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Which Soil Sensor Should I Choose?

Dr. Colin Campbell, METER soil scientist, explains soil sensor differences, pros, cons, and things to consider when choosing which sensor will best accomplish your research goals. Use the following considerations to help identify the perfect sensor for your research.  Explore the links for a more in-depth look at each topic.

Scientists often measure soil moisture at different depths to understand the effects of soil variability and to observe how water is moving through the soil profile.

CHOOSE THE RIGHT MEASUREMENT

  • Volumetric Water Content:  If a researcher wants to measure the rise and fall of the amount (or percentage) of water in the soil, they will need soil moisture sensors. Soil is made up of water, air, minerals, organic matter, and sometimes ice.  As a component, water makes up a percentage of the total.  To directly measure soil water content, one can calculate the percentage on a mass basis (gravimetric water content) by comparing the amount of water, as a mass, to the total mass of everything else.  However, since this method is labor-intensive, most researchers use soil moisture sensors to make an automated volume-based measurement called Volumetric Water Content (VWC). METER soil moisture sensors use high-frequency capacitance technology to measure the Volumetric Water Content of the soil, meaning they measure the quantity of water on a volume basis compared to the total volume of the soil.  Applications that typically need soil moisture sensors are watershed characterization, irrigation schedulinggreenhouse management, fertigation management, plant ecology, water balance studies, microbial ecology, plant disease forecasting, soil respiration, hydrology, and soil health monitoring.
  • Water potential:  If you need an understanding of plant-available water, plant water stress, or water movement (if water will move and where it will go), a water potential measurement is required in addition to soil moisture. Water potential is a measure of the energy state of the water in the soil, or in other words, how tightly water is bound to soil surfaces. This tension determines whether or not water is available for uptake by roots and provides a range that tells whether or not water will be available for plant growth. In addition, water always moves from a high water potential to a low water potential, thus researchers can use water potential to understand and predict the dynamics of water movement.

Understand your soil type and texture

In soil, the void spaces (pores) between soil particles can be simplistically thought of as a system of capillary tubes, with a diameter determined by the size of the associated particles and their spatial association.  The smaller the size of those tubes, the more tightly water is held because of the surface association.

Clay holds water more tightly than a sand at the same water content because clay contains smaller pores and thus has more surface area for the water to bind to. But even sand can eventually dry to a point where there is only a thin film of water on its surfaces, and water will be bound tightly.  In principle, the closer water is to a surface, the tighter it will be bound. Because water is loosely bound in a sandy soil, the amount of water will deplete and replenish quickly.  Clay soils hold water so tightly that water movement is slow. However, there is still available water.

Note: Use the PARIO soil texture analyzer to automate soil texture identification.

Two measurements are better than one

In all soil types and textures, soil moisture sensors are effective at measuring the percentage of water. Dual measurements—using a water potential sensor in addition to a soil moisture sensor—gives researchers the total soil moisture picture and are much more effective at determining when, and how much, to water.  Water contendata show subtle changes due to daily water uptake and also indicate how much water needs to be applied to maintain the root zone at an optimal level.  Water potential data determine what that optimal level is for a particular soil type and texture.

Get the big picture with moisture release curves  

Dual measurements of both water content and water potential also enable the creation of in situ soil moisture release curves (or soil water characteristic curves) like the one below (Figure 1), which detail the relationship between water potential and water content.  Scientists and engineers can evaluate these curves in the lab or the field and understand many things about the soil, such as hydraulic conductivity and total water availability.

Figure 1. Turfgrass soil moisture release curve (black). Other colors are examples of moisture release curves for different types of soil.

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Apply now for the 2018 Grant A. Harris Fellowship

University of Idaho graduate student and past Grant A. Harris Fellowship recipient, Adrianne Zuckerman, is taking a different approach to stream restoration than the traditional approach, channel manipulation, which often requires heavy equipment and major disruption to the riparian area.

