Both the amount and the availability of water in soil is important to plant roots and soil-dwelling organisms. To describe the amount of water in the soil we use the term water content. To describe the availability we talk of water potential. In thermodynamics, the water content would be referred to as the extensive variable and the water potential as the intensive variable. Both are needed to correctly describe the state of water in soil and plants.
In addition to describing the state of water in the soil, it may also be necessary to know how fast water will move in the soil. For this, we need to know the hydraulic conductivity. Other important soil parameters are the total pore space, the drained upper limit for soil water, and the lower limit of available water in a soil. Since these properties vary widely among soils, it would be helpful to establish correlations between these very useful parameters and easily measured properties such as soil texture and bulk density. This paper will present the information needed for simple models of soil water processes.
When we talk with scientists at conferences they often want to know the difference between TDR versus capacitance or FDR. We’ve written a paper about this in our app guide that has been pretty popular, but it can be difficult to find on our website. Here is an introduction and a link if you are interested in learning more.
TDR Sensor Installation (Giulio Curioni, School of Civil Engineering, Univ. of Birmingham)
Capacitance and TDR techniques are often grouped together because they both measure the dielectric permittivity of the surrounding medium. In fact, it is not uncommon for individuals to confuse the two, suggesting that a given probe measures water content based on TDR when it actually uses capacitance.
10HS capacitance sensor
With that in mind, we will try to clarify the difference between the two techniques. The capacitance technique determines the dielectric permittivity of a medium by measuring the charge time of a capacitor, which uses that medium as a dielectric. We first define a relationship between the time, t, it takes to charge a capacitor from a starting voltage, Vi , to a voltage V, with an applied voltage, Vf. Read more….
Watch the webinar
In this webinar, Dr. Colin Campbell discusses the details regarding different ways to measure soil moisture and the theory behind the measurements. In addition, he provides examples of field research and what technology might apply in each situation. The measurement methods covered are gravimetric sampling, dielectric methods including TDR and FDR/capacitance, neutron probe, and dual needle heat pulse.
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.
Who are the people behind EnvironmentalBiophysics.org? We are a group of scientists passionate about measuring the environment. Our day job is to design new environmental sensors and improve existing ones for our customers. Sensors we’ve developed are routinely used to gather data all along the soil-plant-atmosphere continuum. We even have a sensor sitting on Mars (more about that later). Contributors to the articles on EnvironmentalBiophysics.org include:
Gaylon S. Campbell
Dr. Gaylon S. Campbell has been a research scientist and engineer at METER for 16 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. He is a recipient of the prestigious Fellow Award for outstanding scientific contributions from both the American Society of Agronomy (ASA) and Soil Science Society of America (SSSA).
Colin S. Campbell
Dr. Campbell has been a research scientist at METER for 14 years following his Ph.D. at Texas A&M University in Soil Physics. He is currently serving as Vice President of Research, Development, Engineering, and Software. He is also adjunct faculty with the Dept. of Crop and Soil Sciences at Washington State University where he co-teaches Environmental Biophysics, a class he took over from his father, Gaylon, 14 years ago. Dr. Campbell’s early research focused on field-scale measurements of CO2 and water vapor flux but has shifted toward moisture and heat flow instrumentation for the soil-plant-atmosphere continuum and irrigation management. He recently received the prestigious Fellow Award for outstanding scientific contributions from the American Society of Agronomy (ASA). His latest work is focused on combining remote sensing and in situ data to provide a more complete picture of water availability to crops.
Douglas R. Cobos
Dr. Cobos is a Research Scientist and the Director of Research and Development at METER. He also holds an adjunct appointment in the Department of Crop and Soil Sciences at Washington State University where he co-teaches Environmental Biophysics. Doug’s Masters Degree from Texas A&M and Ph.D. from the University of Minnesota focused on field scale fluxes of CO2 and mercury, respectively. Doug was hired at METER to be the Lead Engineer in charge of designing the Thermal and Electrical Conductivity Probe (TECP) that flew to Mars aboard NASA’s 2008 Phoenix Scout Lander. His current research is centered on instrumentation development for soil and plant sciences.
Christopher P. Lund
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 Decagon, 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.
Lauren Bissey Crawford
Lauren Crawford has been the product manager of the soil moisture sensor line at METER for seven years. She earned her masters in Hydrogeology at Washington State University. Her early Ph.D. work in ecology at Washington State University and her background in environmental biophysics and hydrology help her to work with METER research scientists and engineers to develop soil moisture sensors and data loggers targeted to solve specific problems in agriculture and environmental research.
