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Posts tagged ‘Irrigation’

Apr 7

Green Roofs—Do They Work? (Part II)

Innovative soil scientist, John Buck, and his team have discovered that green roofs have more capacity than people imagined (see part I).  Below are some of the challenges he sees for the future, and the type of measurements he suggests researchers take, as they continue to validate the effectiveness of these urban ecosystems.

Green and whited plant on a garden rooftop with orange rocks

A green roof is essentially a garden on a roof, but rather than growing plants in soil, installers use a synthetic substrate made of expanded shale, expanded clay, crushed brick, or other highly porous, lightweight material.

New Challenges for Green Roofs

Green roof results are promising, but they present a new challenge:  making sure the plants have enough water. The crux of the challenge is that the lightweight, expanded shale/clay substrate material, the standard in green roof design, does a good job of soaking up the water, but has some peculiar properties that are unlike typical soils.  Specifically, the expanded shale and expanded clay media tend to be dominated by sand and fine gravel-sized particles that provide a high proportion of macropores, but the interior porosity of the large particles is dominated with micropores.  That pore size distribution leads researchers to two important questions— How much water will be readily available for plant growth? And, will the unsaturated hydraulic conductivity be adequate to avoid starving the roots under high-evaporative demand by allowing water to flow to roots from the bulk soil? These are critical questions as green roof technologies continue to evolve.

Overhead close up of garden roof plant

Researchers wonder, will the unsaturated hydraulic conductivity be adequate to avoid starving the roots under high-evaporative demand.

Measurements Required for Green Roof Validation

Still, Buck has learned a great deal from his work.  Considering the wild spatial distribution of summer storms, quantitative green roof performance studies require that rainfall be measured locally. Monitoring of soil volumetric moisture content measurements in concert with rainfall and soil lysimeter measurements of drainage, reveal the degree of total and capillary saturation, drainage rate, and porosity available for storage. Soil water potential sensors, placed within the capillary fringe of water ponded over subsurface drainage layers, can provide useful insights regarding the dryness of the drainage layer and overlying soil, as well as the available storage of stormwater within the drainage layer.

Direct measurement of soil drainage using lysimeters is a key supplemental measurement on green roof performance quantification projects because there is an unmeasured component of water storage where drought-resistant alpine succulents (typically Sedum species) are used on green roofs.  The Sedum plants can absorb up to 10 mm of rainfall equivalent in their plant tissues.

Plants poking out of the soil in front of a house

Measurement of soil drainage using lysimeters is a key supplemental measurement on green roof performance quantification projects.

Other Projects and Future Plans

At ground level, Buck is quantifying the performance of intensive stormwater infiltration areas known as rain gardens, bioretention areas, or more generically, infiltration-based stormwater best management practices (Infiltration-based BMPs).  When monitoring infiltration-based stormwater BMPs, Buck has used similar tools to those used on green roofs, but has added water-level sensors and piezometers.  Buck has found that ancillary measurements of electrical conductivity, often available on water content sensors, along with surface and pore water sampling, can be used to document transformations taking place in infiltration systems.  These measurements now combine to show that green roofs and infiltration-based BMPs are indeed making a difference to urban environments and contributions to CSOs.  The challenge now is how to implement this technology more widely.  But, with the validation now in hand, that job should be quite a bit easier.

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Apr 1

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Green Roofs—Do They Work?

Green roofs are being built in large cities to provide stormwater management, reduce the urban heat island effect, and improve air quality—but are they effective?   John Buck, an innovative soil scientist based in Pittsburgh, Pennsylvania, has been trying to quantitatively answer this question in many different cities using soil monitoring equipment in order to determine the efficacy and best types of green infrastructure for managing stormwater.  

Garden on a rooftop with flowers and a city around it

A green roof installation site at the Allegheny County Office Building in Pennsylvania.

Why Green Roofs?

In older cities, stormwater runoff is typically combined with sewage flows, and these combined waters are treated at a sewage treatment plant during dry weather and light rain events. Unfortunately, during more substantial storms (sometimes just a few mm of rain) the combined flows exceed the ability of the sewage treatment plant, and are discharged without treatment to surface waters as “combined sewage overflows” (CSOs). One of the ways to mitigate CSOs is to capture and store stormwater to keep it out of the combined sewer.  

