them a passive capillary lysimeter. And with a passive capillary lysimeter, so we have the wick that we’ve optimized— that we’ve chosen the length to apply a hanging water column to pull the tension on the soil water. And the static tension that we chose is chosen to optimize the water collection efficiency. And then we also add what we call the divergence control tube to the top of the lysimeter to minimize the divergence and convergence from the soil. And so essentially what the divergence control tube is, is just a cylinder, either stainless steel or plastic PVC, something along those lines, has the soil encapsulated in it. And what what it’s actually doing is it’s extending the tension that the wick is applying up to the top of the lysimeter. So an example if you had a 60 centimeter wick and a 60 centimeter divergence control tube, the total tension that you’re applying is around 11 kPa. And so here’s kind of just a breakdown of that with a wick in contact with the soil and the divergence control tube. And so here’s some examples of the effects that the length of the diversion control tube has on the collection efficiencies of the lysimeter. And these are data that was collected by Glendon Gee and published in the paper that’s cited below. So in this example, we have a sandy soil, and if we have just a 20 centimeter divergence control tube, we get close to 100% collection efficiencies at all of these different examples of deep drainage rates, so all the way from one millimeter per year up to 10,000 millimeters per year. Now, so if we go up to a 60 centimeter divergence control tube length, then we are getting 100% collection efficiencies across all of these different flow rates.

LEO RIVERA 13:22
Now, if you were to do the same example with a clay type soil, now we run into the issue where we get good collection efficiencies in some of these flux rates, so mostly in the higher flux rates, the 1000 millimeters per year and the 10,000 millimeters per year. But we don’t get very good collection efficiencies at the low flux rates, so in the drier conditions, you could say. Now this is an example of an unstructured clay. So if you are working in a structured clay that has good macropores and those types of things, then that’s a different condition and you actually would probably get higher collection efficiencies. But this is just an example of an unstructured clay. And so that just kind of goes to talk about some of the limitations with passive capillary lysimeters. It’s not going to work on all soil types. They’re really optimally made for sandy soils or well structured soils. So that’s just something you have to remember.

LEO RIVERA 14:26
Some other improvements that we’ve made to passive capillary lysimeters is one, eliminating moving parts. Moving parts have the tendency to clog or stop working, especially when they’re put underground and you can have soil particles moving through. So that’s something that we really had to get rid of to improve the system. So one of the first tools that we had available to help improve this is what we call the dosing siphon. And so that worked really well because essentially you just have a tube designed where when the water level gets to a certain level in the chamber, the pressure actually forces it to drain out, so without any moving parts. And then we just use a capacitance type sensor to measure the water level inside of the chamber and determine when it’s tipped. And so that was a really great change in the way the measurements were made and helped eliminate a lot of the issues. Now, there are still issues present with say flooding due to saturated conditions, if you had a seasonably high water table. And also the sensor, we had limited access to the sensor. And so, if we did have flooding, it could cause a sensor to not work for a period of time. And in some cases, it could cause a sensor to fail. So we knew there was an issue with that. So let’s find another technique, another way to measure this to help improve those issues. So the way we did this is we changed to a sealed system with a center access port on the side. And so here’s an example of that. And this helped eliminate the issue with the flooding due to seasonably high water tables. And because the sensor was accessible from the surface, it was easy to do maintenance on the sensor or replace the sensor if it ever does fail. And so it really helped with a lot of the issues that we had in the past with even the dosing siphon. And what’s really nice about the passive capillary lysimeters is with proper design of the installation, you can resume normal tillage practices over the lysimeter, which you can’t do with all types of lysimeters, especially the laying lysimeters that I will talk about shortly.

