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Posts from the ‘Soil moisture sensors’ Category

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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Feb 25

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

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 (see part I).  This week, read about her challenges installing lysimeters below the root zone, how she used a GPS system that can find the lysimeters within a half-inch of accuracy, and her surprising conclusions.

Irrigation sprinkler line set up in a grassy field

This relatively small ecological region has gone from 60 high capacity wells in 1960 to over 2,500 today.

Below the Root Zone

Nocco says getting the lysimeters below the root zone was a major challenge.  “We tried a couple of things, but we settled on installing all the lysimeters with an 18-inch auger that would drill a hole slightly bigger than the whole lysimeter.  We dug an 80 cm trench to the top of the monolith zone. Then, we pounded the drain gauge divergence control tube to 1.4 m to obtain an intact monolith, wherever it was possible to do so. We also stratified soil moisture sensors at 10, 20, 40, and 80 cm.  We used heavy equipment to slowly lift out the monolith, dig out the soil below, and place it back in, keeping  track of all of the different soil horizons, and backfilled as close to the bulk density as we could.”

Researcher filling a hole with dirt and a tube with dirt

Passive capillary lysimeter installation

Finding the Lysimeters with GPS

Typically, scientists bury lysimeters close to the edge of the field so they are easy to locate, but Nocco was concerned that they would prejudice their data due to the donut effect of center pivot irrigation: more irrigation hits the center of the field with less irrigation toward the edges. She comments, ”When I installed the first ten lysimeters, I had not yet come up with a way to find everything. Those instruments are all about 15 meters from the field edge so that I could triangulate measurements and find them during cultivation.  But then I met an extension scientist at the university who had access to an RTK GPS system, which can locate instrumentation within a half-inch of accuracy.  With his help and training, we were able to install the rest of the lysimeters at more random spots throughout the field.”

Irrigation sprinkler line set up in a pastor or field

Nocco was concerned that they would prejudice their data due to the donut effect of center pivot irrigation.

Surprising Conclusions

Nocco says that ET and differences in crop physiology do not explain or account for all of the variability that she saw in groundwater recharge.  Her team did a particle size analysis on the soils adjacent to the lysimeters, and she comments, “We thought that the greater the relative sand content in the soils, the more recharge we would have seen, but what we are seeing is the opposite.  The particle size analysis reveals a negative linear correlation between potential recharge and sand content. The more silt there is in these lysimeters, the more volume of recharge.  What I’m curious about now is if we’re seeing a greater volume of recharge in the siltier spots from flux convergence.  I’m trying to obtain the time series data from the pressure transducers to see if maybe the sandier areas had less potential recharge, but perhaps drained faster.  I have seen a correlation between antecedent soil moisture content and particle size (with no correlation based on crop type).  So it also looks like the siltier soils are holding more water when the rain comes through.”

What’s Next?

Eventually, Nocco plans to use field-generated estimates of groundwater recharge and ET to parameterize and validate a dynamic, agroecosystem model, Agro-IBIS, simulating hydrological responses to climate and land use changes over the past 60 years. Nocco will then share the water-energy budgets and water quantity/climate simulations with stakeholders in the Wisconsin Central Sands area.

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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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Feb 4

Predicting the Stability of Rangeland Productivity to Climate Change

Dr. Lauren Hallett, researcher at  the University of California, Berkeley, recently conducted a study testing the importance of compensatory dynamics on forage stability in an experimental field setting where she manipulated rainfall availability and species interactions. She wanted to understand how climate variability affected patterns of species tradeoff in grasslands over time and how those tradeoffs affected the stability of things like forage production across changing rainfall conditions.

field with species tradeoffs standing in the brush

Species tradeoffs could help mitigate the negative effects of climate variability on overall forage production.

Species Tradeoff

A key mechanism that can lead to stability in forage production is compensatory dynamics, in which the responses of different species  to climate fluctuations result in tradeoffs between functional groups over time. These tradeoffs could help mitigate the negative effects of climate variability on overall forage production.  Dr. Hallett comments, “In California grasslands, there’s a pattern that is part of rangeland dogma, that in dry years you have more forbs, and in wet years you have more grasses. I wondered if you could manage the system so that both forbs and grasses are present in the seed bank, able to respond to climate.  This would perhaps buffer community properties, like soil cover for erosion control and forage production in terms of biomass, from the effects of climate variability.”

