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Posts from the ‘Agriculture’ Category

Aug 7

New Medium Scale Soil Moisture Measurement Technique

Between dielectric soil moisture sensors with a volume of influence measured in liters and remote sensing systems which measure soil moisture on the scale of kilometers, there is a gap—a gap Dr. Larry Murdoch of Clemson University has been working to fill. In this post, read about the DELTA (Displacement Extensometer for Lysimetric Terrain Analysis), an instrument that measures water content measurements over an area with a 25 m radius.

Close up picture of cracked and dried soil

Dr. Murdoch became interested in how much water content was in the vadose zone (the unsaturated soil above the water table). He wondered if he could use a strain measuring technique to quantify it.

A New Idea:

Dr. Murdoch was a graduate student in structural geology and geomechanics in the mid-1980s, working on the mechanics of hydraulic fractures in soil.  He developed techniques for environmental “fracking” to clean up contaminated soil, long before the recent applications by the oil industry that have caused fracking to become a household word.  Fracking causes movements in soil, and Dr. Murdoch developed methods for measuring those movements in order to monitor fracture displacement. This led to work on sensitive borehole extensometers that could measure small strains in rock during well testing.

In the course of his hydrology work, Dr. Murdoch became interested in how much water content was in the vadose zone (the unsaturated soil above the water table). He wondered if he could use the strain measuring technique to quantify it.  He decided to bore a hole into the vadose zone and insert a simplified extensometer device that could measure the strain as the soil expands and contracts.  This would allow him to gauge the weight change of the overburden.  Then, because other mass changes are relatively minor compared to the water in the soil, that weight change would enable him to determine water content.

Since soil compresses more than bedrock, Dr. Murdoch developed a method where he inserted two anchors and cylinders that are pressed up against the soil borehole.  In the middle of these cylinders is a fiberglass rod held tight by the bottom anchor which is able to move inside the top anchor.  The anchors move up and down from the stress on the soil, and this movement is transferred to the rod where it can be measured with a high-resolution displacement transducer.

Diagram of the Delta (Displacement Extensometer for Lysimetric Terrain Analysis)

Diagram of the DELTA (Displacement Extensometer for Lysimetric Terrain Analysis)

Dr. Murdoch’s device is so sensitive that when it is buried 6 m, it will register clear strain signals as his student walks over it. The weight of a person causes around 50 nanometers of displacement at the Clemson Field site, but the instrument itself can resolve displacement approaching 1 nanometer. And the diameter of measurement on the surface is about 4 times the depth.  So if you install the system at 7m, you’d be measuring about a 25 m diameter circle on top.

Like almost all other water content techniques, the challenge is removing all other confounding factors that affect the measurement. It has been said that all sensors are temperature sensors first.  Not surprisingly, one thing that causes errors in the system is temperature, though Dr. Murdoch’s team has dealt with that by getting the system deep in the soil and putting the electronics near it so the temperature change is small.  Barometric pressure also produces cyclical loading of soil mass and requires correction over a range of periods. And, since the calculation of water content requires an estimate of the soil elasticity, changes in soil moisture also may affect the measurement. Considerable work has been done and significant progress has been made in dealing with these and other issues with the extensometer approach.

picture of a field with a barn in the distance and the ski orange and grey

An advantage of the system is its ability to be buried. In order to plow, for example, all you have to do is pull the sensor up, take off the top plastic casing, and cap it, and the grower can drive a plow over the top.

Strengths:

The amazing thing is that Dr. Murdoch’s system can resolve less than a millimeter of rain water falling on the soil surface, and it can match trends over time. In addition, you can easily calibrate the system by getting your 190-pound student to walk over the top of it and then checking that the compressibility of the soil matches that weight.

Another advantage of the system is its ability to be buried.  In order to plow, for example, all you have to do is pull the sensor up, take off the top plastic casing, and cap it, and the grower can drive a plow over the top. Finding the installation can be challenging, so it must be located by precision GPS or survey equipment prior to burial. But, if done correctly, the site can be monitored for long periods of time.

Though not yet a final technology, the Delta extensometer did correlate well with point measurements of water content and shows a lot of promise. The instrument was developed with funding from the National Science Foundation. Colby Thrash, a grad student at Clemson, has done much of the recent work. Dr. Murdoch’s team will publish a paper describing the technique soon in Water Resources Research.