 

 

Zuckerman set out to understand how vegetation lining the stream bank impacts habitat quality for anadromous salmon and steelhead in Washington’s Methow River, which flows through the eastern Cascades. Zuckerman wanted to know how tree species composition affects the amount of nutrients available to the benthic insect community, since they are a critical food source for young salmonid fish.

When Zuckerman began investigating methods for measuring leaf contribution to the stream, she found that leaf litter traps were the standard equipment. Leaf litter traps are time consuming to set and maintain, and data analysis consists of frequent visits to the field followed by extensive time in the lab processing leaf material.

Looking for an alternative method, she discovered the LP-80 ceptometer: a lightweight, field-portable instrument for measuring leaf area index. Using the LP-80, Zuckerman was able to rapidly assess the leaf area contribution of each tree species along the riparian corridor. Using this information, it was relatively straightforward for her to estimate the contribution of each tree species to the stream food web.

Zuckerman’s research will help land managers and other researchers understand the importance of riparian vegetation for maximizing the food available to salmonid fish species. Improvement and maintenance of optimal stream-side vegetation composition should ultimately help to enhance salmon populations in the Pacific Northwest.

Receive $10,000 in METER research instrumentation

The Grant A. Harris Fellowship awards $40,000 in research instrumentation (four $10,000 awards) annually to graduate students studying any aspect of agricultural, environmental, or geotechnical science.  METER is now accepting applications for the 2018 award. Learn more about how to apply for the Grant A. Harris Fellowship here.

Get more information on applied environmental research in our

A comparison of water potential instrument ranges

Water potential is the most fundamental and essential measurement in soil physics because it describes the force that drives water movement.

Water potential helps researchers determine how much water is available to plants.

Making good water potential measurements is largely a function of choosing the right instrument and using it skillfully.  In an ideal world, there would be one instrument that simply and accurately measured water potential over its entire range from wet to dry.  In the real world, there is an assortment of instruments, each with its unique personality.  Each has its quirks, advantages, and disadvantages.  Each has a well-defined range.

Below is a comparison of water potential instruments and the ranges they measure.

water potential instrument ranges

A comparison of water potential instrument ranges

To learn more about measuring water potential, see the articles or videos below:

 

Improving the Efficiency of Ground-Source Heat Exchange Systems

In an effort to find sustainable energy solutions for heating and cooling buildings, many homeowners, companies, and university campuses are turning to ground-source heat exchange systems (GSHE) to reduce energy usage and greenhouse gas emissions. GSHE systems are designed to take advantage of the moderate and nearly constant temperatures in the ground as the exchange medium for space heating and cooling and to heat water for domestic use.

Ground-source heat exchange systems

Some universities are exploring the development of GHSE systems.

In these systems, water or specially formulated geothermal fluid is circulated through plastic pipes (i.e., ground loops) installed in vertical boreholes. In the winter, geothermal loops tap heat from the ground, while in the summer, heat from the surface is transferred into the ground. Currently, the application of ground-source heat exchange systems reduces overall carbon emissions by up to 50%, and according to the U.S. Department of Energy, they are up to 4 times more efficient than gas furnaces.

But are GSHE systems as efficient as they claim to be? The answer, according to researchers at the University of Illinois at Urbana-Champaign (UIUC), is that it depends. Drs. Yu-Feng Forrest Lin and Andrew Stumpf and their associates at the Illinois State Geological Survey (a division of the Prairie Research Institute) at the UIUC and their collaborator, Dr. James Tinjum from the University of Wisconsin–Madison (UWM), are working on a project funded by the UIUC Student Sustainability Committee (SSC) to improve the efficiency of GSHE systems. They also hope to show that ground-source heat exchange systems systems could be included in the University’s multifaceted sustainability plan to reduce carbon emissions on campus to zero by 2050. Members of their research team are trying to determine whether GSHE systems would be feasible for heating and cooling buildings on campus with the existing subsurface geologic conditions.