A green roof is essentially a garden on a roof, but rather than growing plants in soil, installers use a synthetic substrate made of expanded shale, expanded clay, crushed brick, or other highly porous, lightweight material with high infiltration rates.  During a storm event, water will soak into the air-filled pore space in the substrate, which acts like a sponge to soak up the rain. Excess water will flow into a subsurface drainage layer and will leave the roof garden via existing roof drains. Because a substantial fraction of the stormwater is stored in the substrate, it can later dissipate through evapotranspiration instead of contributing to stormwater volume and CSOs.

Researcher kneeling testing soil with a soil sensor

Researchers are using soil moisture sensors for measuring temperature, bulk electrical conductivity and volumetric water content in green roofs and green infrastructure.

Finding Answers

Designers and regulators want to know how well green roofs work and if they are being over-engineered. They want answers to questions such as: “What sort of substrate should I be using? What type of plants can survive green roof conditions? Will I need to irrigate the green roof when there are no storms to water the plants?” and, “Will the green roof work as well during a one-inch storm that occurs over a half hour versus a five-inch storm that occurs over five days?”  

Buck is using soil lysimeters and modified tipping bucket rain gauges to measure the quantity, intensity, and quality of water coming into and going out of the green roofs.  He also tracks weather parameters and calculates daily evapotranspiration of landscapes.  Using soil sensors, he measures electrical conductivity (dissolved salts), volumetric water content, and temperature.  He has installed data loggers that send data to the web via GSM cellular connection, allowing stakeholders access to the data in real-time.  This data telemetry provides additional data security, immediately updated results, instant feedback of system problems, and an easy way to share data with others.

Green Roof Runoff Reduction graph

Visualized data of the 87% annualized runoff reduction at Phipps Conservatory green roof site in Pittsburgh, PA.

What Has Been Learned?

Buck discovered that green roofs have much more capacity than people ever imagined.  At The Penfield Apartments in St. Paul, Minnesota, the green roof retained enough water to reduce runoff to about half of a conventional roof, and the peak intensity of the runoff was about one-quarter of what it would have been without the green roof.  At Phipps Conservatory in Pittsburgh, there was an 87% annualized runoff reduction and almost no runoff from typical summer rain events.  Buck comments, “Interestingly, on the Penfield project, we expected better hydrologic performance where soils were thicker, but there was no difference, or results were slightly the reverse of expectations. That reversal was likely due to the confounding influence of irrigation, which was probably non-uniform and not metered or measured by the rain gauge.”

Next week:  Read about some of the challenges John Buck sees for the future, and what kind of measurements he suggests researchers make, as they continue to validate the effectiveness of these urban ecosystems.

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Mar 23

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Can Wastewater Save The United Arab Emirates’ Groundwater? (Part II)

With very little recharge and irrigation comprising 75% of groundwater use, natural water resources in the United Arab Emirates region are disappearing fast (see part I).  Wafa Al Yamani and her PhD advisor, Dr. Brent Clothier, are investigating using treated sewage effluent and groundwater for irrigating the desert forests along UAE motorways.

Abu Dhabi from the ocean looking at the city

Abu Dhabi

Infiltrometers Predict Dripper Behavior:

Wafa and her team used what they call, “the Ankeny twin head method” for site evaluation with infiltrometers, and they’ve been able to use it to predict dripper behavior.  They begin with the head at -60 mm, do a series of measurements to measure steady infiltration, and repeat the process at -5 mm.  They use those measurements to solve Woodings equation which has two unknowns: saturated hydraulic conductivity and capillarity.  Dr. Clothier says, “We’ve done it at two heads, and we can use Woodings equation to solve for the slope of the exponential conductivity curve.  Hence, I can predict with time, the movement of the wetting front away from the dripper.   That’s been very useful to work out what volume of soil we’re wetting.  It tells us if we should have one or two drippers.  In this forest, we think we can get away with two drippers because if they irrigate for two hours, the radius of the wet front will be 20 cm, and the depth will be about 40 cm, which is a sufficient volume of water for the tree roots.”  Dr. Clothier says they also constructed a small dyke around the drippers so they could contain the water inside the drip zone in case of hydrophobicity or uneven sand.  

researcher recording data while sitting on the floor

Wafa on site, using the twin head method.