Leo Rivera
Leonardo Rivera, METER’s Hydrology Product Market Manager, 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.
Chris Chambers
Chris Chambers is the Agriculture and Environmental Application Specialist at METER. Chris earned a B.S. in Forestry from the University of Illinois in 1995 and spent the next 6 years as a vagabond. After traveling the world and having many fantastic experiences, he returned to his studies as a graduate student and pursued research in physiological ecology and biogeochemistry. He completed an M.S. in Forest Resources before METER put his skills and experience to work to support our customers and help them get the data they need.
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.
During a recent semester at Washington State University, a film crew recorded all of the lectures given in the Environmental Biophysics course. The videos from each Environmental Biophysics lecture are posted here for your viewing and educational pleasure.
The primary text used throughout the Environmental Biophysics class is “An Introduction to Environmental Biophysics” by Gaylon Campbell and John Norman.
Environmental Biophysics textbook
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.
In the video below, METER soil scientist Dr. Colin Campbell discusses an often-misdiagnosed water content signal that looks like typical diurnal temperature cycling but is actually due to a phenomenon called hydraulic redistribution. He shows how easily these patterns can be seen in ZENTRA Cloud data management software.
Hello, my name is Colin Campbell. I’m a research scientist here at METER Group. And today we’re going to be digging into some water content data that I collected over the last summer. This is a field that’s planted in spring wheat, it’s about 700 meters across. And we’ve set up six measurement sites. At each one of these sites, we’re making several measurements, but the ones we’re going to talk about today are just water content. And while we’ve installed water content sensors at 15, 45, and 65 centimeters, we’re just going to focus on the 65-centimeter water content sensors. These sensors are the METER TEROS 12 soil moisture sensors, so they also measure electrical conductivity and temperature, and we’re going to look at temperature as well because that figures into this discussion.
So this field was planted in April of 2019. And not a lot interesting goes on at the 65-centimeter depth through April, May, and June. But as we get into July, the wheat is reaching maturity, and they essentially are going to cut off the irrigation water here on July 22. So up to July 22, there’s really not a lot of movement in the water content. One of the sites decreases a little bit, but each line is flat. What I noticed as I was looking at this particular graph is after this long period of very flat data, after June 22 when the irrigation was cut off, we start to see some movement in the water content at this depth Not only is there movement down, but there’s a daily movement of the actual water content signals, all but this top light green line. And it made me wonder, what’s going on?
Diurnal water content fluctuations are not always due to temperature.
Initially, whenever you see a diurnal movement, you suspect that it’s caused by temperature. It’s been said that every sensor is probably a temperature sensor first, and a sensor of whatever we’re really interested in second. In this case, we can look to see what the temperature is doing at that depth. Here’s soil temperature, at 65 centimeters, and even though there’s just a little bobble in the line, the line is almost completely flat. We see the seasonal trends in temperature, but really no diurnal temperature cycling. And this scale is also fairly small. So back to our 65-centimeter water content. If it’s not temperature that’s affecting these lines, then what is it?
I’ve seen this before in an experiment that I did years ago in a non-irrigated wheat field. We were measuring down at 150 centimeters, and when the water had been used up in the upper levels of the soil profile, the roots of the wheat plant just simply went down to 150 centimeters and started taking water up. So this is what I assume is also happening here. The wheat has extended its roots down to 65 centimeters, since its irrigated wheat. That’s not too deep, but wheat doesn’t necessarily need to get its roots down super deep. And as the wheat accesses that water, we’re seeing these daily drops in water. But then we’re seeing just a slight increase in water. Here on July 28, we’re seeing that water go up slightly. And so why is this happening? We might understand how the water is being taken out of the soil, but why do we see a slight increase in the water content (just a few tenths of a percent)?
What I think is happening, in this case, is that it’s not temperature, but actually, roots are growing down into this area, and they’re probably growing around the sensor. As we change from day to night, we see a release in the elasticity of the water in the xylem, and maybe just a little bit more water down in the roots as they’re the transpiration pull of the day is lessened and stops overnight. The stomates are closed, and we see just a little bit of water coming back into the roots and possibly into the soil.
Now there was a big discussion many years ago about whether this was something called hydraulic lift where trees could take up water from deep in the soil profile and essentially give it back to plants near the surface. And although it was a great debate, it was never proven that this actually happened: water being spread from deeper locations to more shallow locations by roots. But this is probably hydraulic redistribution where we just have roots filling with water, and when they are filled, we see a little bit in the water content sensor.