Download the “Researcher’s complete guide to soil moisture”—>

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Jul 31

34 Comments

Water Potential Versus Water Content

Dr. Colin Campbell, soil physicist, shares why he thinks measuring soil water potential can be more useful than measuring soil water content.

A horsetail plant showing possible signs of guttation where the water potential in the soil overnight is high enough to force water out of the stomates in the leaves.

A horsetail plant showing possible signs of guttation where the water potential in the soil overnight is high enough to force water out of the stomates in the leaves.

I know an ecologist who installed an extensive soil water content (VWC) sensor network to study the effect of slope orientation on plant available water.  He collected good VWC data, but ultimately he was frustrated because he couldn’t tell how much of the water was available to plants.

He’s not alone in his frustration. Accurate, inexpensive soil moisture sensors have made soil VWC a justifiably popular measurement, but as many people have discovered, a good hammer doesn’t make every soil water problem a nail. I like to compare water potential to temperature because both are considered “intensive” variables that define the intensity of something.

People often try to quantify their own environment, because those measurements define comfort and happiness.  Long ago, they discovered they could make an enclosed glass tube, put mercury inside, and infer this intensive variable called temperature from the changes in the mercury’s volume. This was an obvious way to define the comfort level of a human being.

Thermometer laying on top of wood

People discovered they could make an enclosed glass tube, put mercury inside, and infer an intensive variable called temperature.

They could have measured the heat content of their surroundings.  But they would have discovered that while heat content would be higher in a larger room and lower in a smaller room, you would feel the same comfort level in both rooms.  The temperature measurement helps you know whether or not you’d be comfortable without any other variables entering into the equation.

Similar to heat content, water content is an amount. It’s an extensive variable.  It changes with size and situation. Consider the following paradoxes:

  • A soil with fairly low volumetric water content can have plenty of plant-available water and a soil with high water content can have almost none.
  • Gravity pulls water down through the profile, but water moves up into the soil from a water table.
  • Two adjacent patches of soil at equilibrium can have significantly different water content.

In these and many other cases, water content data can be confusing because they don’t predict how water moves.  Water potential measures the energy state of water and thus explains realities of water movement that otherwise defy intuition. Like temperature, water potential defines the comfort level of a plant.   Similar to the room size analogy for temperature, if we know the water potential, we can know whether plants will grow well or be stressed in any environment.

sand with plants poking out and a blue sky in the background

Soil, clay, sand, potting soil, and other media, all hold water differently.

Plants don’t understand the concept of a content in terms of “comfort” because soil, clay, sand, potting soil, and other media, all hold water differently.  Imagine a sand with 30% water content. Due to its low surface area, the sand will be too wet for optimal plant growth, threatening a lack of aeration to the roots, and flirting with saturation.  Now consider a fine textured clay at that same 30% water content. The clay may appear only moist and be well below optimum “comfort” for a plant due to the surface of the clay binding the water and making it less available to the plant.

Water potential measurements clearly indicate plant available water, and, unlike water content, there is an easy reference scale. We know that plant optimal runs from about -2-5 kPa which is on the very wet side, to about -100 kPa, at the drier end of optimal.  Below that plants will be in deficit, and past -1000 kPa they start to suffer.  Depending on the plant, water potentials below -1000 to -2000 kPa cause permanent wilting.

So, why would we want to measure water potential? Water content can only tell you how much water you have.  If you want to know how fast water can move, you need to measure hydraulic conductivity.  If you want to know whether water will move and where it’s going to go, you need water potential.

Learn more

Soil moisture is more than just knowing the amount of water in soil. Learn basic principles you need to know before deciding how to measure it. In this 20-minute webinar, discover:

  • Water content: what it is, how it’s measured, and why you need it
  • Water potential: what it is, how it’s different from water content, and why you need it
  • Whether you should measure water content, water potential, or both
  • Which sensors measure each type of parameter

Many questions about water availability and movement are best answered by measuring water potential.  To find out more, watch any of the virtual seminars below, or visit our new water potential website.

Download the “Researcher’s complete guide to water potential”—>

Water Potential 101: Making Use of an Important Tool

Water Potential 201:  Choosing the Right Instrument

Water Potential 301: How to Push Your Instruments Past their Specifications

Water Potential 401: Advances in Field Water Potential

Find out when you should measure both water potential and water content.