Treated Effluent Resolves Salinity Issues

Historically, the UAE pumped their sewage effluent into the Arabian Gulf, but recently, there has been a shift toward seeing it as a valuable water resource, not only for the desert forest, but for irrigation of fruit crops and date palms.  Dr. Clothier says, “Once we started getting our results we realized we were irrigating with groundwater that had high salinity, about 10 dS/m, and that treated sewage effluent had only 0.5 dS/m.  This was an important discovery because with the high salinity groundwater, you have to over-irrigate to maintain a salt leaching fraction.  However, when we apply the treated sewage effluent, we immediately see a response in the trees because it has 1/20th of the salt load.”

Dr. Clothier says that there is one problem with the trees responding so well to the sewage effluent.  The treated sewage effluent makes the trees grow taller and faster, so if the ecosystem service you want from the desert forest is that they’re 4-6 meters high, it becomes an issue.  He adds,”This is actually a positive problem, because we can now induce deficit irrigation, thereby creating a larger resource of treated sewage effluent in order to irrigate far more forests.”

Large white irrigation tanks sitting in sand in the desert

Researchers irrigated with water from these tanks which stored groundwater and treated sewage effluent.

What’s The Future?

Dr. Clothier says they started with a pilot study in the UAE in 2014, and it was so successful that they ended up with two fully-funded four-year projects, one on treated sewage effluent, and one investigating the irrigation of date palms. He says they have another 3 ½ years of work in the UAE on these projects, and in the end, their goal is to develop a model for forestry irrigation and soil salinity management, along with developing capability for the measurement and modeling of irrigation impacts on sustainable forestry.  They have recently developed a prototype of a computerized decision support tool for irrigation which will provide sustainable irrigation advice to optimize water use.  The support tool takes into account the need to maintain salt leaching, and actual irrigation records can be entered to enable real-time use.

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Mar 7

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Data loggers: To Bury, or Not To Bury

Globally, the number one reason for data loggers to fail is flooding. Yet, scientists continue to try to find ways to bury their data loggers to avoid constantly removing them for cultivation, spraying, and harvest.  Chris Chambers, head of Sales and Support at METER always advises against it.  He warns,  “Almost all natural systems, even arid ones, will saturate at least once or twice a year—and it only takes once.”  Still…there are innovative scientists who have had some success.

A prototype buriable logger container made from a paint can with sensors attached

A prototype buriable logger container, made from a paint can, PVC elbow, silicone, epoxy putty, and desiccant. Photo Credit: NDSU | Soil Sciences | Soil Physics

The Good

Radu Carcoana, research specialist and Dr. Aaron Daigh, assistant professor at North Dakota State University, use paint cans to completely seal their data loggers before burying. They drill ports for the sensor cables, seal them up, and when they need to collect data, they dig up the cans.  Chambers comments, “So far it looks promising, but we had a long discussion about the consequences of getting any water in those cans. I don’t know what they were sealing the ports with, but they were pretty confident that they could even dunk their paint cans under water.”  The North Dakota research team buried the paint cans last fall, and Chambers says he’s reserving judgment until spring.  Radu comments, “The picture above is just the concept.  The story will continue in April when we see the North Dakota winter toll.” (See update).

The Bad

Chambers has good reason for his skepticism.  If a logger gets saturated even once, its life will be short.  And even if it doesn’t get completely flooded, there is still risk.  As water gets into the enclosure that encases the logger, the resulting high humidity can damage the instrument.  Chambers says, “If loggers that are mounted on a post get a small amount condensation or water inside, they’ll be fine.  But the buried ones have no escape route for water vapor.  If they get wet or are exposed to water vapor even once, they are going to fail. We’ve seen horror stories time and time again. It’s just not a good environment for electronics.”

Five gallon white bucket with rocks and dirt in it

One group of scientists tried burying their loggers in five-gallon buckets.

The Ugly

Chambers likes to relate a cautionary tale about some scientists in Seattle, who buried their data loggers in five-gallon buckets with lids.  They taped their loggers to the lid, but when they dug the buckets up, they were half full of water, and the loggers were dead.  This is because as the buckets filled with water, the loggers were continuously exposed to water-condensing conditions.  After the loggers were repaired, the scientists re-buried them. But, six weeks later, their buckets were again half full of water, and their loggers were dead.