Tradeoff in a green field, aerial view

In areas experiencing moderate grazing, there was a strong species tradeoff between grasses and forbs.

Manipulating Species Composition

Dr. Hallett capitalized on the pre-existing grazing manipulation that her lab had done over the previous four years.  The grazing she replicated for this study was experimentally controlled, making it easier to ensure consistency.  She built rainout shelters where she collected the water and applied it to dry versus wet plots.  She also manipulated species composition, allowing only grasses, only forbs, or a mix of the two.  These treatments allowed her to study changes in cover and biomass.

Hallett used soil moisture probes and data loggers to characterize the treatment effects of this experiment and to parameterize models that predict rangeland response to climate change.  She says, “I wanted to verify that my rainfall treatments were getting a really strong soil moisture dynamic, and I found the shelters and the irrigation worked really well.”  Along with above-ground vegetation, she collected soil cores and looked at nutrient differences in conjunction with soil moisture.  Since her field site is located within the Sierra Foothills Research and Extension Center, Dr. Hallett was able to rely on precipitation data that was already measured on-site.  

Results

Dr. Hallett found that in areas experiencing moderate grazing, there was a strong species tradeoff between grasses and forbs.  She comments, “I had a seedbank that had both functional groups represented, and those tradeoffs did a lot to stabilize cover over time.”

When Dr. Hallett replicated the experiment in an area that had a history of low grazing, she found that the proportion of forbs wasn’t as high in the seedbank.  As a consequence, there was a major loss of cover in the dry plots.  She explains, “When the grass died, there weren’t many forbs to replace it, and you ended up with a lot of bare ground. The areas that were lightly grazed had more litter, so initially, the soil moisture was okay, but as the season progressed into a dry condition and the litter decomposed, there wasn’t enough new vegetation to stabilize the soil.”  As a result, Dr. Hallett thinks in low-grazed areas it’s important to have an intermediate level of litter. She says, “You need enough litter to increase soil moisture, but not so much that it would suppress germination of the forbs because as the season progresses and gets really dry, if you don’t have forbs in the system, you lose a lot of ground cover.”

Surprises Lead to A New Study

Dr. Hallett was surprised that within her three treatments there seemed to be differences in when the functional groups were drying down the soil.  This inspired new questions, leading her to use her dissertation data to generate a larger grant through the USDA.  Her new study will perform extensive rainfall manipulations to measure the effects of early-season versus late-season dryout, and vary species within those parameters.  She says, “One of the reasons you have grass years versus forb years is the timing of rainfall.  For instance, if you have a really dry fall, you tend to have more forbs because their seedlings are more drought resistant.  Conversely, if you have a wet fall, you tend to see more grasses because you have continual germination throughout the season. So, the timing of rainfall matters in terms of what species are in the system.  We are going to look at the coupling between the species that gets selected for the fall versus what would be able to grow well in the spring, and we will be studying how that affects a whole range of things such as ground cover, above-ground production for forage, below-ground investment of different functional groups, and how these things might relate to nutrient cycling and carbon storage.”

You can read more about Dr. Hallett’s rangeland research and her current projects here.  

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Jan 14

Using Soil Moisture Sensors on Humans?

Dr. Stuart Campbell, professor of Biomedical Engineering at Yale University has been toying with the idea of using soil moisture sensors to measure tissue edema in human subjects.

Patients had wrapped in gauze with a tube

Tissue edema occurs when too much fluid leaks from your capillaries into your tissue.

He says he got the idea from Dr. Ken Campbell, former professor of Bioengineering at Washington State University:  “I was explaining to Dr. Campbell about the sensors METER makes, and he pointed out that there are many diseases where you might want to measure someone’s tissue edema, and it would be interesting to see if you could use a soil sensor in a wearable device to help doctors monitor swelling in their patients, much like a heart monitor monitors heart activity.”

Tissue Edema:

Tissue edema occurs when too much fluid leaks from your capillaries into your tissue. Capillaries, the smallest blood vessels in your body, are somewhat leaky, allowing the exchange of nutrients and waste between the tissue and the blood. The fluid that surrounds the blood cells is free to exchange across the capillaries, and edema will occur when too much fluid leaks out of the circulatory system into the tissue.  Edema can be caused by things like heart disease, pregnancy, or standing on your feet all day.  

