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Mar 23

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

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

Abu Dhabi from the ocean looking at the city

Abu Dhabi

Infiltrometers Predict Dripper Behavior:

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

researcher recording data while sitting on the floor

Wafa on site, using the twin head method.

Treated Effluent Resolves Salinity Issues

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

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

Large white irrigation tanks sitting in sand in the desert

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

What’s The Future?

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

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Mar 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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Mar 9

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Data loggers:  To Bury, or Not To Bury (part II)

There are many reasons why you should never bury your data logger.*  Most scientists who try it, fail (see part 1).  However, there is one innovative team at Washington State University who has found a way to overcome many of the problems which plague buried data loggers. They now happily collect data from the road, sitting in the cab of a truck.

Orange plastic container with a data logger in it

The research team houses their data loggers in a water-resistant case marked with a radio ball marker and surface flagging.

Collecting Data by Radio

Caley Gasch decided she wanted to bury data loggers in an actively managed field at the Cook Agricultural Farm, so they weren’t constantly taking down data loggers for cultivation, spraying, and harvest. She says, “We wanted a better system because after we took the data loggers down, they often did not get put back up for weeks, leaving giant gaps in our data.  The idea of burying the data loggers and simply reading them by radio had crossed our minds, but we were stymied by four questions.”  

How would we bury the dataloggers so that we could find them again?  

To solve this problem, the team buried the data loggers with a radio power identifier ball, originally made by the 3M Corporation, for locating buried power lines.  She says, “It’s a radio monitor that transmits a radio signal, and we have an instrument that we can then use to find them.”  Caley buries a radio marker with the data logger so that if the flag that marks the logger location gets removed by farming equipment or the weather (which always seems to happen), she still has the ability find the buried data logger.

Orange box with cable in the port thats water and air sealed

How the cables fit through the ports.

How would we avoid filling them with water, especially on a large scale?  

Caley says she’s had success keeping water out of all but three of her forty-two data loggers. She says the shallow soil in those three locations gets easily saturated in the winter, so they are still trying to modify the system. However, the method they have developed works well for the other 39 data loggers.

Their method is to place the data loggers inside a pelican case, which is a plastic, water-tight box.  She says, “We modify the boxes so the sensor cables leaving the data logger can exit the box through cable entry connectors which we tighten down with a plastic screw.  We make a watertight seal where the cable can go in and out of the box, and we also add some heat shrink tubing on the cables themselves to tighten that connection. We put silica desiccant packs inside of the pelican box along with the data logger to keep the humidity low.  This will collect any condensation that builds up or even soak up small amounts of water that leak in.”  Caley says that any water leakage they have had is probably through the ports where they’ve modified the pelican box for cable entry, but in most locations, it’s not a problem.  

Sealed port on the orange data logger boxes

A sealed port.

How could we get radio signal to transmit out of the soil far enough?

Caley says, typically, she can connect with the radio signal up to 100 meters away from the loggers when they are buried.  She adds, “We have successfully connected to loggers that are 0.5 km away, but it depends on the landscape, the amount of water in the soil, the season, the kind of crop that’s growing, and the terrain that’s between the scientist and the data logger.  We have to get closer to most loggers.  100 meters is convenient enough for the farms that we are working on.  The roads are within that distance to each of the loggers, so we never have to actually leave the vehicle to collect our data.”

How long will the batteries last?

Caley says they’ve gotten away with only changing the batteries once a year. She usually collects data twice each year and changes the batteries in the spring.  She says, “By the time March comes around the batteries are pretty close to being dead, but we’ve been successful with just five alkaline AA batteries lasting about a year.”

One Challenge:

In some cases, the loggers haven’t been buried deep enough, and farm equipment crushed them, or the seeder penetrated the boxes.  Caley says, “We just have to make sure they are buried deep enough. We typically bury them at least 30 cm deep, and that seems to work pretty well with the current farm equipment.”

Dirt coating a data logger box sitting on top of piled up dirt

A buried data logger that has been dug up.

For the Future:

Caley has a new idea for modifying the locations that are prone to flooding.  She will keep the loggers buried most of the year, and then dig them up during the winter.   “After the harvest in the fall, when the grower gives us permission, we will go out and dig up the boxes and mount the dataloggers on a short post, so they can spend the winter above ground.  Then, after the soil has dried a little in the spring, but prior to seeding to minimize disturbance, we will bury them again.”  Caley says that even though digging them up in the winter is more work, it’s worth her time.  She concludes, “It’s still worth it to bury the loggers during the growing season so we don’t continually have data gaps while growers are seeding, spraying, or making a pass over the field.”

*Note:  METER’s (formerly Decagon) official position is that you should never bury your data logger.  But we couldn’t resist sharing a few stories of scientists who have figured out some innovative ideas which may or may not be successful if tried at other sites.

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Mar 7

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Data loggers: To Bury, or Not To Bury

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

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

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

The Good

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

The Bad

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

Five gallon white bucket with rocks and dirt in it

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

The Ugly

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

One Success Story So Far

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

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

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Feb 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 11

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Estimating Relative Humidity in Soil: How to Stop Doing it Wrong

Estimating the relative humidity in soil?  Most people do it wrong…every time.  Dr. Gaylon S. Campbell shares a lesson on how to correctly estimate soil relative humidity from his new book, Soil Physics with Python, which he recently coauthored with Dr. Marco Bittelli.