Globally, the gap between the energy production and consumption is growing wider. To promote sustainability, University of California San Diego PhD candidate and ASCE GI Sustainability in Geotechnical Engineering committee member, Tugce Baser, Dr. John McCartney, Associate Professor, and their research team, Dr. Ning Lu, Professor at Colorado School of Mines and Dr. Yi Dong, Postdoctoral Researcher at Colorado School of Mines, are working on improving methods for borehole thermal energy storage (BTES), a system which stores solar heat in the soil during the summer months for reuse in homes during the winter. Baser says, “We are running out of finite energy resources. We need to come up with new strategies to use free and renewable energy resources such as solar energy for a sustainable future.”
Baser’s BTES design.
How it works
BTES systems are an approach to provide efficient renewable resource-based thermal energy to heat buildings. They are configured to store thermal energy collected from solar thermal panels during the summer and discharge the heat to buildings during the winter. They function by circulating a fluid within a closed-loop pipe network installed in vertical boreholes to inject heat collected from solar thermal panels. During winter, cold fluid is circulated through the heat exchangers to recover the heat from the subsurface and distribute it to the buildings. Baser explains, “The subsurface provides an excellent medium to store this heat due to the relatively lower thermal conductivity and lower specific heat capacity especially when the soil layer is in the vadose zone. Lower thermal properties allow us to concentrate the heat in a specific array and the heat losses to the environment are potentially low. These systems typically include an insulation layer and a hydraulic barrier near the ground surface to reduce heat and vapor losses to the atmosphere.”
BTES construction.
Why do we need improved methods?
Baser and her team are trying to improve the understanding of heat storage mechanisms and evaluate changes in the rate of heat transfer and heat storage in the vadose zone where the soil is unsaturated. The goal of the project is improve conventional methods by generating models to fit different soil types and situations. She says, “The European community introduced us to the borehole thermal energy storage systems to provide heat specifically for domestic use, but there is still a chance for us to design them more efficiently by having a full understanding of the thermal response of these systems that is specific to the ground material and subsurface conditions. The primary objective of this research is to understand the mechanisms of coupled heat transfer and water flow in unsaturated soil profiles during the heat injection and subsequent heat extraction into these different arrays and different dimensions of borehole heat exchangers.”
Solar panels.
Baser and her team working on designing numerical models based on finite element method which improve some of the numerical models in the literature used to characterize the thermal response of the systems. The new models add new considerations, such as the heat pipe effect in different soil types. Baser explains, “Because thermal and hydraulic properties of soils are highly coupled and are specific to soils, the thermal response of a BTES system will be different when it is installed in different types of soils. For example, you see the heat pipe effect where there is evaporation and subsequent condensation in fine grained soils rather than coarse soils because in coarse grain soils the pore characteristics are different. The duration of the heat pipe effect (or convective cycle) is longer in fine grain soils. We conclude that considering coupled heat transfer and water flow in the thermal response of Borehole Thermal Energy Storage system is important.”
In-ground heat exchanger
Experiments in the field and in the lab help verify the new models
To fully understand heat transfer mechanisms and water flow in unsaturated soils, the research team installed two different SBTS systems at different scales, one in Golden, Colorado School of Mines campus, and the other at the UC San Diego research campus. Baser says, “The subsurface characteristics of both sites are different, and this gives us the opportunity to investigate the impact of the different soil layers on the thermal response experimentally in a full scale. In addition, the scales of each Borehole Thermal Energy Storage system are different, and we also apply different heat injection rates. We have used these data to further validate our coupled heat transfer and water flow model so that we can use it for design purposes.”
Soil moisture sensor locations.
Baser started with laboratory heating experiments, in which soil in a large tank is heated by heat exchangers. She installed soil moisture sensors to measure volumetric water content and the temperature and then used the KD2 pro thermal property analyzer (recently updated to TEMPOS) to monitor thermal properties during heating experiments to characterize the coupled thermo-hydraulic relationships. For the field experiments the team uses soil moisture sensors equipped with temperature sensors and the KD2 pro to monitor subsurface temperature fluctuation because during the summertime the air temperature is higher, thus ambient air temperature fluctuation and penetration may become significant.