What Makes the Fluid Leak?

In soil, water moves from high water potential to low water potential. Similarly, there are forces inside the circulatory system that cause the transfer of fluid between capillaries.  Your blood vessels have a certain amount of pressure that is generated by your heart.  If your blood pressure goes up, it can cause edema. Dr. Stuart Campbell says, “The actual fluid pressure is part of what decides how much fluid is pushed out, but it’s not that simple. Your blood has large proteins that are too big to get out of the capillary. That means the more water that leaves the capillary and moves into the tissue, the more concentrated those proteins become, which lowers the water potential (or osmotic potential) of the blood. This delicate balance is what prevents too much water from leaking out.  However, if you have a disease that tips this balance, either through high blood pressure or a condition that allows those proteins to leak out of the capillary, edema would occur because you don’t have the osmotic potential pulling the water back into the capillary and keeping the proper balance.”

Stuart Campbell operating on the heart for a medical procedure

Dr. Campbell thought it would be interesting to figure out if he could monitor the edema of heart tissue during one of the procedures.

The Heart Experiment:

Dr. Campbell decided to see if a soil sensor would work to measure animal tissue when he was working as a summer student in the Visible Heart Lab at the University of Minnesota.  Campbell says, “Similar to a human heart transplant, this lab is able to keep pig hearts alive outside the body.  The problem, however, is that they use a manmade solution instead of blood, and that imitation blood is not ideal. If the composition of the fluid is not perfectly adjusted, you can have problems with your experiments.  I thought it would be interesting to figure out if we could monitor the edema of the heart tissue during one of the procedures. I hooked up the soil probe and used it in one experiment where I put it in contact with the heart while it was beating.  There was, in fact, a change in output of that signal during the experiment.  But, because I only got one chance at it, it was inconclusive as to whether this was indicative of an imbalance in the composition of our artificial blood substitute.”  

An Anecdotal Experiment:

Still curious to see if the idea would work, Dr. Campbell decided to try one more experiment: this time on his wife who was experiencing edema symptoms after childbirth.  He says, “It occurred to me that this was an opportunity to try out the soil moisture probe one more time to measure tissue edema.  So each day, I would measure her ankles, putting the probe in flat contact with her skin while tightening a strap gently.”  Dr. Campbell says he watched the swelling go down as the numbers on the probe got smaller, and comments, “It was anecdotal evidence that at least in extreme cases, you might be able to get the soil probe to work.  But I still have questions, such as, how would you make sure that the probe was always touching the skin in the same way?  And, if the person got sweaty, would that change the soil probe reading?”  

Nurse measuring heart rate

There are millions of people in this country who have heart failure.

Why the Experiments Should Continue:

Though Dr. Campbell hasn’t had time to pursue the experiment further, he feels that if the idea works, it has the potential to improve lives and save our nation billions of dollars.  He says, “There are millions of people in this country who have heart failure.  Maybe they’ve had a blockage in one of their coronary arteries, or perhaps their heart is worn out because of age. You can tell when someone is in heart failure because when they lie down to go to sleep at night, all that fluid makes its way slowly from the ankles, through the legs, the torso, and eventually into the chest. The problem is that the lungs are very delicate, and when you have edema in the lungs, it’s almost like you have pneumonia.  This type of sensor could be an easy way for people to monitor themselves and manage their fluid intake and diet after they get home from the hospital.”  Dr. Campbell says this helps the economy because if people don’t manage their fluids, they have to return to the hospital so they can be supervised to eat correctly and regain the proper fluid balance. This ends up costing the economy billions of dollars unnecessarily.  He concludes, “Perhaps people just need to follow instructions, but it’s possible with better monitoring that the situation can be improved.”

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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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Dec 4

Understanding the Influence of Coastal Fog on the Water Relations of a California Pine Forest

Forests along the California coast and offshore islands experience coastal fog in summer, when conditions are otherwise warm and dry. Since fog-water inputs directly augment water availability to forests during the dry season, a potential reduction of fog due to climate change would place trees at a higher risk of water stress and drought-induced mortality.  Dr. Sara Baguskas completed her Ph.D. research in the geography department at UC Santa Barbara on how variation in fog-water inputs impact the water relations of a rare, endemic tree species, Bishop pine, located on Santa Cruz Island in Channel Islands National Park. The goal of her study was to enhance our ability to predict how coastal forests may respond to climate change by better understanding how fog-water inputs influence the water budget of coastal forests.

