Hydrology 101: The Science Behind the SATURO Infiltrometer

Dr. Gaylon S. Campbell teaches the basics of hydraulic conductivity and the science behind the SATURO automated dual head infiltrometer.

Master the basics

Dr. Gaylon S. Campbell teaches the basics of hydraulic conductivity and the science behind the SATURO automated dual-head infiltrometer. In this 30-minute webinar learn:

  • What is hydraulic conductivity?
  • Porous mediums
  • What determines hydraulic conductivity
  • Why you should care about hydraulic conductivity
  • How is hydraulic conductivity measured?
  • Lab instruments
  • Field instruments
  • The method behind SATURO: dual-head infiltrometer
  • Comparison: Double-ring and SATURO dual-head methods

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Our scientists have decades of experience helping researchers and growers measure the soil-plant-atmosphere continuum.


Dr. Gaylon S. Campbell has been a research scientist and engineer at METER for 19 years following nearly 30 years on faculty at Washington State University.  Dr. Campbell’s first experience with environmental measurement came in the lab of Sterling Taylor at Utah State University making water potential measurements to understand plant water status.  Dr. Campbell is one of the world’s foremost authorities on physical measurements in the soil-plant-atmosphere continuum.  His book written with Dr. John Norman on Environmental Biophysics provides a critical foundation for anyone interested in understanding the physics of the natural world.   Dr. Campbell has written three books, over 100 refereed journal articles and book chapters, and has several patents.


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Welcome to our discussion on the dual head infiltrometer. Let me take just a minute before we get into that to introduce METER. METER has several locations throughout the world but the main ones are Pullman, Washington and Munich, Germany. Both of those locations, we do engineering and manufacturing. You have about 200 employees in Pullman, about 35 in Munich. And there are two main subdivisions in the company METER Food and METER Environment. METER Food focuses mainly on instrumentation for food measurements. METER Environment does, both above ground and below ground instrumentation for measuring environmental variables. In METER Environment, I’ve just shown a number of the instruments that we build. On the left is a particle size— automated particle size analyzer. Next thing over with the three prongs is the soil moisture sensor. Next one over is a matric potential or soil suction sensor. The larger thing with the blue ring around it is a mini weather station that measures 12 variables. Next over with the black top is a data logger, solar power data logger that has cell connections. And finally, on the right is our dewpoint potentiometer that measures soil suction over the whole range. So that gives you a feeling, those aren’t all of the instruments, but a few of them that we do.

The one that we want to talk about today specifically is the automated dual head infiltrometer. Measures field saturated hydraulic conductivity. And we just have it shown schematically here, a device that sits on the soil surface, another device that controls it, and a source of water. Start out just by talking about what hydraulic conductivity is. It’s a measure of the ability of a porous medium to transmit water. And to get a bit of a feeling for what that means, if we let I indicate the water flux, the amount of water per unit area per unit time, we can say that that’s equal to k, a hydraulic conductivity multiplied by the gradient in head, dhdz. The head gradient is the force causing water to move in soil and K is the proportionality factor between that driving force and the flux of water in the soil. We can expand the head out into its two main components. hm is the the matric head, and hg is the gravitational head. And so we have matric forces causing water to move through soil and gravitational forces. Now the matrix— or the gravitational gradient dhg dz is just equal to one. And initially as we apply water the soil matric forces are pretty strong and draw water into the soil pretty rapidly. But if we let infiltration occur for a long, long time, to where the soil is pretty wet that gradient in matric head goes to zero. And so, at long times the infiltration rate is roughly equal to the hydraulic conductivity and that can give you some feeling for actually what the hydraulic conductivity means if we were to apply water for a long time that the rate at which the water would infiltrate into soil would be about equal to the hydraulic conductivity.