One Success Story So Far

There is one innovative group at Washington State University, however, who can be considered successful.  Postdoctoral research associate Caley Gasch decided she wanted to bury data loggers in the Cook Agricultural Farm, an actively managed field, so they weren’t constantly taking down loggers and causing large gaps in their data.  

Next week: Find out how she was able to solve many of the problems that prevent successful deployment of data loggers underground.

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Feb 17

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Irrigation and Climate Impacts to the Water-Energy Balance of the WI Central Sands (Part I)

Due to controversy over the growing number of high capacity wells in the Wisconsin Central Sands, University of Wisconsin PhD student, Mallika Nocco, is researching how agricultural land use, irrigation, and climate change impact the region’s water-energy balance.  She and her team have uncovered some surprising results.

Fisher women leans in for a kiss with a class 1 trout she caught

A class 1 trout stream has sufficient natural reproduction to sustain populations of wild trout at or near carry capacity.

Water Use Debate

There are class 1 trout streams in the Central Sands region, and some people worry that the increasing number of high capacity wells used for agriculture will reduce the water levels in those streams.  “Lake Huron has lost about 11 feet of water since 2000,” says one resident of the Central Sands area, “and water levels are continuing to drop.” In 2008, the small well he used to pump drinking water went dry, and he blames the high capacity wells.” (Aljazeera America)  On the other side of the debate, agriculture irrigated by these wells is extremely valuable to the state, and growers have taken quite a bit of time to understand the water cycle and their role in it. You can read about their water management goals and accomplishments here.

Updating Former Research

Irrigated agriculture wasn’t prevalent or profitable in the Wisconsin Central Sands until groundwater irrigation with high capacity wells became feasible in the 1950s.  Since then, this relatively small ecological region has gone from 60 high capacity wells in 1960 to over 2,500 today.

Mallika Nocco is studying potential groundwater recharge from irrigated cropping systems that use the wells, hoping to understand if the irrigation water is lost or returned to the groundwater.  She says, “Until now, we’ve been relying on models validated by two lysimeters in the 1970s. Champ Tanner (one of the fathers of environmental biophysics) designed the weighing lysimeters, and they were very accurate, but we wanted to do a larger scale study with multiple crops to get a handle on interannual variability and to improve our understanding of recharge in the region so we can do a better job of managing irrigation and groundwater.”

Lysimeter installation into a dirt and a field

Lysimeter installation into actively managed fields presented challenges that the research team had to overcome.

Measuring Recharge

Nocco used twenty-five drain gauge lysimeters to capture vadose zone flux under potato and maize cropping systems.  She monitored soil water (and temperature) flux by stratifying water content sensors from the soil surface to a depth of 1.4 meters.  She also estimated evapotranspiration (ET) using a porometer to measure stomatal conductance, in addition to obtaining micrometeorology, leaf area index, and gas exchange measurements.

Nocco and her team had to put their sensors in to avoid cultivation, so they extended the drain gauge PVC that comes up to the soil surface and removed it any time there was major fieldwork, whether it was tillage or planting, so that the area over the lysimeter got the same treatment as the rest of the agricultural fields.

Below the Root Zone

Nocco says getting the lysimeters below the root zone was a challenge.  Next week, read about how she solved that challenge, how she used a GPS system to find the lysimeters within a half-inch of accuracy, and about her surprising conclusions.

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Jan 7

Using The Salt Balance Approach to Measure Soil Drainage

Understanding the amount of drainage that comes out of the bottom of the root zone and infiltrates into groundwater recharge is a very difficult measurement to do well. Drain gauges do a good job of it but on a small scale. Large lysimeters do an even better job, but are extremely expensive and complex.  There is an economical alternative, however, called the salt balance approach to measuring drainage.