Fog on Trees

Dr. Baguskas’ study seeks a better understanding of how fog-water inputs influence the water budget of coastal forests.

Fog Manipulation

Santa Cruz Island supports the southern extent of the species range in California, thus it is where we would expect to see a reduction in the species range in a warmer, drier, and possibly less foggy future. To advance our mechanistic understanding of how coastal fog influences the physiological function of Bishop pines, Dr. Baguskas conducted a controlled greenhouse experiment where she manipulated fog-water inputs to potted Bishop pine saplings during a three-week drydown period. She installed soil moisture (VWC) sensors horizontally into the side of several pots of sapling trees at two different depths (2 cm and 10 cm) and exposed the pines to simulated fog events with a fog machine.

In one group of plants, Baguskas let fog drip down to the soil, and in another treatment, she prevented fog drip to the soil so that only the canopies were immersed in fog.  She adds, “Leaf wetness sensors were an important complement to soil moisture probes in the second treatment because I needed to demonstrate that during fog events, the leaves were wet and soil moisture did not change.” Additionally, Baguskas used a photosynthesis and fluorescence system to measure photosynthetic rates in each group.

Fog in pine trees from the ground

The fog events had a significant, positive effect on the photosynthetic rate and capacity of the pines.

Results

Dr. Baguskas found that the fog events had a significant, positive effect on the photosynthetic rate and capacity of the pines.  The combination of fog immersion and fog drip had the greatest effect on photosynthetic rates during the drydown period, so, in essence, she determined that fog drip to the soil slows the impact of drydown.  

“But,” she says, “when I looked at fog immersion alone, when the plant canopies were wet by fog with no drip to the soil, I also saw a significant improvement in the photosynthetic rates of these plants compared to the trees that received no fog at all, suggesting that there could have been indirect foliar uptake of water through these leaves which enhanced performance.”  An alternative interpretation of that, Baguskas adds, is that nighttime fog events reduced soil evaporation rates, resulting in less evaporative loss of soil moisture.

Dr. Baguskas says her “canopy immersion alone” data are consistent with other research: Todd Dawson, Gregory Goldsmith, Kevin Simmonin, Carter Berry, and Emily Limm have all found that when you wet plant leaves, it has a physiological effect, suggesting the plants are taking water up through their leaves and not relying as much on soil moisture.  (These authors performed different types of experiments, but their papers serve as reference studies). Baguskas says, “My results suggest that is what’s going on, but it’s not as definitive as other studies that have actually worked on tracking the water through leaves using a stable isotope approach.”  

Lessons Learned

Though Dr. Baguskas did not monitor soil temperature in this study, she says that in the future, she will always combine temperature data with soil moisture data.  She comments, “Consistently, the soil moisture in the “canopy-immersed only” plants was slightly elevated over the soil moisture in the control plants.  It made me wonder if this was a biologically meaningful result. Does it support the fact that if plants are taking up water through their leaves, they don’t rely on as much soil moisture?  Or did my treatment change soil temperature, and is that having a confounding effect on my results?  What I’ve learned from this, is that in the future I will always use soil probes with temperature sensors because you may not know until you see your results if temperature might be important.”

Future Fog Studies

Baguskas is a USDA-NIFA postdoctoral Research Fellow working with Dr. Michael Loik in the Environmental Studies Department at UC Santa Cruz. She continues to study coastal fog, but now in strawberry fields. Her current research questions are focused on integrating coastal fog into water-use decisions in coastal California agriculture. She loves the work and continues to rely on soil moisture sensors to make meaningful and reliable environmental measurements in the field and greenhouse.

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Nov 14

Sensor Data Improves Cherry Production

In July of 2013, Lav Khot and his team were in the field looking at how cherries were picked, weighed, and transported, when suddenly a helicopter began circling around a nearby orchard block.  When Dr. Khot asked the grower about it he said, “There was a rain last night, and we are trying to dry the tree canopies.” The grower told Khot that cherries are susceptible to cracking if moisture stays on the fruit too long, so they hire helicopters to fly over their orchards to remove water from the fruit and leaves, hoping to prevent fruit loss.