LEO RIVERA 16:53
So the next step up from the passive capillary lysimeter would be the control tension lysimeter. And so you can kind of think of it as a similar construction to the pan lysimeter, where we have in this case, say a ceramic plate installed in the soil at a certain depth. But the one difference from there — the main important difference — is that the tension in the lysimeter is actively controlled based on the soil water tension. So we have a tensiometer measuring the actual soil water tension and how it changes over time, and we use a vacuum system to actively control the vacuum that’s applied to the lysimeter to match how conditions are changing in the soil. And so this really is the most accurate method for measuring deep drainage because we’re constantly matching the changes in the field dynamics and so it becomes a really powerful tool for measuring deep drainage. Now, one of the issues in the past with this is there was really no system ready to go that made this easy to set up. Typically it required logger programming and so it took some expertise and time to get this set up. They weren’t very inexpensive and oftentimes were very power hungry. So those were some of the limitations. But again, like we said, this is probably one of the most accurate drainage measurement methods. But it did have its drawbacks with it being expensive and fairly complex. Now there are new turnkey systems that make this actually easier to implement and set up. There are vacuum systems set up, ready to go where they will automatically read the tensiometer and actively change the vacuum level based on the user settings to match the changes in the water potential of the soil, and so these new systems actually make it easier to implement a control tension lysimeters.

LEO RIVERA 19:05
Now, the next step up from that would be a weighing lysimeter. So, with a weighing lysimeter, one of the advantages of a weighing lysimeter is we can get a large surface area measurement. And so in this picture here, this is a station in, I believe, Spain, where they have multiple large weighing lysimeters set up at this station to do different studies of crops and how they can affect the water balance and do drought stress studies and a variety of different studies because of the things that weighing lysimeters allow us to do. It can be deep enough to encompass the root zone. You can make them as deep as a one and a half meters. I’ve seen some weighing lysimeters as deep as three meters, so you can have some different flexibility there that you can play with to really encompass the root zone that you’re trying to work with or the soil that you’re trying to work with. With the weighing lysimeter, you use a precision load cell that is continuously weighing the lysimeter. So not only are you getting the deep drainage measurement, which is an important component and a hard to measure component of the water balance, but now because we’re able to continuously weigh the soil weight, or the lysimeter weight, we are now able to actually measure storage of water in the soil. And so that’s another important component of the water balance that we’re able to measure with weighing lysimeters. And with the weighing lysimeters we’ll do the same techniques that we use with the control tension lysimeters. The lower boundary of the lysimeter is controlled with an active suction system that is changing the suction based on the changes in the native soil.

LEO RIVERA 20:59
So an important part about lysimeters and especially with these weighing lysimeters is our goal is to match field water dynamics. So in this example, we have a weighing lysimeter with a tensiometer inside of the lysimeter itself, and a tensiometer in the native soil next to it. And so our goal is to match how water is going to be moving through the soil to get an accurate understanding of what’s actually happening, or how the crops would actually interact with the actual soil and not just the soil inside of our lysimeters. And so say for an example, we have water coming in through the soil, say a precipitation event or an irrigation event, the lysimeter is actively controlled to what’s happening in the native soil. And so we get a good accurate estimate of how the water is actually moving through the soil. Now, say for example, we’re actually running into a different setting where we have drier conditions out, we haven’t had a whole lot of precipitation, and actually we’re getting capillary rise of water, say from the groundwater, and the crops are actually able to pull that up in the native soil. So we’re actually getting this rise of water. Now, in the lysimeter, the lower boundary is closed off from the groundwater and any potential upward rise of water. So we could run into issues with lysimeters, where the soil in the lysimeter is actually drier than the native soil because we weren’t able to actually pull the groundwater up. So in this case, we now use a bidirectional pump, which would actually pump water back into the lysimeter to continuously maintain the same water potential at the lower boundary as the water potential in the native soil at that same depth. And so now we’re actually able to accurately estimate the actual capillary rise of water, and really just gives us a better estimate of the actual evapotranspiration of the crop in the native soil. And so this is a really powerful addition to weighing lysimeters.

LEO RIVERA 23:19
So the next thing or the next issue that we ran into with lysimeters was the disconnect with the actual temperature dynamics of the soil. And so in an ideal setup, we want the temperature dynamics in the lysimeter to match what is actually happening in the field. This is especially important if you’re doing studies on changes in different nutrients or other compounds in the soil. And really just I mean the temperature dynamics of the soil affect a lot of things. So it’s important to try and match what’s actually happening in the soil. And in the past, we weren’t able to do this. In the past, we’ve had lysimeters installed in large rooms that were disconnected from the soil. And so there was no influence from the actual temperature dynamics in the native soil in this small column. And another issue could be, say, for example, you have these lysimeters in a large room, that room has a doorway that’s open to the atmosphere. And if somebody opens that door, especially in a hot climate, say in a desert climate, where normally the soil at that depth is still fairly cool. But if you open the door and you get a huge movement of the heat from the outside into that room, you actually begin heating the soil. And so you’re really definitely at that point in time not accurately matching the temperature dynamics in the native soil. So to change that what people have done, like UMS out of Germany, they’ve moved to porous concrete well rings, so the soil is completely isolated from the access well, except for a small tube that runs the cables and tubes to the loggers and control systems. And because we have porous concrete wells, they actually allow for what’s called evaporation enthalpy between the native soil and the lysimeter to help create a thermal equilibrium. And so this has really helped better match the field temperature dynamics.