Hydraulic soil can be either saturated or unsaturated. And so, the hydraulic conductivity can be either saturated hydraulic conductivity or the unsaturated. In this graph, the vertical line vertical axis is at zero head, and values to the right of that indicate saturated conductivity values. Values to the left indicate unsaturated values. And if we take the line on the right in the center, that would be maybe a value that you might find for sandy soil. Now, one thing to point out is that that vertical axis is a logarithmic axis. Think of differences in that as order of magnitude differences, factors of 10, not factors of one or two. And so, if we look at say a poorly structured clay soil, the lower line it would have a saturated conductivity quite a bit lower than sandy soil would. The other hand if that clay soil had good structure, if they were aggregates and large pores between those aggregates, then it’s saturated hydraulic conductivity could be somewhat higher than the conductivity even in the sand. Now, if we go to the left side of the graph, where the head is negative, the water potential is negative, the soil starts to desaturate. The pores empty. And as the pores empty, especially as the large pores empty, the hydraulic conductivity decreases dramatically. And especially where there are large pores present. And so, the unsaturated conductivity is always less than and in most cases, much, much less—orders of magnitude less—than it is when the soil is saturated.

So, what determines the hydraulic conductivity of soil? We’ve just talked about two of those factors, the soil texture—coarse textured soils have higher hydraulic conductivity than fine texture soils typically—and the soil structure—structured soil typically will have large pores present in it and structureless soil smaller pores, bio pores, root channels or or animal burrows can have a big effect on hydraulic saturated hydraulic conductivity, if they’re filled with water, unless they come all the way to the surface so that they can fill with water then they can actually decrease conductivity rather than increase it. And compaction or the density of the soil certainly will have a big effect on hydraulic conductivity. And then as we’ve mentioned, the water content or the water potential of the soil is important. Now why do we care about hydraulic conductivity? Well, it impacts almost everything that soils are used for and so, crop production has to do with how the water gets into the soil, how the water gets out of the soil, how the water flows to the roots of a crop, so has an important impacts there. Irrigation and drainage related to how water gets into and out of soil. Hydrology, both urban and hydrology of native lands in the water movement in soil, water moving into and out of soil. Landfill performance, we design landfills to minimize the water movement out of the landfill, to minimize maybe or capture water before it flows in. We need to know the hydraulic conductivity to do those calculations. When we’re designing stormwater systems, infiltration is an important thing. And even in applications like soil health, that a soil has poor hydraulic properties is not a healthy soil.

Let’s talk a bit about methods for measuring hydraulic conductivity. Mentioned just quickly a couple of laboratory methods that are useful. These are used pretty often for characterizing soil hydraulic properties, but they require that we go to the field, take a soil core, try to take that in such a way that we disturb it minimally, and then bring that core back to the laboratory to make the measurement. METER has a device that will measure the saturated conductivity of that core. Inside the blue part of this apparatus, you put a soil core that you’ve brought in from the field, you fill up the tube with water, you establish steady flow of water through that soil core, and you measure the rate of flow, and from that calculate the saturated conductivity. Another device that also is something that we build and sell here is called the HYPROP. It measures the unsaturated hydraulic properties of a sample of soil. Again, we go to the field, we get a soil core, we bring it back to the laboratory and put it on this device. The device has two tensiometers inside it to measure the soil suction. The water evaporates from the top of the soil column. To establish a gradient within the column, we measure the rate of water loss from the column using the balance. And then from the rate of water loss and measurements of tension, we’re able to determine both the moisture release curve and an unsaturated hydraulic conductivity function for the soil. Very few methods exist for getting that unsaturated conductivity function, but this does it nicely.

Now I’ve just put one of those curves that comes out of that here. It’s the moisture release curve for a soil, a blue silt loam in this case. The volumetric water content is plotted on the vertical axis and the logarithm, or the water potential or suction is plotted on the horizontal axis as log scale. Now I don’t have a graph here that shows the unsaturated conductivity function but we get similar detail to this with that function. Now if we want to go to the field to make a measurement of hydraulic conductivity and call that the field saturated hydraulic conductivity, we have several options. One option is this single ring infiltrometer. It’s just a single cylinder that we drive into the soil, it can have various diameters, maybe from 10 to 50 centimeters or sometimes even bigger than that. We can either maintain a constant water head in this and measure the water flow into it or we can do it as a falling head method, let the head decrease as water infiltrates and measure that. It’s pretty simple—there are some publications even that the call this the beer can method.

