Traveling around the world, I’ve seen many ways to install soil moisture sensors. Digging a trench to the required depth and inserting the sensors into the sidewall is certainly the most common technique. But using a shovel takes a lot of effort, especially in rocky soil. To solve this problem, I like to use an auguring tool because of its ability to dig through soil to deeper depths without taking a lot of time. Also, the footprint of an augured hole is also only a few inches, which makes for a much cleaner installation. Still, borrowing an auger from the lab next door and heading to the field may not be the best option. This is what we did on the Cook Farm project a few years back.
Standard bucket auger (image: www.atlanticsupply.com)
The Cook Agricultural Farm is a 37 Ha managed research site near Pullman, Washington where a combined team of Decagon and WSU scientists installed 150 water contentsensors over 30 sites a few years ago. At each site, we used the techniques outlined in METER’s installation video, which can be found here. However, the hardest thing about this installation was that we used some borrowed, standard bucket augers to bore the holes. These had a cutting surface along the bottom and an enclosed cylinder to hold the soil. Once we filled that bucket, we had a difficult time getting the soil out which really slowed the installation.
Ben digging soil out of the bucket auger during the Cook Farm Installation, 2009.
Recently while traveling to Germany, I learned about the Edelman Auger. The company that makes these (Eijkelkamp), says that most people in America use bucket augers to bore into fine soils which is needlessly time consuming. Edelman Augers, originally designed by the Army to dig latrines, will save time and labor.
Edelman auger.
At first, I was skeptical. It only had two cutting blades that ran up the auger in kind of loop; how would the soil lift out of the hole? However, when I tried one later in the day, the auger cut through the soil, making a 10 cm hole with very little effort, and as I removed it, the soil came out easily. It wasn’t hard to get the soil out from between the blades because there was no enclosed cylinder for the bucket. I wish I’d known about this auger when I was trying to install sensors at the Cook Farm.
So, here are a few tips about augers to help you pick the best one for your work:
The Eijkelkamp Edelman augers are best for silty soils to clay soils so pick this one if you’re working in sites with these types of soils. It’s also great for digging a quick latrine.
Bucket augers are best for sandy soils because of the enclosed cylinder will help lift the loose sand out of the borehole.
If you’re trying to install your soil moisture sensors in very rocky soils, try a stony soil auger. It has big blades to help move small rocks and lift them out of the hole.
With the recent news coverage of the SMAP (Soil Moisture Active Passive) satellite launch, researchers may wonder: what does remote sensing mean for the future of in situ measurements? We asked two scientists, Drs. Colin Campbell and Chris Lund, for answers to this complex question. Here’s what they had to say:
Image: www.jpl.nasa.gov
What is SMAP?
SMAP is an orbiting earth observatory that estimates soil moisture content in the top 5 cm of soil over the entire earth. The mission is three years long with measurements taken every 2-3 days. This will allow seasonal changes around the world to be observed over time, improving our ability to manage water resources and better parameterize land surface models. SMAP determines the amount of water found between the minerals, rocky material, and organic particles found in soil by measuring the ability of radar to penetrate the soil. The wetter the soil is, the less the radar will penetrate. SMAP has two different sensors on the platform: an L band aperture radar with a resolution of about a kilometer when it’s looking straight down (the pixel size is about 1 km by 1 km), combined with a passive radiometer with about 40 km of resolution. This combination creates a synthetic product that takes advantage of the sensitivity of the radiometer.
What does SMAP mean for in situ soil water content measurement?
It’s all about scale: In some ways, comparing in situ to SMAP measurements is like comparing apples to…well…mountain-sized apples. The two forms of measurement use vastly different scales. In situ soil moisture sensors measure water content at the volume of several liters of soil, maximum. Even the sensor with the largest field of sensitivity, the neutron probe, can only integrate a volleyball-sized volume. On the other hand, SMAP measures at a resolution of 1 km2, which is larger than the size of a quarter section, a large field for many farmers. Global soil moisture maps will allow scientists using SMAP to look at big picture applications like weather, climate and hydrological forecasting, drought, and flooding, while more detailed in situ measurements will tell a farmer when it’s time to water, or help researchers discover exactly why plants are growing in one location versus another. The difference in spatial scale makes the two forms of measurement useful for very different research purposes and applications. However, there are applications where the two measurements can be complementary. Most notably, in situ measurements are often temporally rich while being spatially poor. But, SMAP can be used to scale in situ measurements to areas where in situ measurements are absent. In situ measurements can also be used as a source of validation data for SMAP-derived values for any location where both in situ and SMAP measurements overlap. Thus, there is opportunity for synergy when pairing SMAP and in situ measurements.
Satellite image in Winter.
What can SMAP do that in situ measurement can’t?
Scientists say they’ve seen a relationship between the top 5 cm of soil moisture and some factors related to climate change and weather. Because in situ soil sensors sample across a spatial footprint of a few meters, it can be very difficult to use their data to say anything about processes occurring across broad spatial scales; two liters of soil is not going to tell you anything about weather or flooding. SMAP can help us better understand the interaction between the land surface and atmosphere, improving our understanding of the global water cycle as well as regional and global climate. This will help with forecasting crop yield, pest pressure, and disease…that’s big picture research.