Soil profile underneath canola

Soil profile underneath canola

The Salt Balance Approach

Since the majority of non-fertilizer salts in the soil solution don’t get taken up by plants, this salt can be used in soil as a conservative tracer.  This means that whatever salt is applied to the soil through rainfall or irrigation water is either stored in the soil or leaches through the profile with the soil water, enabling us to use conservation of mass in our salt balance analysis. The electrical conductivity of water (ECw) is directly proportional to the salt concentration, so ECw can be used in place of salt concentration in this analysis.  If you measure the EC of the water that’s applied to the soil, either through irrigation or precipitation,  as well as the EC of the water that’s coming out of the bottom of your profile, then you can calculate what fraction of the applied water is being transpired by the plants, and what fraction is draining out of the bottom.  This method is useful for measuring water balance at field sites.

To illustrate this concept, let’s work through a simple example.  A particular field received 40 cm of water through precipitation and irrigation.  The average ECw of the precipitation and irrigation water is 0.5 dS/m.  Measurements of ECw draining from the soil profile below the root zone indicate an ECw of 2.0 dS/m.  The drainage or leaching fraction can be easily calculated as :

ECw(applied) / ECw(drained) = 0.5 dS/m / 2.0 dS/m = 0.25

The amount of water drained can also be easily calculated as:

Leaching fraction * applied water = 0.25 * 40 cm = 10 cm

Measuring Pore Water EC (ECw)

One challenge to this approach is the measurement of water electrical conductivity itself.  Bulk EC is a relatively simple measurement, and several types of soil water content sensors measure it as a basic sensor output.  However, the electrical conductivity of water, called pore water EC (ECw), is more complex.  Pore water EC requires that it be either estimated from the bulk EC and soil water content or that a sample of pore water be pulled from the soil matrix and measured.  When estimated, pore water EC can contain considerable error.  In addition, removing a water sample and measuring the pore water EC is not easy. 

To learn more about measuring EC, read our EC app guide.

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Oct 31

Water Potential/Water Content:  When to Use Dual Measurements

In a previous post, we discussed water potential as a better indicator of plant stress than water content.  However, in most situations, it’s useful to take dual measurements and measure both water content and water potential.  In a recent email, one of our scientist colleagues explains why: “The earlier article on water potential was excellent.  But what should be added is an explanation that the intensity measurement doesn’t translate directly into the quantity of water stored or needed. That information is also required when managing water through irrigation.  This is why I really like the dual measurement approach. I am excited about the possibilities of information that can be gleaned from the combined set of water content, water potential, and spectral reflectance data.”

Field plantation with a sprinkler in the middle of it

Potato field irrigation

Managing Irrigation

The value of combined data can be illustrated by what’s been happening at the Brigham Young University Turf Farm, where we’ve been trying to optimize irrigation of turfgrass (read about it here). As we were thinking about how to control irrigation, we decided the best way was to measure water potential.  However, because we were in a sandy soil where water was freely available, we also guessed we might need water content. Figure 1 illustrates why.

Turf farm data concerning water potential diagram

Figure 1: Turf farm data: water potential only

Early water potential data looks uninteresting; it tells us there’s plenty of water most of the time, but doesn’t indicate if we’re applying too much.  In addition, if we zoom in to times when water potential begins to change, we see that it reaches a stress condition quickly.  Within a couple of days, it is into the stress region and in danger of causing our grass to go into dormancy.  Water potential data is critical to be able to understand when we absolutely need to water again, but because the data doesn’t change until it’s almost too late, we don’t have everything we need.

Turf farm data dual measurements data diagram

Figure 2: Turf farm data, volumetric water content only

Unlike water potential, the water content data (Figure 2) are much more dynamic. The sensors not only show the subtle changes due to daily water uptake but also indicate how much water needs to be applied to maintain the root zone at an optimal level. However, with water content data alone, we don’t know where that optimal level is. For example, early in the season, we observe large changes in water content over four or five days and may assume, based upon onsite observations, that it’s time to irrigate. But, in reality, we know little about the availability of water to the plant. Thus, we need to put the two graphs together (Figure 3).

Water potential and vol. water content diagram

Figure 3: Turfgrass data: both water potential and volumetric water content together.

In Figure 3, we have the total picture of what’s going on in the soil at the BYU turf farm. We see the water content going down and can tell at what percentage the plants begin to stress.  We also see when we’ve got too much water: when the water content is well above where our water potential sensors start to sense plant stress. With this information, we can tell that the turfgrass has an optimal range of 12% to 17% volumetric water content. Anything below or above that range will be too little or too much water.  