Baser also uses thermistor strings that include six thermistors at different depths and thermistor pipe plugs, voltage input modules, and flow meters. She says, “Thermistor pipe plugs and flow meters are used in the manifold to monitor the inlet and outlet fluid temperatures and flow rates in each loop to calculate heat transfer rate into the ground. Flow meters were installed to control flow in each loop because you don’t want to over or underload the borehole loops. The amount of energy that you collect from the solar loop and the amount of energy that you inject into the ground can be used to define the efficiency of the system.” Baser says thermistor strings help monitor the ground temperature during the summer heat loading at different depths. They’re also used to monitor borehole wall temperature over time. The team installed one thermistor string 9 meters away from the heat storage array to see if far field is affected by the heat transfer within the array.
Insulation prevents heat loss to the environment.
The new models will save money in future Borehole Thermal Energy Storage design
Baser says building numerical models and solving them was very complicated and time consuming, but they’ve had good results. She explains, “We’ve recently proved, both experimentally and numerically, that considering coupled thermal and hydraulic relationships are very important for thermal response analysis. Thus, our recommendation is that it’s fine to use the analytical models and user-friendly numerical models that consider constant thermal properties in the design analyses for saturated soils. However, in unsaturated soils, there is a very high possibility that the contribution of heat transfer evaporation and condensation would be missing and the Borehole Thermal Energy Storage system would be oversized, costing a significant amount of money. When dealing with soils in the vadose zone, coupled thermo-hydraulic constitutive relationships in the modeling efforts need to be considered.”
You can learn more about Tugce Baser’s research here.
In Italy, on January of 2014, one of the Secchia river leveesfailed, causing millions of dollars in flood damage and two fatalities. Concerned withpreventing similar disasters, scientists and geotechnical engineers are using soil sensors to investigate solutions in a project called, INFRASAFE (Intelligent monitoring for safe infrastructures) funded by the Emilia Romagna Region (Italy) on European Funds.
Professor Alberto Lamberti, Professor Guido Gottardi, Department of Civil, Chemical, Environmental, and Materials Engineering, University of Bologna, along with Prof. Marco Bittelli, University of Bologna professor of Soil and Environmental Physics, installed soil sensors along some transects of the Secchia river to monitor water potential and piezometric pressure. They want to study properties of the compacted levee “soil”, during intense flooding. Bittelli comments, “Rainfall patterns are changing due to climate change, and we are seeing more intense floods. There is a concern about monitoring levees so that we can, through studying the process, eventually create a warning system.”
Trench for burying sensor cables.
What Are The Levees Made Of?
Amazingly, some of these levees are very old, built at the beginning of the second millennium to protect the Secchia valley population from floods. “These rudimentary barrages were the starting point of the huge undertakings, aiming at the regulation and stabilization of the river, which were gradually developed and expanded in the following centuries…building up a continuous chain all along the river.” (Marchii et. al., 1995)
Vegetation in the Secchia River floodplain.
Unlike natural soil with horizons, the soil that makes up the levees is made up of extremely compact clay and other materials, which will pose challenges to the research team in terms of sensor installation. The team will use soil sensors to determine when the compacted material that makes up the levees gets so saturated it becomes weak. Bittelli says, “We are looking at the mechanical properties of the levees, but mechanical properties are strongly dependent on hydraulic properties, particularly soil water potential (or soil suction). A change in water potential changes the mechanical properties and weakens the structure.” This can happen either when a soil dries below an optimal limit or wets above it; the result is a weakened barrier that can fail under load.
Here the team uses an installation tool to install water content sensors.
Soil Sensors Present Installation Challenges
To solve the installation problems, the team will use a specialized installation tool to insert their water content sensors. Bittelli says, “Our main challenge is to install sensors deep into the levees without disturbing the soil too much. It’s very important to have this tool because clearly, we cannot dig out a levee; we might be the instigator of a flood. So it was necessary for us to be able to install the sensors in a relatively small borehole.” The researchers will install the sensors farther down than the current tool allows, so they are modifying it to go down to eight or ten meters. Bittelli explains, “We used a prototype installation tool which is two meters long. We modified it in the shop and extended it to six meters to be able to install water content sensors at further depths.”
Another challenge facing the research team is how to install water potential sensors without disturbing the levee. Marco explains, “We placed an MPS-6 (now called TEROS 21) into a cylinder of local soil prepared in the lab. A sort of a muffin made of soil with an MPS-6 inside. Then we lowered the cylinder into the borehole, installed the sensor inside, and then slid it down into the hole. Our goal is to try and keep the structure of the soil intact. Since the cylinder is made of the same local soil, and it is in good contact with the borehole walls, hydraulic continuity will be established.”