The productivity of a forest also may depend on the general soil moisture measured by SMAP. For instance, if we got an idea of the soil moisture and greenness of a forest, we could tie together the approximate water availability and the resulting biomass accumulation with incoming solar radiation. Better biomass accumulation models could lead to better validation of global carbon cycle models.
SMAP will also be able to detect dry areas across the U.S. and challenges they might present. Surface runoff that leads to flooding could also be predicted as scientists will be able to see where soils reach saturated conditions.
In other applications, people working on global water or energy budgets have to parameterize the land surface in terms of how wet or dry it is. That’s the big advantage of SMAP’s relatively new data sets. Any time you’re running a regional climate model you have to parameterize what the soil moisture is in order to partition surface heat flux into sensible and latent heat flux. If there’s a lot of available water, it’s weighted more toward evaporation and less toward sensible heat flux. In areas where there’s little available water and low evaporation, you get high surface temperatures and sensible heat flux. So SMAP will be important for model parameterization as we haven’t had a good global data set for soil moisture until now.
In situ sensors show how much water is lost from the root zone and what is still left.
What can in situ sensors do that SMAP can’t?
In irrigated agriculture, farmers need to know when and how much to irrigate. In situsensors give them this information by showing how much water was lost from the root zone and what is still left. SMAP is unable to tell you what’s down in the root zone; it only reaches to 5 cm. Additionally, 1 km resolution is larger than most irrigation blocks. These factors mean that it will be difficult to make irrigation decisions from SMAP alone.
Scientists using in situ sensors are concerned with the soil moisture available in a local area because their time resolution is excellent and they have the ability to resolve what’s happening in particular conditions related to crops or natural systems. Natural systems are often heterogeneous, meaning there may be adjacent areas with different types of vegetation including trees, shrubs, and grass. Tree roots may grow deep while grass roots are shallow. Being able to look over all these different areas without averaging them together, as SMAP does, is critical in some applications.
What about geotechnical applications? Literature suggests SMAP output can help predict landslides. It is more likely that it can only see when the soil is generally saturated and generate a warning. But in slopes that are at risk of landslides, in situ monitoring with sensors such as tensiometers to measure positive pore water pressure may be more useful for determining when a slide is imminent.
SMAP, like in situ water content measuring systems, is also limited by the fact that it measures the amount, not the availability, of water. If it measures 23% water content in a certain area, that measurement may not tell us what we want to know. A clay soil at 23% VWC will be close to wilting point while a sand would be above the plant optimal range. SMAP doesn’t measure the energy status of water (water potential), so even if SMAP tells us a field has water content, that water might not be readily available. Water availability must be determined through a pedo-transfer function or moisture release curve appropriate for a specific soil type (It is possible to overlay SMAP data on soil type data to estimate energy state, but this might not be fine enough resolution to be useful).
Complementary Technology
How do SMAP and in situ instruments work together? The key is ground truthing in situ soil moisture measurements with SMAP type satellites and vice versa. Ground-based measurements at specific locations can be matched with satellite information to extrapolate over a field and gain confidence in the small continuous scale alongside the larger infrequent scale. It’s analogous of a video camera recording one plant continuously while a single shot camera snaps whole-field pictures every day. With the SMAP “single-shot” we can say, something changed from time A to time B, but we don’t know what happened in the middle (rain event, etc.). In situ measurements will tell us the details of what happened in between each snapshot. Putting both data sets together and matching trends, we can show correlation and complete the soil moisture picture. Basically, In situ measurements provide temporally rich information about soil moisture from a postage stamp-sized area of earth’s surface (driven by highly localized conditions), whereas SMAP gives us the ability to monitor broad scale spatiotemporal patterns across all of earth’s surface (driven by synoptic conditions).
I often hear researchers complain about the accuracy of our TEROS 21 water potential sensors. We still have room to improve, but we’ve certainly come a long way! People have been attempting to make water potential measurement in the field for over 100 years. The following is a brief overview of the evolution and history of water potential measurements over that time.
Pre-MPS-1 prototype.
Livingston Discs
The Livingston disc, developed in 1908, was one of the first attempts at determining water potential in the field. The Livingston Disc was actually a primitive, manual version of the technology used in our MPS6 ceramic disc. Here is how it worked: first, you’d weigh the dry disk, then put it in the soil and let it equilibrate. After that, you would dig it up and weigh it again. Using the water retention curve of the disc, you could then determine the water potential.
Gypsum block
In the 1940s gypsum block sensors were invented as the first solid matrix equilibrium technique for water potential. This method tried to continuously sense water potential with a simple electrical conductivity measurement in a solid porous (and naturally occurring) gypsum matrix. However, because naturally occurring gypsum doesn’t have a consistent pore size distribution and it degrades over time, the instrument was not very accurate.