Take our Soil Moisture Master Class

Six short videos teach you everything you need to know about soil water content and soil water potential—and why you should measure them together.  Plus, master the basics of soil hydraulic conductivity.

Watch it now—>

Download the “Researcher’s complete guide to soil moisture”—>

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Apr 16

7 Comments

Do the Standards for Field Capacity and Permanent Wilting Point Need to Be Reexamined?

We were inspired by this Freakonomics podcast, which highlights the book, This Idea Must Die: Scientific Problems that are Blocking Progress, to come up with our own answers to the question:  Which scientific ideas are ready for retirement?  We asked METER scientist, Dr. Gaylon S. Campbell, which scientific idea he thinks impedes progress.  Here’s what he had to say about the standards for field capacity and permanent wilting point:

Canola Field right next to an eroded soil cliff

A layered soil, a soil that has a fine-textured horizon on top of a coarse-textured soil, will hold twice as much water as you’ll predict from the -⅓ bar value.

Idea:

The phrase, “this idea must die,” is probably too strong a phrase, but certainly some scientific ideas need to be reexamined, for instance the standard of -⅓ bar (-33 kPa) water potential for field capacity and -15 bars (-1500 kPa or -1.5 MPa) for permanent wilting point.

Where it came from:

In the early days of soil physics, a lot of work was done in order to establish the upper and lower limit for plant available water.  The earliest publication on the lower limit experiments was by Briggs and Shantz in 1913. They planted sunflowers in small pots under greenhouse conditions, letting the plants use the water until they couldn’t recover overnight, after which they carefully measured the water content (WC).  The ability to measure water potential came along quite a bit later in the 1930s using pressure plates.  As those measurements started to become available, a correlation was found between the 15 bar pressure plate WCs and the WCs that were determined by Briggs and Shantz’s earlier work.  Thus -15 bars (-1.5 MPa) was established as the lower limit of plant available water.  The source of the field capacity WC data that established a fixed water potential for the upper limit is less clear, but the process, apparently, was similar to that for the lower limit, and -⅓ bar was established as the drained upper limit water potential in soil.

Sunflowers against a blue sky

Briggs and Shantz planted sunflowers in small pots under greenhouse conditions, letting the plants use the water until they couldn’t recover overnight, after which they carefully measured the water content (WC).

Damage it does:  

In practice, using -15 bars to calculate permanent wilting point probably isn’t a bad starting point, but in principle, it’s horrible. Over the years we have set up experiments like Briggs and Shantz did and measured water potential. We have also measured field soils after plants have extracted all the water they can.  Permanent wilting point never once came out at -15 bars or -1.5 MPa.  For things like potatoes, it was approximately -10 bars (-1 MPa), and for wheat it was approximately -30 bars (-3 MPa).  We found that the permanent wilting point varies with the species and even with soil texture to some extent.

Of course, in the end it doesn’t matter much as the moisture release curve is pretty steep on the dry end, and whether you predict it to be 10 or 12% WC, it doesn’t make a huge difference in the size of the soil water reservoir that you compute.

However, on the field capacity end of the scale, it matters a lot.  If you went out and made measurements of the water potentials in soils a few days after a rain, it would be an absolute accident if any of them were ever -⅓ bar (-33 kPa).  I’ve never seen it.  A layered soil, a soil that has a fine-textured horizon on top of a coarse-textured soil, will hold twice as much water as you’ll predict from the -⅓ bar value.  On the other hand, if you’re getting pretty frequent rains or irrigation, that field capacity number becomes irrelevant. Thus, -⅓ bar may be a useful starting point for determining field capacity, but it’s only a starting point.

Why it’s wrong:

Field capacity and permanent wilting point are dynamic properties.  They depend on the rate at which the water is being extracted or the rate at which it’s being applied.  They also depend on the time you wait to sample after irrigation. Think of the soil as a leaky bucket.  If you were trying to carry water in a leaky bucket and you walked slowly, the bucket would be empty by the time you get the water where you want it. However, if you run fast, there will still be some water left in the bucket.  Similarly, if a plant can use water up rapidly, most of it will be intercepted, but if a plant is using water slowly, the water will move down past the root zone and out the bottom of the soil profile before the plant can use it.  These are dynamic phenomena that you are trying to describe with static variables.  And that’s where part of the problem comes.  We need a number to do our calculations with, but it’s important to understand the factors that affect that number.