Soil water potential and volumetric water content diagram

Figure 4: Turfgrass soil moisture release curve (black). Other colors are examples of moisture release curves for different types of soil.

Dual measurements will also allow you to make in situ soil moisture release curves like the one above (Figure 4), which detail the relationship between water potential and water content.  Scientists can evaluate these curves and understand many things about the soil, such as hydraulic conductivity and total water availability.

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Oct 9

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Water Content helps Turf Growers find Water/Nutrient Balance

Many athletes don’t like artificial turf. They say it’s hot, uncomfortable to run on, causes burns when you slide or fall on it, and changes the way a ball moves.  Professional women’s soccer players even started a lawsuit over FIFA’s decision to use artificial turf in the 2015 Women’s World Cup.

Soccer players running after the soccer ball on a green field

Soccer players on natural turf.

Some universities—including Brigham Young University—have responded to athlete concerns by using natural turf fields for practice and in their stadiums. But the challenge is to develop plants and management practices for natural turf that help it stand up to frequent use and allow it to perform well even during the difficult fall months. It’s a perfect research opportunity.

BYU turf professor and manager of BYU sports turf, Bryan Hopkins and his colleagues in the Plant and Wildlife Department, have been able to set up a new state-of-the-art facility to study plants and soil in both greenhouse and natural conditions. The facility includes a large section of residential and stadium turfgrass.  

Before Soil Sensors

Initially, BYU maintained the turf farm grass on a standard, timer-based irrigation control system, but over time they realized that understanding the performance of their turf relative to moisture content and nutrient load is crucial. Last year during Memorial Day weekend their turf farm irrigation system stopped working when no one was around to notice.  During those four days temperatures rose to 40 C (100 F), and the grass in the field slipped into dormancy due to heat stress. In response, Dr. Hopkins began imagining a system of soil moisture sensors to constantly monitor the performance of the turf grass.  He wanted not only to make sure the turf never died but also to really understand the elements of stress so they could do a better job growing healthy turf.

Sensors Give a Clear Picture

Soon afterward, a team of scientists, including fellow professor Dr. Neil Hansen, installed volumetric water content (VWC) and matric potential sensors at two different sites: one in the sports turf and one in a residential turf plot.  Each plot had two installations of sensors at 6 cm and 15 cm, along with VWC only at 25 cm, to measure water moving beyond the root zone. Combining these measurements, they could clearly see when the grass was reaching stress conditions and how quickly the turf went from the beginning of stress (in terms of water content and time) to permanent wilting point. In addition, ancillary measurements of temperature and electrical conductivity provide an opportunity for modeling surface and root zone temperature as well as fertilizer concentration dynamics.

Researcher digging a dirt canal and installing sensors

Installing water content sensors at the BYU turf farm.

Errors Revealed

What the researchers learned was that they were using too much water. Dr. Colin Campbell, a METER research scientist who worked with BYU on sensor installation, comments, “We found in the first year that the plants never got stressed at all. So this year, the researchers allowed the water potential (WP) at 6 cm to drop into the stress range (~ -500 kPa) while observing WP at 15 cm (-50 kPa to -60 kPa). We hope this approach will reduce irrigation inputs while creating some stress in the grass in order to push the roots deeper.”

What’s happening with the water?

Dr. Campbell’s favorite part of the sensor data was the detailed picture it gave of what was happening with the water in the sandy soil (Figure 1). He says, “Most people believe that they have an intuitive feel for water availability in soil.  If we were only using water content sensors, seeing a typical value of 20% would lead us to believe we were comfortably in the middle of the plant available range (A).  But in this study, using our colocated soil water content and soil water potential sensors, the data showed readings over 15% VWC were too wet to affect the WP (B). However, once WP visibly changed, it quickly moved toward critical stress levels (C, -1500 kPa is permanent wilting point); it only took two days for the water potential to change from -8 kPa to -1000 kPa.  A subsequent dry period (D) shows similar behavior, but this time the 15 cm WP drops to near -1000 kPa.”