Ground-source heat exchange systems

Diagram showing ~50% reduction of energy using GHSEs (from USEPA)

The UIUC is not the first university to explore the development of GSHE systems. For example, Ball State University recently replaced its coal-powered heating and cooling system on campus with a large district-scale GSHE system. Other universities with similar systems include the Missouri Institute of Science and Technology and the University of Notre Dame. These ground-source heat exchange systems are specifically designed to meet future energy needs. However, as Dr. Stumpf notes, “Historically, quite a few large district-scale systems have not achieved their projected efficiencies. Some systems have even overheated the ground, forcing them to go off-line. We’re trying to come up with a way to make borehole fields more efficient and prevent these hazards from occurring.”

Why do some ground-source heat exchange systems not meet their efficiency targets?

Dr. Stumpf explains that many times, the contractors that install ground-source heat exchange systems do a single conductivity measurement in the borehole. Or they run a thermal response test (TRT) and then use these calculations to determine the conductivity of the geologic materials at the proposed site. In many cases, however, especially for district-scale GSHE systems with multiple large borefields and a complex geology, this information does not adequately characterize the site conditions. He states, “Because only limited measurements are taken, many systems have developed problems and are unable to keep up with the thermal demands.”

Ground-source heat exchange systems

University of Illinois campus.

To assist contractors and other groups involved in designing and installing ground-source heat exchange systems, the UIUC research team is studying the thermal conditions in a shallow geoexchange system and collecting data from geologic samples from a 100-m-deep borehole located on the UIUC Energy Farm. A fiber-optic distributed temperature sensing (FO-DTS) system is being used to collect detailed temperature measurements in this borehole during and after a TRT. The FO-DTS system is an emerging technology that utilizes laser light to measure temperature along the entire length of a standard telecommunications fiber-optic cable. By analyzing the laser’s backscattered energy, the team can estimate temperatures along the entire sensor cable as a continuous profile. The ground temperature can be measured every 15 seconds, in every meter along the cable, with a resolution from 0.1 to 0.01 °C (depending on the measurement integration time). These data can be integrated with the TRT results, ultimately providing a better understanding of the subsurface thermal profile, which will lead to increasing the efficiency of the GSHE system.

Continuous core collected from the 100-m borehole was subsampled to measure the thermal properties of the subsurface geologic units, and testing was performed at the UWM with a thermal properties analyzer. The resulting information will provide a better understanding of how thermal energy is stored and transported in the subsurface.

Ground-source heat exchange systems

Geologic and geophysical logs from the borehole at the UIUC Energy Farm

How is the UIUC Energy Farm site unique? 

Dr. Stumpf states that the ground under the UIUC Energy Farm includes various geologic materials that conduct heat differently and require some additional design considerations. He explains, “The upper 60 m of the borehole was drilled into glacial sediment, including till, outwash (sand and gravel), and lake sediment (silt and clay), which have different thermal conductivities. Flowing groundwater in the sand and gravel units also increases the thermal transport. Conversely, the bottom 40 m of the borehole penetrated Pennsylvanian-age bedrock, mostly shale and siltstone, which included layers of coal. Unlike the other lithologies, coal has a very low thermal conductivity and is therefore not optimal for a GSHE system. The most efficient GSHE systems avoid low-conductivity geologic units and are optimized to take advantage of flowing groundwater. 

To learn more about this research project, visit the UIUC sustainability project site or the ISGS blog.

How to Create a Full Soil Moisture Release Curve

Two Old Problems

Soil moisture release curves have always had two weak areas: a span of limited data between 0 and -100 kPa and a gap around field capacity where no instrument could make accurate measurements.

Soil moisture release curve

Using HYPROP with the redesigned WP4C, a skilled experimenter can now make complete high resolution moisture release curves.

Between 0 and -100 kPa, soil loses half or more of its water content. If you use pressure plates to create data points for this section of a soil moisture release curve, the curve will be based on only five data points.

And then there’s the gap. The lowest tensiometer readings cut out at -0.85 MPa, while historically the highest WP4 water potential meter range barely reached -1 MPa. That left a hole in the curve right in the middle of plant-available range.