Rye Field

Rye field

What do we do now:

What I hope we can do is better educate people. We should teach that we need a value we call field capacity or permanent wilting point, but it’s going to be a dynamic property.  We can start out by saying: our best guess is that it will be -⅓ bar for finer-textured soils and -1/10 bar (-10 kPa) for coarser-textured soils. But when we dig a hole and find out there is layering in the profile or textural discontinuities, we’d better adjust our number.  If we’re dealing with irrigated farmland, the adjustment will always be up, and if we’re dealing with dryland or rain-fed agriculture where the time between water additions is longer, we’ll use a lower number.

Some Ideas Never Die:

One of the contributors to the book, This Idea Must Die, Dr. Steve Levitt, had this to say about outdated scientific ideas, and we agree:  “I love the idea of killing off bad ideas because if there’s one thing that I know in my own life, it’s that ideas that I’ve been told a long time ago stick with me,  and you often forget whether they have good sources or whether they’re real. You just live by them. They make sense. The worst kind of old ideas are the ones that are intuitive. The ones that fit with your worldview, and so, unless you have something really strong to challenge them, you hang on to them forever.”

Harness the power of soil moisture

Researchers measure evapotranspiration and precipitation to understand the fate of water—how much moisture is deposited, used, and leaving the system. But if you only measure withdrawals and deposits, you’re missing out on water that is (or is not) available in the soil moisture savings account. Soil moisture is a powerful tool you can use to predict how much water is available to plants, if water will move, and where it’s going to go.

In this 20-minute webinar, discover:

  • Why soil moisture is more than just an amount
  • Water content: what it is, how it’s measured, and why you need it
  • Water potential: what it is, how it’s different from water content, and why you need it
  • Whether you should measure water content, water potential, or both
  • Which sensors measure each type of parameter

Take our Soil Moisture Master Class

Six short videos teach you everything you need to know about soil water content and soil water potential—and why you should measure them together.  Plus, master the basics of soil hydraulic conductivity.

Watch it now—>

Download the “Researcher’s complete guide to soil moisture”—>

Download the “Researcher’s complete guide to water potential”—>

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Apr 4

Double Ring Infiltrometers Versus DualHead Infiltrometers

Several years ago I had the chance to work at the USDA ARS Research Watershed in Riesel, Texas. The goal of my research was to look at the effects of land use and landscape position on water infiltration.  Within the research watershed there is preserved and maintained native prairie, improved pasture, and conventional tilled areas, which have been in existence for 75 years. Thus we were able to use infiltrometers to study the long-term effects of those different land uses, along with the effect of landscape position within the same soil type.

Double Ring Lysimeters

Texas Infiltrometer setup

My research focused on the Houston Black Soil Series, which is a clay-rich soil with a high shrink-swell capacity. This soil type has key economic importance, as it is present in much of Texas’ USDA prime farmland.  To achieve our objectives, we began by mapping soil bulk electrical conductivity using an EM38 device (electromagnetic geo-surveying instrument).  The maps we created allowed us to look for areas of variability in water content, depth to parent material, clay content, and salinity.  Then we randomly selected three zones within the catinas (full hill slope including summit, back slope, and front slope) and flagged them with GPS points.  Our goal was to make infiltration measurements at all of the landscape positions on the slope and compare them to the same landscape positions within each land use type.

We found that the native prairie had the highest infiltration rates because the soil maintained its strong structure and macropores which allowed water to conduct well through the soil.  We also found some differences by landscape position that were consistent within the different catinas.  As water would run down the catina, erosion would transport soil and organic matter off the shoulder and back slope and deposit it on the foot slopes.  Even though they were mapped as the same soil type, the differences in erosion and reduction of organic matter affected the ability of these different positions to transport water.

