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Posts from the ‘Geophysics’ Category

5 ways site disturbance impacts your data—and what to do about it

Lies we tell ourselves about site disturbance

When it comes to measuring soil moisture, site disturbance is inevitable. We may placate ourselves with the idea that soil sensors will tell us something about soil water even if a large amount of soil at the site has been disturbed. Or we might think it doesn’t matter if soil properties are changed around the sensor because the needles are inserted into undisturbed soil.

site disturbance

The key to reducing the impact of site disturbance on soil moisture data is to control the scale of the disturbance.

The fact is that site disturbance does matter, and there are ways to reduce its impact on soil moisture data. Below is an exploration of site disturbance and how researchers can adjust their installation techniques to fight uncertainty in their data.

Non-disturbance methods don’t measure up—yet

During a soil moisture sensor installation, it’s important to generate the least amount of soil disturbance possible in order to obtain a representative measurement. Non-disturbance methods do exist, such as satellite, ground-penetrating radar, and COSMOS. However, these methods face challenges that make them impractical as a single approach to water content. Satellite has a large footprint, but generally measures the top 5-10 cm of the soil, and the resolution and measurement frequency is low. Ground-penetrating radar has great resolution, but it’s expensive, and data interpretation is difficult when a lower boundary depth is unknown. COSMOS is a ground-based, non-invasive neutron method which measures continuously and reaches deeper than a satellite over an area up to 800 meters in diameter. But it is cost prohibitive in many applications and sensitive to both vegetation and soil, so researchers have to separate the two signals. These methods aren’t yet ready to displace soil moisture sensors, but they work well when used in tandem with the ground truth data that soil moisture sensors can provide.

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Double Ring vs. SATURO: Two Infiltrometers Go Head to Head

The SATURO and the double-ring infiltrometer are both ring infiltrometers that infiltrate water from the surface into soils. Overall, they compare fairly well (see comparison). The main difference is how they deal with three-dimensional flow in the Kfs calculation. The SATURO uses the multiple-ponded head analysis approach to get a more direct estimation of alpha, which is used to determine how the soil pulls the water laterally. The double-ring infiltrometer uses a larger outer ring to act as a buffer from three-dimensional flow. This requires more water, and literature suggests that it doesn’t perform well. Also, with a double-ring infiltrometer, there is still a need to estimate alpha in the equations. This is typically done from a look-up table based on soil type and often results in error.

Infiltrometers

The SATURO is an automated infiltrometer which uses the multiple-ponded head analysis approach.

How do SATURO readings compare to double-ring infiltrometer readings?

We compared the SATURO with a 6-inch (15.24 cm) inner ring diameter against a double-ring infiltrometer with a 6-inch (15.24 cm) inner ring diameter and an outer ring with a 12-inch (30.48 cm) diameter.

Hydrology 301: What a Hydraulic Conductivity Curve Tells You & More

Hydraulic conductivity is the ability of a porous medium (soil for instance) to transmit water in saturated or nearly saturated conditions. It’s dependent on several factors: size distribution, roughness, tortuosity, shape, and degree of interconnection of water-conducting pores. A hydraulic conductivity curve tells you, at a given water potential, the ability of the soil to conduct water.

One factor that affects hydraulic conductivity is how strong the structure is in the soil you’re measuring.

For example, as the soil dries, what is the ability of water to go from the top of a sample [or soil layer in the field] to the bottom. These curves are used in modeling to illustrate or predict what will happen to water moving in a soil system during fluctuating moisture conditions. Researchers can combine hydraulic conductivity data from two laboratory instruments, the KSAT and the HYPROP, to produce a full hydraulic conductivity curve (Figure 1).

Hydraulic conductivity curve

Figure 1. Example of hydraulic conductivity curves for three different soil types. The curves go from field saturation on the right to unsaturated hydraulic conductivity on the left.  They illustrate the difference between a well-structured clayey soil to a poorly structured clayey soil and the importance of structure to hydraulic conductivity especially at, or near, saturation.

In Hydrology 301, Leo Rivera, Research Scientist at METER, discusses hydraulic conductivity and the advantages and disadvantages of methods used to measure it.

Watch the webinar below.

 

 

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Lab versus in situ soil water characteristic curves—a comparison

The HYPROP and WP4C provide the ability to make fast, accurate soil moisture release curves (soil water characteristic curves-SWCCs), but lab measurements have some limitations: sample throughput limits the number of curves that can be produced, and curves generated in a laboratory do not represent their in situ behavior. Lab-produced soil water retention curves can be paired with information from in situ moisture release curves for deeper insight into real world variability.

soil water characteristic curves

Soil water characteristic curves help determine soil type, soil hydraulic properties, and mechanical performance and stability

Moisture release curves in the field? Yes, it’s possible.

Colocating matric potential sensors and water content sensors in situ add many more moisture release curves to a researcher’s knowledge base. And, since it is primarily the in-place performance of unsaturated soils that is the chief concern to geotechnical engineers and irrigation scientists, adding in situ measurements to lab-produced curves would be ideal.

In this brief 20-minute webinar, Dr. Colin Campbell, METER research scientist, summarizes a recent paper given at the Pan American Conference of Unsaturated Soils. The paper, “Comparing in situ soil water characteristic curves to those generated in the lab” by Campbell et al. (2018), illustrates how well in situ generated SWCCs using the TEROS 21 calibrated matric potential sensor and METER’s GS3 water content sensor compare to those created in the lab.

Watch the webinar below:

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Lab vs. field instruments—when to use both

Whether researchers measure soil hydraulic properties in the lab or in the field, they’re only getting part of the picture. Laboratory systems are highly accurate due to controlled conditions, but lab measurements don’t take into account site variability such as roots, cracks, or wormholes that might affect soil hydrology. In addition, when researchers take a sample from the field to the lab, they often compress soil macropores during the sampling process, altering the hydraulic properties of the soil.

Field sensors

Roots, cracks, and wormholes all affect soil hydrology

Field experiments help researchers understand variability and real time conditions, but they have the opposite set of problems. The field is an uncontrolled system. Water moves through the soil profile by evaporation, plant uptake, capillary rise, or deep drainage, requiring many measurements at different depths and locations. Field researchers also have to deal with the unpredictability of the weather. Precipitation may cause a field drydown experiment to take an entire summer, whereas in the lab it takes only a week.

The big picture—supersized

Researchers who use both lab and field techniques while understanding each method’s strengths and limitations can exponentially increase their understanding of what’s happening in the soil profile. For example, in the laboratory, a researcher might use the PARIO soil texture analyzer to obtain accurate soil texture data, including a complete particle size distribution. They could then combine those data with a HYPROP-generated soil moisture release curve to understand the hydraulic properties of that soil type. If that researcher then adds high-quality field data in order to understand real world field conditions, then suddenly they’re seeing the larger picture.

Field instruments

Table 1. Lab and field instrument strengths and limitations

Below is an exploration of lab versus field instrumentation and how researchers can combine these instruments for an increased understanding of their soil profile. Click the links for more in-depth information about each topic.

Particle size distribution and why it matters

Soil type and particle size analysis are the first window into the soil and its unique characteristics. Every researcher should identify the type of soil that they’re working with in order to benchmark their data.

Field instruments

Particle size analysis defines the percentage of coarse to fine material that makes up a soil

If researchers don’t understand their soil type, they can’t make assumptions about the state of soil water based on water content (i.e., if they work with plants, they won’t be able to predict whether there will be plant available water). In addition, differing soil types in the soil’s horizons may influence a researcher’s measurement selection, sensor choice, and sensor placement.

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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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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

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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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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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