New Technology Closes the Gap

Read more

How to Assess Maximum Potential Biomass Production—Simplified

The conversion of light energy and atmospheric carbon dioxide to plant biomass is fundamentally important to both agricultural and natural ecosystems.

biomass

Potato field

The detailed biophysical and biochemical processes by which this occurs are well understood. At a less-detailed level, however, it is often useful to have a simple model that can be used to understand and analyze parts of an ecosystem. Such a model has been provided by Monteith (1977). He observed that when biomass accumulation by a plant community is plotted as a function of the accumulated solar radiation intercepted by the community, the result is a straight line. Figure 1 shows Monteith’s results.

biomass

Figure 1. Total dry matter produced by a crop as a function of total intercepted radiation (from Monteith, 1977).

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Can a Leaf Wetness Sensor be a Rain Detector?

The PHYTOS 31 Leaf Wetness Sensor was designed to measure the presence and duration of water on leaf surfaces. However, Dr. Bruce Bugbee, professor of Crop Physiology at Utah State University, noticed that his leaf wetness sensor revealed interesting phenomena associated with some precipitation events. Here is what he observed on a recent day at the USU Environmental Observatory in Logan, Utah

leaf wetness sensor

It is possible to have a day with numerous 0.1 mm increments of rain, followed by some evaporation, in which a rain gauge would not record any rain during the day.

“Recent data from our weather station provided two examples of the offset in measurement associated with tipping bucket rain gauges. It started raining on campus last night at exactly 20:00 hours, as indicated by the response of the leaf wetness sensor (Figure 1). The first 0.1 mm tip of the rain gauge occurred about 25 minutes later (Figure 2). The resolution for most high-quality tipping bucket rain gauges is listed as 0.1 mm, but this is not the resolution for the first 0.1 mm of rain.

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How to Protect your Soil Moisture Sensors from Lightning Surge

We occasionally see soil moisture sensors damaged by lightning.  Here’s what to do to protect them.

The secondary products of a lightning strike include electromagnetic pulses, electrostatic pulses, and earth current transients.

lighting

Surge suppression components typically perform their suppression function by temporarily short circuiting the voltage between two wires, several devices, or ground.

Electromagnetic pulses are created by the strong magnetic field that is formed by the short term current flow taking place in the lightning strike. With current flows as high as 510kA per microsecond, these currents create very large magnetic fields. These short term magnetic fields then induce voltages onto wires and cables.

Electrostatic pulses are created by electrostatic fields that accompany a thunderstorm. Any cable suspended above the earth during a thunderstorm is immersed in the electrostatic field and will be electrically charged. Quick changes in the charges stored in both the clouds and earth take place whenever there is a lightning strike. The charge on the cable must now be discharged or neutralized. Unable to find a path to ground (earth), it breaks down insulation and component in its efforts to return to earth.

Earth current transients are the direct result of the neutralization process that immediately follows the end of lightning strike. Neutralization is accomplished by the movement or redistribution of charge along or near the earth’s surface from all the points where the charge had been initially induced to the point where the lightning strike has just terminated. Earth current transients create a shift in potential across a ground plan, often called a “ground bounce”.

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Stop Hiding Behind a Shield

Get better air temperature accuracy with this new method

Accurate air temperature is crucial for microclimate monitoring

The accuracy of air temperature measurement in microclimate monitoring is crucial because the quality of so many other measurements depend on it. But accurate air temperature is more complicated than it looks, and higher accuracy costs money. Most people know if you expose an air temperature sensor to the sun, the resulting radiative heating will introduce large errors. So how can the economical ATMOS 41’s new, non-radiation-shielded air temperature sensor technology be more accurate than typical radiation-shielded sensors?

We performed a series of tests to see how the ATMOS 41’s air temperature measurement compared to other sensors, and the results were surprising, even to us. Learn the results of our experiments and the new science behind the extraordinary accuracy of the ATMOS 41’s breakthrough air temperature sensor technology.