Double ring infiltrometer chart

We chose to customize existing double ring infiltrometers to make these measurements because there wasn’t anything automated on the market.  If I was going to conduct my research in a reasonable amount of time, I had to come up with a system where I could run a lot of measurements relatively easily.  As a result, we bought three double-ring infiltrometers and modified them with pressure sensors and some larger controlled ports.  The resulting setup was huge; the outer ring on each infiltrometer was 60 cm in diameter and the entire instrument was very heavy.  We were constantly refilling the instrument water reservoirs. In fact, this setup required so much water that we had to pull a 1,900-liter water tank on a trailer wherever we were taking measurements.

Our goal was to save time by running all three infiltrometers concurrently, but it still took a LONG time.  Even though we had automated the instruments, they required a lot of monitoring; sometimes I had to fill our 1,900-liter water tank twice in a day. One measurement at one site took anywhere from 1.5 hours to 3 hours depending on when we reached steady state. We spent so much time out in the field that we were actually caught on film in one of the Google Maps picture flyovers!   Even after all this field time, the data analysis was overwhelming, despite a relatively seamless approach to handle it all.

One huge infiltrometer setup

Our huge setup caught on google maps

I often dreamed of making a tool that would be a lot easier for me and others to use. When I joined Decagon (now METER), it gave me an opportunity to do just that.  Our design goals were to make an infiltrometer that required less water and simplified the data analysis.  We rejected the double ring design in favor of a single ring approach because research has shown that the outer ring doesn’t buffer three-dimensional flow like it’s supposed to. (Swartzendruber D. and T.C. Olson.  “Sand-model study of buffer effects in the double-ring infiltrometer” Soil Sci. Soc. Am. Proc. 25 (1961), 5-8)

We also wanted to simplify the analysis of three-dimensional flow.  With a constant head control in a single ring, there are equations that you use to correct for it.  But you have to guess at things like soil type and structure which leads to inaccuracies.  Multi-head analysis has been around for decades. It involves establishing constant water heights (heads) at multiple levels and looking at the difference in the infiltration rates to calculate the sorptivity. Thus, parameters that are normally estimated from a table can actually be measured, and infiltration results will be independent of users.

Still, there can be problems with the multiple head approach. Increasing the water height when infiltrating into a really low conductivity soil may take 1 to 2 hours to drain back to the original height. We didn’t want to make this measurement take longer than necessary, so instead of using additional water, we used air pressure to simulate higher water levels which can be added or removed very quickly.

So, thanks to the instrument hardships I endured in my past efforts to obtain infiltration measurements, we now have an easy-to-use dual-head infiltrometer (now called the SATURO), that can do the analysis of infiltration rates and saturated hydraulic conductivity on the instrument itself (it gives sorptivity and alpha, based on the soil type and structure, and makes the correction onboard).  Thus, if a scientist needs a value right away, it’s there. But, if like me, they wanted to dig deeper through the data, all the measured values can still be downloaded for more careful analysis.  Together, it’s a simple tool for both scientists and consultants who need to make these measurements.  And they won’t get caught on Google Maps like me, because they’ve had to spend their whole life in the field taking measurements.

Below is a video of the dual-head infiltrometer in action.

Get more information on applied environmental research in our

Download the “Researcher’s complete guide to soil moisture”—>

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Mar 9

Could This Farming Practice Make Food Grown in Fukushima Safe?

March 11, 2015 marks four years since the Fukushima disaster.  What have we learned?

Shortly after the Fukushima disaster, we donated some of our sensors to Dr. Masaru Mizoguchi, a scientist colleague at the University of Tokyo.  He is using the equipment to contrive a more environmentally friendly method to rid rice fields in the villages near Fukushima of the radioactive isotope cesium 137.

Over the last three years, government contractors removed 5 cm of topsoil from fields in order to extract the radioactive isotope. The topsoil has been replaced with sand.  The problem with this method is that it also removes most of the essential soil material, leaving the fields a barren wasteland with little hope of recovery anytime soon.  Topsoil removal may also prove ineffective because wild boars dig up the soil to root for insects and larvae.  This presents a problem in the soil stripping method, as it becomes impossible to determine exactly where the 5 cm boundary exists.  In addition, typhoons and heavy rains erode the sand surface raising safety and stability concerns.

Trash Bags Full of Radioactive Topsoil

Currently, bags full of radioactive topsoil are stacked into pyramids in abandoned fields. An outer black bag layer filled with clean sand is placed around the outside to prevent radiation leakage. The government has promised that these bags will be removed and taken to a repository near the destroyed reactor, but many people don’t believe that will happen as the bags themselves only have a projected life of 3-5 years before they start to degrade. More of these pyramids are being built around Iitate village every day, which is a source of uneasiness for many people that are already cautious about returning.