Bright red cherries on a tree

The economic impact of solving the cherry cracking problem could be huge as growers now suffer heavy losses each year.

Fresh market cherries are a lucrative business. That’s how the growers can afford the approximately $25K it costs to rent the helicopters every season. They try to do everything that they can to stop any cracking or splitting, but interestingly, Dr. Khot says grower decisions are influenced completely by emotion. “If there is a rain event, the farmer will become anxious, and they will hire pilots to fly the helicopter.”  

Dr. Khot wondered if he could help the cherry growers make their decisions based on real data instead. He and his postdoc, Dr. Jianfeng Zhou, are using leaf wetness sensors to determine if and how long water is present on the tree canopies after a rain event. Dr. Khot hopes that the data from these sensors will help growers decide whether or not it makes sense to fly the helicopter.

Why the cherries split

Not all varieties of cherry crack, but high sugar content varieties do as the skin is thin during maturation.  There are two hypotheses associated with fruit splitting or cracking:

Irrigation:  High water availability in the soil as the fruit is maturing (a few weeks before harvest) encourages trees to take up more water and causing the fruit to split.

Rainwater:  Rain collects in the cherry stem bowl or hangs off the bottom and is slowly absorbed into the fruit along the osmotic potential gradient. The fruit will start to split due to increased pressure inside the skin.

Dr. Khot and his team will use soil moisture sensors to investigate the first hypothesis with the object of improving irrigation management, especially as harvest approaches.  And he’s getting some support:  “Dr. Matt Whiting (colleague at the Center for Precision and Automated Agricultural Systems, Washington State University) is helping us understand this cracking phenomenon from the soil perspective. He is doing work on deficit irrigation (reducing the rate of irrigation below optimal) towards harvest time and seeing how that relates to cracking.  Also, the WSU CAHNRS ERI (Emerging Research Issues grant), which supports high-risk research, has funded us $75,000, along with Decagon who is supporting us with their sensors.”

Last Year’s Research

There are two approaches to drying canopies. One uses a sprayer that produces a cross-wind that moves sideways through the canopies, while the other uses the downwash from helicopter blades.  Last year, Dr. Khot and his research assistant experimented with crosswind velocities to see how much wind was being generated and how much water was really being dispersed.   Dr. Khot commented, “Last season we went out to the WSU orchard and ran the sprayer at two settings in order to see how water was removed and how much wind was coming through the canopies for a given amount of time.” They had good success at both removing the water from the trees and measuring it with the leaf wetness sensors.  But, they started the measurements after the cherries had matured, so weren’t able to tie it to cracking.

This Year’s Experiment

One issue with using helicopters is that they are extremely dangerous.  Accidents are not uncommon, and unfortunately pilots have died.  This year the team will also evaluate the efficacy of a mid-size, unmanned helicopter in order to test if it can produce enough downwash to dry the cherries and compare it with manned helicopters.  Dr. Khot says, “The helicopters are large and difficult to fly close to the canopies, but we can program the unmanned drone to fly close to the canopy and get rid of the water safely.” Digital Harvest and Yamaha, who are supporting this aspect of the research, have received an exemption from the FAA so they can test their unmanned helicopter.

Differing Tree Architectures

Dr. Khot’s team did their first experiments on traditional cherry tree architectures (imagine a typical tree), but this year they will perform their experiments on trees that are trained into a “Y” shape, or completely vertical.

Cherry production trees

These trees represent traditional tree architecture, but this year researchers will perform their experiments on trees that are trained into a “Y” shape, or completely vertical.

Researchers have developed these new architectures for ease of harvesting and management, but Dr. Zhou says that there will be less canopy variability and thus more interpretable results compared to the traditional tree architecture where wind velocity is more heterogeneous throughout the canopy.

Economic Impact  

Dr. Khot says the economic impact of solving the cherry cracking problem could be huge as growers now suffer heavy losses each year.  One former grower underscored this when he noted they lost one crop in every four. But, there could be other benefits as well. The implications of this research could lead to solving other grower problems such as disease and pest management.   “WSU already has a good AgWeatherNet program where we monitor the weather outside the trees at different locations, but not inside the canopies. If we had some smart sensing equipment like the leaf wetness sensor sitting in the canopy monitoring the wetness level over a 24-hour cycle, then we could develop some models based on the wetness and relate them to the number of pests at different locations in the orchard.  That is something every grower can benefit from.”