Researchers placed an MPS-6 into a cylinder of local soil prepared in the lab.
Unlike installing water content sensors, matric potential sensors don’t need to be installed in undisturbed soil but only require good contact between the sensor and the bulk soil so liquid water can easily equilibrate between the two. The researchers are also contemplating using a small camera with a light so they can see from above if the installation is successful.
Find Out More
The researchers will collect data at two experimental stations, one on the Po river, and one on the Secchia River. So far, the first installation was successfully performed, and data are collected from the website. Bitteli says the first installation included water content, temperature, and electrical conductivity sensors, water potential sensors, and tensiometers connected to a wireless network that will transmit all the data to a central office for analysis.
You can read more about this project and how it’s progressing here.
Get more information on applied environmental research in our
Many researchers carefully choose the right instrumentation for their projects, but when it comes to installing the soil sensor into the soil, they are less than careful about the process. Researchers need to know how to install sensors in a way that will allow them to get the most accurate data the instruments are capable of.
Georg von Unold
Georg von Unold has almost two decades of experience installing all types of soil sensors and a German eye for precision that is unmatched in our experience. As the president and founder of UMS (now METER Ag), a German company that develops and manufactures precision soils instrumentation, and a close friend, we thought there would be no one better to share a couple of ideas on careful installation. Here’s what he had to say:
Pick the Right Place to Install your Sensors
When we develop research instrumentation we look at the accuracy and the resolution of our instruments from a technical point of view. However, the heterogeneity of research sites can be so vast that we have to take care to select a research site that is representative from a scientific point of view of the results we would like to publish. We do this first by analyzing the biosphere above the soil that is visible to us, and then perhaps doing some auguring into the soil at various sites to investigate what might be going on in different areas of the field. If you are researching on a farm, it is important to ask the grower where he’s had good and bad harvest results, where he’s needed to irrigate, and where he’s had problems with erosion. Always interview people who know the history and specifics of the sites first, because if the sites are flooded or at risk for landslides, it will be a bad choice for long-term monitoring. Investigating the right place for your sensors before you install will save you time and help you obtain the most applicable and accurate data for your research.
We knew that gravel would have bad capillary contact because the stones would have holes between them.
Be Careful with the Way you Install Sensors
One of our research projects used tensiometers to try and determine how water flowed through gravel. We knew that gravel would have bad capillary contact because the stones would have holes between them. So we decided to make a slurry of fine material from this gravel soil and put it in the installation hole so that the tensiometer would have better capillary contact. It was a good idea, but it led to misleading results. What we ended up with was a kind of water reservoir with fine material around the tensiometer which had nothing to do with the true moisture situation in the gravel. The tensiometer gave us wonderful readings: very constant but with no dynamics that would have been typical for a gravel soil. When we took it into the lab to investigate, we realized we’d built an artificial soil around our tensiometer. We weren’t measuring the gravel but were measuring our artificial error which we had created so carefully. The other thing we found is that over the course of time our slurry would move away from the tensiometer, and within a few years, the tensiometer would be simply hanging in a big gap. This project also contained fine, heavy soils. Eventually, we realized that we needed an auguring tool that would not push the soil away or compact the soil where we placed the tensiometer because compaction would mean different hydraulic behavior. So we asked our friends at a Dutch company to make us an auger that was shaped in a form that wouldn’t change the natural soil density that we wanted to measure.
It is important to be careful when you install sensors. For example, if you have a clay soil and you auger a bigger hole than your tensiometer, you will have a water tube around your sensor. If your soil flooded, the water would flow down your shaft to where your tensiometer is placed, and then what are you measuring? Thus it is necessary to seal the shaft or to prevent access of surface water to a deeper horizon.
You need to remember that if you want to measure temperature at a depth of one meter below the surface, the thermal conductivity is strongly dependent on the kind of soil and the moisture of the soil.
Beware of Simple Mistakes
You can also make simple mistakes with other types of soil sensors, such as temperature probes. You need to remember that if you want to measure temperature at a depth of one meter below the surface, the thermal conductivity is strongly dependent on the kind of soil and the moisture of the soil. If, for example, you put a temperature probe wired with copper wires in a dry sand or gravel, you will get an average value of the temperature of the sunlight exposed hot cable. The reason is that the copper is leading the temperature down to where you measure and has a much higher conductivity compared to dry, coarse soil. Thus it is important to think through your installation processes because it is likely you will have a different installation method in a clay soil versus a gravel soil.