The water that flows out doesn’t flow directly downward though because there are matric forces, so flows both downward and laterally. And in an effort to try to deal with that and make the flow more one dimensional, people have used a double ring infiltrometer as shown here. So there are two rings, the inner one is just like the one we just described. The idea of the outer one is that we maintain a head in that, the same as the head on the inner one, and that that outer one is supposed to make the flow fro, the inner infiltrometer one dimensional so it just goes down. I’m skeptical of that, but at least that’s the idea of it. So one of our researchers here at METER did his master’s degree at Texas A&M University, measuring hydraulic conductivity in a fairly large area that had been subjected to different kinds of land use. And the idea was to try to quantify the differences in hydraulic conductivity that resulted from the differences in land use. And the photograph here shows you the equipment that he had to set up to do that, you can see three of the double ring infiltrometers that he used here, and three of the setups that he used to try to maintain a constant head. You can see that there’s quite a bit more equipment that goes along with this, a pickup with a flatbed trailer and large tank on it for hauling water, you can see where he’s had to have some lawn chairs and some shades set up so that there’s a place for him to get out of the sun while taking the measurements, and even a cooler sitting out there with— not sure what’s inside it. But anyway, a lot of equipment has to go along with this set of measurements. He made over 200 measurements of saturated conductivity. And measurement times took close to two hours, a lot of times, and for that experiment, something over 2000 liters of water. So you can see this is not a undertaking for the faint hearted. He was able to show the things that he wanted to show with all that work, though.

Well, we could think of this whole— think through the work that Leo had to go to to accomplish this and think maybe there are some other ways of doing it that might work better. Some of the observations that would have come out of this is that hydraulic conductivity is spatially and temporally variable, way more variable than we’d like it to be. It’s hard to get enough measurements to really establish differences and to be able to model the kinds of things that go on in the field. The double ring infiltrometer takes a lot of time and a lot of water. It might occur to us when we were hauling all of that water, that it might be possible to use mathematics to eliminate the need for that second ring. There probably was a time when we were limited enough in our mathematical capabilities that we would have needed to stick with just one dimension. But those times are long past. And there’s no reason why we can’t do, can’t use mathematics to model flow from a single ring. It’s hard to measure and control infiltration in head. It takes a lot of work a lot of time to do it. Why not just do that electronically? And then finally, sorptivity is the flow that when you initially start an infiltration experiment, the water flows a lot faster than it does after a period of time and that’s because the matric forces are drawing the water into the soil rapidly. And that has to be taken into account in order to get accurate measurements. Why don’t we automate that too?

And so, the goals in designing the dual head infiltrometer were to fully automate the measurement and control to reduce the water requirements to make it portable—portable meaning not needing a pickup in a flatbed trailer necessarily. We still needed to have a wide range of hydraulic conductivities that we can measure. We wanted to be able to characterize soil that would be used in lagoons, manure lagoons, and other places like that, as well as soil that was well structured in a cultivated field. And we wanted to be able to automatically calculate the field saturated conductivity. Now, the equations that apply to this, we can start with the conductivity being equal to the water flux i divided by a function F, capital F. And that function takes into account both the sorptivity and the three dimensional nature of the flow out of a single ring infiltrometer. And we won’t go into the details of that, those are given in a paper by John Nimmo, published in 2009. But we can see that the equation that he came up with here, that F was one plus set of variables. And so if that set of variables of lambda plus d, go to zero, why then f goes to one and we get that equation we presented in the earlier slide, in the third slide, that the conductivity is equal to the infiltration rate. But those other factors take into account the three dimensional nature. Lambda is a characteristic length of the porous medium. And we don’t know what that length is necessarily, and so, will be nice if we can eliminate that from our measurements. D is the depth of ponding the water on the soil surface, and then the small d and small b factors have to do with the insertion depth and the radius of the infiltrometer. And we combine those into the factor delta that is the characteristic of the infiltrometer that we build.