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Nov 9

Screening for Drought Tolerance

Screening for drought tolerance in wheat species is harder than it seems.  Many greenhouse drought screenings suffer from confounding issues such as soil type and the resulting soil moisture content, bulk density, and genetic differences for traits like root mass, rooting depth, and plant size. In addition, because it’s so hard to isolate drought stress, some scientists think finding a repeatable screening method is next to impossible. However, a recent pilot study done by researcher Andrew Green may prove them wrong.

An automatic irrigation setup with green plants sticking out

Automatic Irrigation Setup

The Quest for Repeatability

Green says, “There have been attempts before of intensively studying drought stress, but it’s hard to isolate drought stress from heat, diseases, and other things.”  Green and his advisors, Dr. Gerard Kluitenberg and Dr. Allan Fritz, think monitoring water potential in the soil is the only quantifiable way to impose a consistent and repeatable treatment. With the development of a soil-moisture retention curve for a homogeneous growth media, they feel the moisture treatment could be maintained in order to isolate drought stress.  Green says, “Our goal is to develop a repeatable screening system that will allow us to be confident that what we’re seeing is an actual drought response before the work of integrating those genes takes place, since that’s a very long and tedious process.”

Why Hasn’t This Been Done Before?

Andrew Green, as a plant breeder, thinks the problem lies in the fact that most geneticists aren’t soil scientists. He says, “In past experiments, the most sophisticated drought screening was to grow the plants up to a certain point, stop watering them, and see which ones lived the longest. There’s never been a collaborative approach where physiologists and soil scientists have been involved.  So researchers have imposed this harsh, biologically irrelevant stress where it’s basically been an attrition study.” Green says he hopes in his research to use the soil as a feedback mechanism to maintain a stress level that mimics what exists in nature.

Data acquisition a cabinet setup for green's expanded experiment

Data Acquisition Cabinet setup for Green’s expanded experiment.

The Pilot Study

Green used volumetric water content sensors, matric potential sensors, as well as column tensiometers to monitor soil moisture conditions in a greenhouse experiment using 182 cm tall polyvinyl chloride (PVC) growth tubes and homogenous growth media. Measurements were taken four times a day to determine volumetric water content, soil water potential, senescence, biomass, shoot, root ratio, rooting traits, yield components, leaf water potential, leaf relative water content, and other physiological observations between moisture limited and control treatments.  

Soil Media:  Advantages and Disadvantages

To solve the problem of differing soil types, Andrew and his team chose a homogeneous soil amendment media called Profile Greens Grade, which has been extensively studied for use in space and other applications.  Green says, “It’s a very porous material with a large particle size.  It’s a great growth media because at the end of the experiment you can separate the roots of the plant from the soil media, and those roots can be measured, imaged, and studied in conjunction with the data that is collected.”   Green adds, however, that working with soil media isn’t perfect: there have been hydraulic conductivity issues, and the media must be closely monitored.

What’s Unique About this Study?

Green believes that because the substrate was very specific and his water content and water potential sensors were co-located, it allowed him to determine if all of his moisture release curves were consistent.  He says, “We try to pack these columns to a uniform bulk density and keep an eye on things when we’re watering, hoping it’s going to stay consistent at every depth.  So far it’s been working pretty well: the water content and the water potential are repeatable in the different columns.”

Irrigation setup for the expanded study with research data cabinet

Entire Irrigation setup for the expanded study.

Plans for the Future

Green’s pilot study was completed in the spring, and he’s getting ready for the expanded version of the project:  a replicated trial with wild relatives of wheat. He’s hoping to use soil moisture sensors to make automatic irrigation decisions: i.e. the water potential of the columns will activate twelve solenoid valves which will disperse water to keep the materials in their target stress zone, or ideal water potential.

The Ultimate Goal

The ultimate goal of Green’s research is to breed wild species of wheat into productive forms that can be used as farmer-grown varieties. He is optimistic about the results of his pilot study.  He says, “Based on the very small unreplicated data that we have so far, I think it is going to be possible to develop a repeatable method to screen these materials.  With the data that we’re seeing now, and the information that we’re capturing about what’s going on below ground, I think being able to hold these things in a biologically relevant stress zone is going to be possible.”

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