LEO RIVERA 25:35
And here’s some example data of measurements from inside of one of these lysimeters and the native soil. So if we were to look at the 35 centimeter measurement, you can see that the measurement changes from the lysimeter and the native soil are almost spot on. So very, very close matching of the temperature dynamics. And as we go down to the 60 centimeter depth, we’re still very close to the actual temperature dynamics in the native soil inside of the lysimeters. Go down to the 90 centimeter depth. Now we’re starting to get a little bit of divergence away from the native soil and the lysimeter, but we’re still pretty close. And so that’s a lot better than the way it used to be in the past. And then at the 180 centimeter depth, still fairly close, at some points and times we’re off by about two degrees C. I’m not exactly sure why they were off at those times, and not off other times, but still fairly closely matching the temperature dynamics inside of the native soil.

LEO RIVERA 26:55
So the good and the bad with weighing lysimeters. Weighing lysimeters are the best possible quantification of the hydrologic cycle. We’re able to accurately measure pretty much everything that’s happening with the water in the soil, except for maybe the disconnect between lateral flow. But that’s another issue for another day. And this is really helpful if you’re doing climate change studies, ecohydrology studies, contaminant transport study, or even if you’re just trying to estimate crop coefficients. Really, this is the best tool available for doing them. The drawbacks, they’re very, the installation of these large weighing lysimeters takes large equipment, takes time, it’s not the safest thing. There’s so there’s a lot that goes into installing these. And they’re also maintenance intensive, and they’re very expensive, as well. So in order to get these types of measurements, it’s typically taken a lot of time and money. But now we have things available like small scale weighable lysimeters. And with these small scale weighable lysimeters, we’ve been able to take the same technology that’s used in the larger lysimeters, scale it down, and what this has done is allowed for easier installation and lower cost tools that we can use to get the same quality of measurements from the large weighing lysimeters in these smaller packages. Now they have their limitations as well because of the smaller footprint. But they do help us get closer to being able to more feasibly, with especially the way budgets are limited now, to a more feasible approach for accurately measuring the water balance.

LEO RIVERA 28:52
So one of these tools available is the Smart Field Lysimeter. This was a tool developed by UMS. And with the Smart Field Lysimeter they have different soil column heights available for different applications, depending on your root zone, how deep your soil actually is. And I’ll talk about an example where we’re working with very shallow soils and so the shallow lysimeter was actually a very useful tool. Also, with these Smart Field Lysimeters you have a solar powered base station that essentially allows you to do remote installations. And now again, you’re still gonna have to come out and check on the drainage tanks and a few other things. But these solar powered base stations make it where we can power the entire system without needing to bring in power from the main line or anything like that. We have the same precise lower boundary control with the Smart Field Lysimeters as we do with the large scale lysimeters. And here’s an example of the tool that we use with the Smart Field Lysimeters. And so what you’ll have is this plate at the bottom of the lysimeter. And this would actually be filled with silica flour. And it’s the silica flour that’s been chosen to optimize the air entry point to get a good range of actual suction capabilities. And in this basin, we have these three suction cups that are applying the suction and controlling the lower boundary. And then what’s nice is they’ve actually implemented what’s called a virtual tensiometer with one of these suction cups, where there’s an additional tube running out to a pressure transducer. And so in the same basin, we’re actually able to measure the tension and control the tension at the lower boundary. And so this is a really nice tool, kind of eliminates the need to install another sensor inside of the lysimeter. And with these Smart Field Lysimeters, you have fairly easy excavation of an intact monolith with hand tools. And so this eliminates the need in most cases, unless you’re working with the larger the taller lysimeters, it eliminates the need for equipment, you know tractors or large equipment to actually be able to do the work. This can mostly all be done by hand. And because of the smaller package, you have the easier handling compared to the large lysimeters, except in the case of the tall lysimeters, so an example like this 90 centimeter tall lysimeter. You can do the extraction of the monolith by hand. But you’re going to want to use some type of equipment to actually raise the monolith out of the because it is just too heavy for a person or even two people to pick up out of the soil. So it’s a much safer approach to use tools and equipment to actually pull this out of the soil. The installation can easily be done by hand. Here’s an example of a station that’s being installed here in the Palouse. They’re installing three lysimeters. Almost all of the work was done by hand except for pulling the lysimeters and lowering the lysimeters into their access wells. And what’s really nice is, you know, after our excavation, we tried to minimize the disconnect between the native soil and the lysimeter. And so when everything is complete, the main disconnect between the surface of the lysimeter and the surface of the native soil is just a small rim that’s used to keep water and soil particles out of the access well, and also to allow the soil column to sit free so you can get an accurate weight measurement.

