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Posts from the ‘Agriculture’ Category

Mar 18

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Can Wastewater Save The United Arab Emirates’ Groundwater?

The hyper-arid United Arab Emirates (UAE) has a rapidly dwindling supply of groundwater, and that water is becoming increasingly saline.

Image of the city of Dubai at night on the coast of the UAE

Dubai is situated on the coast of the UAE.

With very little recharge and irrigation comprising 75% of groundwater use, natural water resources in this region are disappearing fast.  PhD candidate Wafa Al Yamani works for the Environmental Agency of Abu Dhabi, which has contracted with Plant and Food Research in New Zealand to investigate using treated sewage effluent and groundwater for irrigating the desert forests along their motorways.  

Sidr tree plantation in the UAE forest in the sand

Sidr trees in the UAE forest.

The Desert Forests

The UAE desalinates all the water for their cities, so the tertiary treated sewage effluent from these cities could be a viable resource, replacing some groundwater for irrigation of the desert forests. These forests perform a wide range of ecosystem services from sand stabilization along all UAE motorways to harboring a great deal of biodiversity.  There is also a cultural association with the forests.  The original ruler of the UAE, Sheikh Zayed, embarked on a program in the 1970s of “greening the desert,” so the people see the desert forests as a legacy of their founder.

Infiltrometer pushing sand and being measured

Infiltrometers were used to examine how the drip irrigation system worked.

Measuring Water Use:

Wafa and her PhD advisor, Dr. Brent Clothier, had a goal to minimize groundwater use and maximize value by quantifying the irrigation needs of the UAE’s five most important desert-forestry species.  They also wanted to determine the impact of treated sewage effluent on forest growth and health.  They used infiltrometers to examine how the drip irrigation system worked.  Dr. Clothier says, “These soils have hydraulic conductivities of between 2 and 5 meters an hour.  They are highly permeable desert sands.  We can find out how wide the bulb (the wetted area underneath an irrigation dripper) is and how deep the water will travel by using an infiltrometer to look at the hydraulic properties of the soil.”  Dr. Clothier has also developed software to predict water movement radially, with depth and with the time that the drippers are on.  He comments, “We’ve now got a setup of two drippers per tree, and we will use that in the future for modeling how the trees are taking up water from the root zone.”

Tree with 20cm dykes accessing the dripper water

Researchers built dykes of 20 cm to stop surface redistribution of dripper water.

The scientists used a heat pulse method to measure tree water-use by comparing sap flow with evaporative demand (ETo).  They used Time Domain Reflectometry (TDR) to measure soil water content, and they have developed a “light stick” using light sensors to detect the shadow area of the trees to measure trees’ leaf area in order to predict the crop factor that will enable prediction of tree water-use from ETo.

Next week:  Find out how Wafa and her team use infiltrometers to predict dripper behavior and how the treated effluent resolves salinity issues.

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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 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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Nov 19

Killing Cheatgrass and Shooting for the Moon

My grandfather, Grant A. Harris, wrote his Ph.D. thesis about the detrimental effects of cheatgrass (bromus tectorum) on rangeland ecology, so I’ve been taught since birth to hate this invasive plant species. So it didn’t surprise me to read that cheatgrass has become the equivalent of an eco-supervillain, wreaking havoc in farmer’s fields, rapidly spreading, and reducing wheat yield—sometimes by fifty percent.  

Microscope focusing on glass panel with sample on it

Washington State University scientist, Dr. Ann Kennedy, has successfully worked with a naturally-occurring soil bacteria that limits the depth of root growth in cheatgrass.

Cheatgrass increases the spread of wildfire, aiding the jump from plant to plant, and it afflicts livestock: lodging in the eyes and mouths of grazing cattle, not to mention having little nutritional value. For years, it’s been tenacious and incredibly prolific, out-competing native grasses and essentially “taking over” the eco-world.  Until now.  This New York Times article spotlights Washington State University scientist, Dr. Ann Kennedy, and her successful work with a naturally-occurring soil bacteria that limits the depth of root growth in cheatgrass, reducing its competitive advantage on the prairie.  

As a scientist, I was intrigued by this article because of what it didn’t say.  Dr. Kennedy, a good friend of mine and a great scientist, once told me that her bacteria experiment was the one she thought least likely to work. She’d looked at it as a kind of “shoot the moon” idea, riddled with “unknowns,” making it risky to spend too much time on.  In fact, she’d only had time to pursue this interesting and challenging experiment because she’d made time for it.

A concentrated solution of bacteria being sprayed on cheatgrass

A concentrated solution of bacteria is sprayed on fields, and over time, the organisms colonize the roots of the cheatgrass.

In a seminar she gave years ago about her work with cheatgrass, Dr. Kennedy shared her simple 60-30-10 prioritization method.  Sixty percent of her research effort was put into core projects she knew would yield publishable papers and keep her lab running.  Thirty percent of her time was spent on challenging projects that were more impactful but less likely to succeed. Finally, she put ten percent of her effort into “shoot the moon” type projects: research that was unlikely to come to fruition, but if successful, would have a dramatic impact in the world.  

In science, it’s easy to get stuck in the purely practical, only spending time on the experiments we know will work. It’s safer and won’t expose us to ridicule when things don’t go the way we hope. But, Dr. Kennedy has proven that there is value in trying things that might fail.  

It’s been more than a decade since I’ve listened to her lecture, but it still impacts the way we do research at METER. Although we spend a lot of time on projects we know will turn into finished instruments, we continue to dream up ways to produce frozen soil moisture release curves or measure leaf water potential. These ideas may not succeed, but if they do, they could have a big impact on the way we make measurements.

As I think about my team’s research priorities and the possibilities of success, I always first consider core projects: What are we really good at?  What will be a sure bet for success?  But because of Dr. Kennedy, I’ll always devote some of my time to more risky endeavors, speculating on what could happen and what might possibly change the world.  

(Read about our most spectacular example of risky research:  the collaboration with NASA’s Jet Propulsion Laboratories to send one of our sensors to Mars.)

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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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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.

Download the “Researcher’s complete guide to water potential”—>

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

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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.”

Download the “Researcher’s complete guide to water potential”—>

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

Get more information on applied environmental research in our

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