In this brief 30-minute webinar, find out:

  • Why you should care about air temperature accuracy
  • Where errors in air temperature measurement originate
  • The first principles energy balance equation and why it matters
  • Results of experiments comparing shielded sensor accuracy against the ATMOS 41
  • The science behind the ATMOS 41 and why its unshielded measurement actually works

 

Register for the Live Event

Date: July 26, 2017 – 8:00 AM – 8:30 AM PDT

Soil Moisture Sensors: Which Installation Method is Best?

Patterns of water replenishment and use give rise to large spatial variations in soil moisture over the depth of the soil profile. Accurate measurements of profile water content are therefore the basis of any water budget study. When monitored accurately, profile measurements show the rates of water use, amounts of deep percolation, and amounts of water stored for plant use.

How to avoid measurement errors

Three common challenges to making high-quality volumetric water content measurements are:

  1. making sure the probe is installed in undisturbed soil,
  2. minimizing disturbance to roots and biopores in the measurement volume, and
  3. eliminating preferential water flow to, and around, the probe.

All dielectric probes are most sensitive at the surface of the probe. Any loss of contact between the probe and the soil or compaction of soil at the probe surface can result in large measurement errors. Water ponding on the surface and running in preferential paths down probe installation holes can also cause large measurement errors.

Installing soil moisture sensors will always involve some digging. How do you accurately sample the profile while disturbing the soil as little as possible?  Consider the pros and cons of five different profile sampling strategies.

Preferential flow is a common issue with commercial profile probes

Profile probes are a one-stop solution for profile water content measurements. One probe installed in a single hole can give readings at many depths. Profile probes can work well, but proper installation can be tricky, and the tolerances are tight. It’s hard to drill a single, deep hole precisely enough to ensure contact along the entire surface of the probe. Backfilling to improve contact results in repacking and measurement errors. The profile probe is also especially susceptible to preferential-flow problems down the long surface of the access tube.

Trench installation is arduous

Installing sensors at different depths through the side wall of a trench is an easy and precise method, but the actual digging of the trench is a lot of work. This method puts the probes in undisturbed soil without packing or preferential water-flow problems, but because it involves excavation, it’s typically only used when the trench is dug for other reasons or when the soil is so stony or full of gravel that no other method will work. The excavated area should be filled and repacked to about the same density as the original soil to avoid undue edge effects.

soil moisture sensors

Digging a trench is a lot of work.

Augur side-wall installation is less work

Installing probes through the side wall of a single augur hole has many of the advantages of the trench method without the heavy equipment. This method was used by Bogena et al. with EC-5 probes. They made an apparatus to install probes at several depths simultaneously. As with trench installation, the hole should be filled and repacked to approximately the pre-sampling density to avoid edge effects.

Multiple-hole installation protects against failures

Digging a separate access hole for each depth ensures that each probe is installed into undisturbed soil at the bottom of its own hole. As with all methods, take care to assure that there is no preferential water flow into the refilled augur holes, but a failure on a single hole doesn’t jeopardize all the data, as it would if all the measurements were made in a single hole.

The main drawback to this method is that a hole must be dug for each depth in the profile. The holes are small, however, so they are usually easy to dig.

Single-hole installation is least desirable

It is possible to measure profile moisture by auguring a single hole, installing one sensor at the bottom, then repacking the hole, while installing sensors into the repacked soil at the desired depths as you go. However, because the repacked soil can have a different bulk density than it had in its undisturbed state and because the profile has been completely altered as the soil is excavated, mixed, and repacked, this is the least desirable of the methods discussed. Still, single-hole installation may be entirely satisfactory for some purposes. If the installation is allowed to equilibrate with the surrounding soil and roots are allowed to grow into the soil, relative changes in the disturbed soil should mirror those in the surroundings.

Reference

Bogena, H. R., A. Weuthen, U. Rosenbaum, J. A. Huisman, and H. Vereecken. “SoilNet-A Zigbee-based soil moisture sensor network.” In AGU Fall Meeting Abstracts. 2007. Article link.

Read more soil moisture sensor installation tips.

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