Desert with trees and brush everywhere

Radioactive waste buried in steel containers will corrode if the humidity is too high.

A number of years ago a former student told me of a meeting he had with some engineers establishing a low-level radioactive waste repository in a desert area. The waste was to be buried, and at least some of it was in steel containers which would corrode if the humidity was too high. The engineers assumed the humidity in the soil would be pretty low because it was a desert, but they didn’t know how low. So, what is the relative humidity in soil? That sounds like it would be a hard thing to find out without measuring it, but it isn’t. Let’s apply a little physics to see what we can find.

The energy required to create an infinitesimal volume of water vapor can be found using the first law of thermodynamics. For an adiabatic system

Thermodynamics Equations

where dE is the energy required, p is the pressure, and dV is the volume change.

The Boyle-Charles law, which gives the pressure-volume relationship for a perfect gas, is

Thermodynamics Equations

where n is the number of moles of gas, R is the universal gas constant, and T is the kelvin temperature. Rearranging terms and taking the derivative of both sides gives

Thermodynamics Equations

This equation can be substituted for dV in the first equation, giving

Thermodynamics Equations

The total energy required to go from a reference vapor pressure, po (the vapor pressure of pure water) to the vapor pressure of the water in the soil, p is

Thermodynamics Equations

We can divide both sides by the mass of water. The left side then becomes the energy per unit mass of water in the soil, which we call the water potential. On the right side, the number of moles per unit mass is the reciprocal of the molecular mass of water, and the ratio of the vapor to the saturation vapor pressure is the relative humidity hr so the final equation is

Thermodynamics Equations

We can rearrange this and take the exponential of both sides, giving

Thermodynamics Equations

In the second version of the equation the molecular mass of water, the gas constant and the temperature (298K) have been substituted.

We can use this equation to find the range of humidities we would expect in soil. When soil is very wet, the water potential is near 0, so the humidity is exp(0) = 1. At the dry end, the soil is dried mainly by plant water uptake. Even desert soils support some vegetation. The soil near the surface will be dried by evaporation, but a few decimeters below the surface the lowest water potentials are those to which plants can dry them. The nominal permanent wilting point (lower limit of plant available water) is -1500 J/kg. Desert vegetation can extract water to lower potentials. If we say their lower limit is -2500 J/kg, then the humidity is

Thermodynamics Equations

so the relative humidity in the soil is around 98%. Sagebrush can go lower than -2500 J/kg. We measured -7000 J/kg under it at the end of the growing season. Even that, though, is around 95% humidity.

The conclusion is that the humidity in the soil is always near saturation, except in a shallow evaporation layer near the surface. I don’t remember what the engineers were expecting. I think anything above 60 or 70% was going to be a disaster for corroding the steel containers. I don’t know whether they believed the calculations or just went on thinking that desert soil is dry.

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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 28

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The Quest for Accurate Air Temperature (Part 2)

In the conclusion to last week’s blog, Mark Blonquist, chief scientist at Apogee Instruments and air temperature measurement expert, explains the complexities of some proposed solutions to the problems that challenge accurate air temperature measurement.

A aspirated raditation shield by Apogee Instruments

An aspirated radiation shield manufactured by Apogee Instruments in Logan, Utah. Multiple models of passive and active shields are available from several manufacturers.

Solution:  Passive Radiation Shield

In addition to an accurate sensor, accurate air temperature measurement requires proper shielding and ventilation of the sensor.  Passive shields do not require power, making them simple and low-cost, but they warm above air temperature in low wind or high solar radiation. Warming is increased when there is snow on the ground due to increased solar radiation load from higher albedo and increased reflected solar radiation. Errors as high as 10 degrees C have been reported in passive shields over snow (Genthon et al., 2011; Huwald et al., 2009).  The figure below shows the differences in error for the two conditions.

Shortwave Radiation > 50 W m-2 diagram

Corrections for Passive Shields

Equations to correct air temperature measurements in passive shields have been proposed, but often require measurement of wind speed and solar radiation, and are applicable to a specific shield design.  Corrections that don’t require additional meteorological measurements have also been proposed, such as air temperature adjustment based on the difference between air temperature and interior plate temperature differences. Others have suggested modifying traditional multi-plate passive shields to include a small fan that can be operated under specific conditions, but using natural aspiration when wind speeds are above an established threshold.

Solution: Active Shields

Warming of air temperature sensors above actual air temperature is minimized with active shields, which are more accurate than passive shields under conditions of high solar radiation load or low wind, but power is required for the fan. The power requirement for active shields ranges from one to six watts (80-500 mA). For solar-powered weather stations, this can be a major fraction of power usage for the entire station and has typically required a large solar panel and large battery. Power requirement and cost are disadvantages of active shields (Table 3), and they have led to the use of less accurate passive shields on many solar-powered stations.