So if we were to make measurements at two ponding depths, we can write two equations in two unknowns, and from those two equations, we can eliminate the lambda factor, which is the fact that the characteristic length of our porous medium, and so we get the hydraulic conductivity in terms of the two ponding depths that we have, and the characteristics of the infiltrometer and the infiltration rates that we measure. So, to give you an idea of how that analysis is done, I have on the left here the pressures that we establish, the two different pressures, and we do this— in the past that’s been done by filling the ring to a greater depth or a lesser depth, but what we do is apply air pressure so that we can go quickly from one to the other and so that we can do that automatically. And then on the right is the flux. And you can see the two infiltration values. We supply the water, control the water at a constant level, then supply water with a pump that can measure that infiltration rate. You can see the two depths that we have, and so we combine those to get the hydraulic conductivity.

So we control the water level electronically, there’s a level gauge inside that monitors that. We have a stepper motor that we can accurately measure and control the infiltration rate to maintain that level of water. Then we adjust the air pressure inside the chamber to get the two different heads that we use for infiltration to get the dual head calculation. We call this device the SATURO and it provides an automated field saturated conductivity measurement. The measurement is done automatically. It’s just set up and left in place, come back and get the result. We vastly reduce the water requirements by both the algorithms that we use and the fact that we use a single ring infiltrometer. It’s portable because the reduced water requirements and we can measure a broad range of infiltration rates and then just gives out the number that you want, the saturated conductivity. So got a couple of pictures here showing the installation process. Starts out with the ring that’s pounded into the ground, you can see that on the left, and on the right is the ring installed and ready to go. Then the cap that’s put on the top of the ring and CO2, there’s an O ring so that we can pressurize that chamber on the top. And so the water level of about 10 centimeters can be established inside that. Tubes are hooked up to the control unit that’s on the right. We set up the program that we want to run in the controller, then hook up the water supply, that’s just a collapsible plastic bottle, and you go off and leave it. And it’s there for a period of time, an hour or two, and makes the measurement that you want. You come back and it’s all finished. You can see here the flux measurement over time, can see that due to the sorptivity, the infiltration rate starts high and decreases over time and then you can see the steps in that as the pressure is applied and infiltration rate goes up, then the pressure goes off and the infiltration goes down. And so we have the are two levels of infiltration or two infiltration rates. And so here again, you can see the two values that it gets.

So we did a little experiment just to compare the double ring infiltrometer with the the dual head infiltrometer, the SATURO. And you can see the things that were used here. We did this experiment on a— we have a soccer field out behind our METER building. And so on the right here you can see several locations where we made these measurements in the soccer field. And we compared the two methods at those locations. You can see here the results, the double ring results that we got, and the SATURO results. And then, pretty good agreement between the two. I mean, if you’re not used to looking at saturated conductivity, field saturated conductivity data, these data may look a little bit noisy, but if you are used to that you’ll be amazed at how good those look. Except for a couple of readings, you can see those were both on the bottom end of the field up against where the ground slopes up rapidly. And in those the SATURO gave larger numbers than the double ring infiltrometer. Now, if we plot those we can— comparing to a one to one line except for those two, the comparison is a good one. So in general, we think that the double ring and the SATURO, our dual method, compared well except in those cases where flow was obviously dominated by large macropores. We think probably in the installation of the double ring that that causes a lot more disturbance and that the macropores there were damaged as we did the installation. So hope that gives you a feeling for how the SATURO works, how the dual head infiltrometer works and gives you a feeling for opportunities where that can be applied to hydraulic conductivity measurements. Thank you.

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