Water potential changes diagram

Figure 1

The plant stress levels were reached surprisingly quickly in this soil because its sand composition has a lot of large pores and not very many small ones (Figure 2). Campbell explains, “The large pores store water that is not held tightly due to low surface area, so the water is freely available. But at around 10% VWC all the water from the large pores is used up. As the soil dries beyond that, the water is held tightly in small pores and becomes increasingly unavailable. This is clear in the moisture release curve.  We see almost no change in water potential as the soil dried to 16% VWC, but from 10% down to 7%, the water potential reached permanent wilting point, and it happened in just over a day.”

VWC and Water potential sensors diagram

Figure 2

What the Future Holds:

The researchers wanted to make sure that if they went down to certain stress levels, they wouldn’t cause harm to the plants, so this year, they installed a weather station to monitor evapotranspiration and calculate irrigation application rates.  They also began measuring spectral reflectance to monitor changes in leaf area (NDVI) and photosynthesis (PRI).  This will enable them to see the impact on the plants as the turf is drying down.  “In the future,” says Campbell, “we hope that both commercial and residential turf growers will be able to more effectively control their irrigation and nutrients based on what we find in this study.”

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Sep 11

Scientists and Greenhouse Growers Collaborate to Help the Environment

Bodies of water across the world face extreme pressure from non-point source pollution.  It’s easy to get overwhelmed by the sheer enormity of this problem, but it didn’t daunt Dr. John Lea-Cox, Research and Extension Specialist for horticulture at University of Maryland.  Dr. Lea-Cox was acutely aware that agriculturally applied fertilizers threatened serious harm to the Chesapeake Bay area near his home. Using an early version of METER’s water content sensors, he began to put together a system that could monitor water status in nursery operations. The effort was based on the work of Dr. Andrew Ristvey (now a colleague at Maryland) who showed water savings of more than 50% during his PhD work using TDR sensors in pots growing ornamental plants.  Dr. Lea-Cox and his colleagues wanted to ultimately develop a network of soil moisture and environmental sensors that would help greenhouse and nursery growers know when to turn on and off their water. Their goal was to reduce nutrient and water use through more efficient application.

Close up of a yellow flower with red tipped petals

How did Dr. John Lea-Cox, Research and Extension Specialist for horticulture at University of Maryland, convince nursery growers to reduce water and fertilizer use?

Convincing Growers

One hurdle facing Dr.Lea-Cox was that water savings didn’t resonate with all growers.  But he soon realized that better irrigation control influenced things growers did care about: higher quality crops, lower mortality rate, and less spending on pesticides.  Dr. Lea-Cox discovered that when he showed growers their moisture sensor data, they were hooked. One snapdragon grower, who found that he could use the sensors to produce a more lucrative A grade crop, said he would not like to go back to the days before sensors. “My gosh, it would be like going back ten years. It would be like trying to measure the temperature in a room without a thermometer. We are totally dependent on them.”

Pink orchids growing in a nursery super green nursery

Orchids grown in a nursery.

Finding Collaborators

Dr. Lea-Cox was not only good at convincing growers, but scientific collaborators as well.  Building on this team’s initial findings, he organized a project to develop water retention curves to tie the amount of water in pots to what was actually available to the plant for several different mixes of potting soil. He realized that moisture measurements were practically useless to growers without a mechanism for viewing them all in one place, so he began to look for collaborators who could build an integrated, wireless system to get root zone information to the nursery grower’s computer and allow them to set irrigation limits and automate their systems based on soil and weather data.  

The resulting collaboration was a group of diverse scientists and commercial growers who could study root behavior, plant-environmental interactions, the performance of the plants, and individual grower interaction with the system.  After a few years of testing, the group received $5M in funding from the Specialty Crops Research Initiative (SCRI) Program over five years to improve horticulture for ornamental plants grown in the U.S.

Lauren Crawford, METER’s soils product manager, says that the resulting collaboration was unique. “It was amazing that an instrumentation company, a research group, and commercial growers were able to work so well together. It was because of the trust we had for each other. We were very transparent about what we were doing, even when we knew that transparency would be difficult. The result was that we were able to make tremendous progress in both science and technology.”

Watch two virtual seminars highlighting SCRI research given by scientists Marc Van Iersel and John Lea-Cox.

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

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Aug 7

New Medium Scale Soil Moisture Measurement Technique

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

Close up picture of cracked and dried soil

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

A New Idea:

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

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

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

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

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

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

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

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

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

Strengths:

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

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

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

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

Get more information on applied environmental research in our

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