Dr. Mizoguchi and his colleagues have come up with a new “flooding” method now being tested in smaller fields that can save the topsoil and organic matter while at the same time removing the cesium, making the land usable again within two years.  The new method floods the field and mixes the topsoil with water, leaving the clay particles suspended. Because the cesium binds with the clay, they can drain the water and clay mixture into a pre-dug pit and bury it with a meter of soil after the water has infiltrated.  After one year of using this method, the scientists saw that the cesium levels in the rice had gone down 89%.  And in situ and laboratory instrumentation have shown that two years after cesium removal, the plants’ cesium uptake is negligible, and the food harvested is safe for consumption.

Researcher standing by a sensor station

Dr. Mizoguchi standing by a sensor station containing Decagon sensors

Dr. Mizoguchi is monitoring the surrounding forests with our canopy and soils instrumentation in order to determine if runoff from the wilderness areas will return cesium to the fields and what can be done about it.  He’s figured out a way to network all the instrumentation and upload data directly to the cloud. Still, even if this technology and new methodology work, will people around the world ever feel safe eating food grown near Fukushima?  Dr. Mizoguchi says, “I believe that the soil is recovered scientifically and technically.  However, harmful rumors will remain in the public mind for a long time, even if we show the data that proves safety.  So we must keep showing the facts on Fukushima based on scientific data.”

Resurrection of Fukushima Volunteers using Dr. Mizoguchi's method to rehabilitate small farms

Resurrection of Fukushima volunteers use Dr. Mizoguchi’s method to rehabilitate small farms

Incredibly, each weekend a volunteer organization of retired scientists and university professors use their own money and time to travel out to small village farms.  There they labor to rehabilitate the land using Dr. Mizoguchi’s method.  One of the recipients of this selfless work is a 72-year-old farmer who took his nonagenarian mother and returned to their home to fulfill her heartfelt plea that she could live out her final years outside the shadow of a highrise apartment (see this story in the video above).  We are honored to be a part of this humanitarian effort.

Download the “Researcher’s complete guide to soil moisture”—>

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Jan 26

Sneak Peek: Remote Sensing in Commercial Agriculture: Perspective on Innovations

Dr. Christopher Lund is a research scientist and product manager for METER’s new irrigation management instrumentation group. He has more than a decade of experience working with land surface flux measurements, terrestrial water budgets, and soil-vegetation-atmosphere transfer scheme modeling. Prior to joining METER, he served as a research scientist on the NASA-CSUMB SIMS (Satellite Irrigation Management Support) Project, a multi-year collaboration between the California Department of Water Resources, NASA, and CSU Monterey Bay providing California growers with novel irrigation decision support tools. Dr. Lund’s current research focuses on developing cost-effective irrigation management instrumentation for commercial markets. Dr. Lund will be giving a talk on innovations in agricultural remote sensing at the Third Professional Workshop on Technology For Irrigation Scheduling.  He will talk about his work with the SIMS team and what growers can do with remote sensing data to estimate things like evapotranspiration.  He’ll also address how to improve those estimates by combining them with field measurements from ground based instrumentation such as soil moisture sensors.

remote sensing in commercial agriculture

Image: USGS Landsat Project Website

“The advantage of satellite remote sensing is that it allows you to look at many fields at once and also integrate across spatial variability.  The down side is it doesn’t give you access to everything you might want for irrigation management, so there are certain things you have to measure on the ground.  When it comes to remote sensing data and ground measurements, I don’t think it’s an either/or situation.  I think the future is hybrid products utilizing both remote sensing and ground based measurements,” he says.

He will also speak on how satellite derived NDVI data can benefit from new inexpensive ground based-sensors like the SRS.  This enables scientists to make sure that their satellite NDVI data accurately reflect what’s happening on the ground.

The seminar will be held at the Third Professional Workshop On Technology For Irrigation Scheduling on February 11, 2015 at the CREA auditorium, Calle Jose Galan Merino Sevilla, Spain.

Download the “Researcher’s complete guide to soil moisture”—>

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