LEO RIVERA 32:52
So now let’s go on to some really cool applications of lysimeters. And one of my favorite applications is this Toreno-Soilcan project that’s been done in Germany. And with this project, they took 126 lysimeters. And these are the large scale weighing lysimeters. And they extracted these lysimeters at 12 different sites. And then what they did is they actually moved some of those soil columns to different parts of Germany, or to different parts of the region to try and simulate the effect that climate change is actually going to have on the soil. And the main objective of this project was to characterize and quantify the effect of climate change on the carbon nitrogen cycle and the carbon and nitrogen storage, the biosphere atmosphere exchange of greenhouse gases. And one of the cool tools that they use for this was a large robotic chamber that moved across lysimeters to measure greenhouse gas exchanges. They also were looking at vegetation and microbial biodiversity, and the temporal dynamics of carbon and nitrogen. And also the need that we want to understand the effect on the hydrology, so water budget, how will seepage water change, and also how well the water retention capacity of the soil change at these different climates? And here’s an example of the layout that they used at most of their sites, except for the site where they had the robotic chamber that moved across the lysimeters. They used the six hexagon layout. And depending on how many lysimeters they had at the different sites, they would have, you know, up to three or four of these stations with six lysimeters around it. And everything ran to one main axis well, where they had the loggers and the pumps controlling the lower boundary and everything. And so this is a really cool example of what you can do with lysimeters, and how this technology can be applied.

LEO RIVERA 35:05
Another example is an example of using the actual, in this case, a small scale weighable lysimeters to better understand the effect of climate change in the Alps, and this is primarily specifically the Alps of northern Italy and Austria. And this area is an important resource to the local economies. So it really is vital that they understand the effects that climate change may have on these areas. And so in this case, they used the Smart Field Lysimeters to quantify potential changes on deep drainage, and even vegetation impacts. And so it was really important that we had minimal disturbance at the site. And also because we were working in the Alps, access to these sites was pretty difficult. And so in this case, the large type lysimeters didn’t really fit in well, especially because the soil wasn’t very deep either. The soil typically ranged from around 40 to 50 centimeters deep. So a big lysimeter would really not fit in well here. So that’s where the shallow Smart Field Lysimeters came in, and where they actually served as a good tool for this area. And so they were able to install multiple of these small lysimeters at different sites in Northern Italy and Austria, without any large equipment, and most of the stuff could be hiked in because of the limited access. So that was a nice advantage of using the small scale lysimeters here.

LEO RIVERA 36:49
And then, one last example I want to talk about, and this is the more recent example for me, and one that I’m more heavily involved in, is a station that’s being used to better understand the water balance in the Palouse, and also better understand the issues with nitrate leaching in the Palouse. And so this is a joint project with the University of Idaho researchers. The PI on this project is Aaron Brooks. And in this case, we’re using three of the Smart Field Lysimeters to look at nitrate leaching and quantify the water balance of the different crops that are grown here in the Palouse, so primarily spring and winter wheat and garbanzo beans. And so this is a really just fun project that we’re working on here to really kind of go on to a bigger project that is dealing with big issues that we have here and that’s nitrate leaching. And that’s a common issue that is found all over the US and lysimeters really provide a powerful tool to understand nitrate leaching and measure and estimate nitrate leaching. So I’ll open it up to questions.