In the 1940’s gypsum block sensors were invented as the first solid matrix equilibrium technique for water potential. Image: www.soilmoisture.com
Tensiometers
In the 1960’s a liquid equilibration technique called tensiometry was discovered that allowed water potential measurement with good accuracy even in the presence of positive pore water pressures. Tensiometers work extremely well in wetter soils with water potentials between 0 and -80 kPa and should be the choice for all wet soil applications, especially above -9 kPa where the MPS6 will not work (the air entry value for its ceramic is -9 kPa). However, when the soil dries out the water under tension in the tensiometer eventually cavitates, causing the output to be useless until they are refilled. Thus solid equilibrium techniques like the TEROS 21 are the best choice across the dry range.
Tensiometers are the most accurate way to measure water potential in the field in the wet range, but are limited to the plant optimal range of about -100 kPa and above.
The Evolution of Ceramic Discs
We learned with the gypsum blocks that one of the challenges in solid matrix water potential measurement is finding a material that will create the same water retention curve every time. In quest of this goal, the ceramic discs in sensors like the TEROS 21 have taken years of development. Because of the limited range of the tensiometer, we wanted to develop a water potential sensor that could measure over a larger range. The hardest part about developing that ceramic was getting a variety of pore sizes so the instrument could read said wide range of water potentials. This started years ago in the lab of Dr. Gaylon Campbell at Washington State University where his technician, Kees Calisendorf, experimented over a long period of time to come up with the perfect recipe.
The MPS1 was our original matric potential sensor released in 2001. It allowed for long-term monitoring in the field because, unlike gypsum, the ceramic did not degrade over time.
Even after we found a consistent ceramic, there were still outliers. So creating a calibration method was essential to making an accurate sensor. The first challenge was to be able to store calibration points in the sensor, which required a microprocessor. The second, and more difficult task, was to establish a method to calibrate large numbers of sensors at once. We tried many different approaches like pressure plate, equilibration over salt solutions, and even centrifugal force, but nothing worked. Finally, in a discussion with our partner, UMS, we discovered the key. We now can accurately calibrate 50 sensors at a time in only 12 hours. Still, even with these advanced techniques, we only have a sensor with an accuracy of plus or minus 10%, but considering the history of how hard it’s been to develop consistent ceramic, this accuracy is exciting for the range that we can get.
The MPS 2 was our second matric potential sensor which offered two-point calibration and a temperature sensor, improving accuracy.
What’s Next?
Now that we’ve created a reliable calibration method, we can turn our attention toward further improving the sensor measurement range as well as its accuracy. Testing different ceramics, or other porous media, may hold the key to a solid equilibrium technique sensor reading all the way to 0 kPa, eventually replacing the need for tensiometers in the field.
The two key innovations in the MPS6 (now called TEROS 21), released in 2014, are the addition of a microprocessor to the sensor and fast, accurate equilibration at multiple points.
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.
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.
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.
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 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.
Months ago, I was reading about the tragic test-flight crash of the Virgin Galactic passenger rocket plane and noting that there was a lot of criticism of that company for it’s “risky space venture.” In the midst of the heartbreak I felt at the deaths of the two pilots, as an innovator, I can somewhat sympathize with the Virgin Galactic Company in the realization that historically, innovation has involved risk. If you think about airplane development in the early 1900’s, every year 20% of pilots died because of malfunctions on airplanes (“By the spring of 1917, the life expectancy of a British pilot was put at eight days.” Van Creveld, The Age of Airpower, p. 28). Today we worry about the safety of air travel much less because people were willing to go through years of painful learning, and because of that learning, we no longer use trains as our fastest transportation.
Thankfully, no one’s life is on the line with Decagon sensor innovation. However, similar to early airplanes and the new rocket plane, when we release an instrument that is completely new (like our circuit board water content sensors), there are occasional problems that are not accounted for in our extensive laboratory and field testing. In light of this, we are grateful for scientists who are willing to give us feedback in order to aid innovation and advancement of science and technology. Here is an example of how scientists became our collaborators in developing a new product that helped to advance their discipline.
In 2000 we made our first water content sensor where we put the circuit board on the sensor itself, rather than a data logger. In advance of its release, we made a version of the sensor that was flexible with the idea that it would have excellent contact by conforming to the soil. In theory, it looked great, but when it was put in the ground, the sensor flexed and popped the components off the circuit board. Thanks to testing feedback, we made more rigid, 20cm long sensors, and the components had no more trouble.
Researchers loved the instrument but wanted shorter ones to put in greenhouse pots.
After this problem was solved, researchers loved the instrument but wanted shorter ones to put in greenhouse pots. So we made the shorter ones. Scientists then became concerned that the sensors were sensitive to salinity in higher electrical conductivity conditions. Coincidentally, components became available to raise the frequency to 70 MHz, which is much less sensitive to electrical conductivity. Thus, because scientists were willing to partner in the development process and learn with us, we have developed a sensor that is affordable, cutting-edge technology which advanced the discipline of soil science.
We love to partner with scientists in the pursuit of knowledge. A researcher at a conference once said to one of our scientists, “I hate your stuff…for the first couple of months. But then you guys take your lumps, make it better, and then I buy all your stuff.” He was saying this in jest because it certainly doesn’t happen with many of the products we release. But when it does, we appreciate the dedicated support of our scientist friends who enjoy learning along with us to make the same kind of advancement that helped the world move from trains to air, and now onward into commercial space travel.