Also, the fan motor can heat air as it passes by. Active shields should be constructed to avoid recirculation of heated air back into the shield.  There is no reference standard for the elimination of radiation-induced temperature increase of a sensor for air temperature measurement, but well-designed active shields minimize this effect.

Advantages and disadvantages of passive and active radiation shield diagram

Table 3: Advantages and disadvantages of passive (naturally-aspirated) and active (fan-aspirated) radiation shields.

There is no reference standard for the elimination of radiation-induced temperature increase of a sensor for air temperature measurement, but well-designed active shields minimize this effect. Radiation-induced temperature increase was analyzed in long-term experiments over snow and grass surfaces by comparing temperature measurements from three models of active radiation shields (the same temperature sensor was used in all shields and were matched before deployment). Continuous measurements for one year indicated that mean differences among shield models were less than 0.1 C over grass and less than 0.3 C over snow. Differences increased with increasing solar radiation, particularly during winter months when there was snow (high reflectivity) on the ground.

Air Temperature: a Complex Measurement

The properties of materials and nearly all biological, chemical, and physical processes are temperature dependent. As a result, air temperature is perhaps the most widely measured environmental variable. Accurate air temperature measurement is essential for weather monitoring and climate research worldwide. The road to accuracy is complex, however, and will continue to be challenging given the trade-off between accuracy and power consumption with passive and active shields.

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Jan 22

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The Quest for Accurate Air Temperature (Part 1)

Mark Blonquist, chief scientist at Apogee Instruments and air temperature measurement expert, explains the difficulties of obtaining accurate air temperature.

Air Temperature gauge at the foot of a tree

The accuracy of air temperature has come a long way.

Accurate air temperature measurements are challenging, despite decades of research and development aimed at improving instruments and methods. People assume that they can use a static louvered radiation shield along with a temperature sensor and start measuring accurate air temperature.  That assumption is good if you are at a site where the wind blows all the time (roughly greater than 3 m/s).  However, if the wind at your field site is below that, you’re going to see errors due to solar heating (See Figure 1).

Wind Speed Graph diagram

Figure 1: Passive Shield Error: Data for 3 different models are graphed.

Challenge 1:  Accurate Sensors

Over the years, thermocouples, thermistors, and platinum resistance thermometers (PRTs) have been used for air temperature measurement, each with associated advantages and disadvantages. PRTs have the reputation as the preferred sensor for air temperature measurement due to high accuracy and stability. However, thermistors have high signal-to-noise ratio, are easy to use and low cost, and have similar accuracy and stability to PRTs. Thermocouples are becoming less commonly used for air temperature measurement because of the requirement of accurate measurement of reference temperature (i.e., meter temperature, data logger panel temperature).

Advantages and Disadvantages of Air Temperature Sensors Chart

Challenge 2: Housing Air Temperature Sensors

The challenge of accurate air temperature measurement is far greater than having an accurate sensor, as temperature measured by an air temperature sensor is not necessarily equal to air temperature. Temperature sensors must be kept in thermal equilibrium with air through proper shielding in order to provide accurate measurements. To do this, housings should minimize heat gains and losses due to conduction and radiation, and enhance coupling to air via convective currents. They must shield it from shortwave (solar) radiant heating and longwave radiant cooling. A temperature sensor should also be thermally isolated from the housing to minimize heat transport to and from the sensor by conduction. The housing should provide ventilation so the temperature sensor is in thermal equilibrium with the air. Also, the housing should keep precipitation off the sensor, which is necessary to minimize evaporative cooling of the sensor. Conversely, condensation on sensors can cause warming. When condensed water subsequently evaporates, it cools the sensor via removal of latent heat (evaporational cooling).

Challenge 3: Size of Sensor

The magnitude of wind speed effects on air temperature measurement in passive shields is highly dependent on the thermal mass (size) of the sensor. Many weather stations have combined relative humidity and temperature sensors, which are much larger than a stand-alone air temperature sensor.  Air temperature errors from larger probes are greater than those from smaller sensors. One study, Tanner (2001), reported results where a common temperature/RH probe was approximately 0.5 degrees C warmer than a common thermistor in a weather-proof housing.

Thermal mass of temperature sensors also has a major impact on sensor response time. Sensors with small thermal mass equilibrate and respond to changes quicker and are necessary for applications requiring high-frequency air temperature measurements.

Thermal Mass measurement table

Challenge 4:  Proper Shielding

In addition to an accurate sensor, accurate air temperature measurement requires proper shielding and ventilation of the sensor. Active, fan aspiration improves accuracy under conditions of low wind but requires power to operate the fan. Passive, natural aspiration minimizes power use but can reduce accuracy in conditions of high solar load or low wind speed.  Radiation shields for air temperature sensors should be placed in an environment where air temperature is representative. For example, air temperature sensors and radiation shields should not be deployed on the tops of buildings or in areas where they will be shaded by structures or trees. Conditions in microenvironments have that potential to be very different from surrounding conditions. Typical mounting heights for air temperature sensors are 1.2 to 2.0 meters above the ground. Typically, radiation shields should be mounted over vegetation.

Up next: Mark Blonquist explains the complexities of some of the proposed solutions to the above challenges in part 2.

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