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Posts tagged ‘Methods’

May 27

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Are Biodegradable Mulches Actually Better for the Environment?

Henry Sintim, PhD student at Washington State University, is investigating whether biodegradable mulches are, in fact, what they claim to be.

Plant row farm with dirt between each row

Application of plastic mulches conserves water, and helps in weed, pest, and disease control.

He and his research team want to understand what leaches into the soil as the mulches degrade and which ones perform as well as polyethylene-made plastic mulches (PEs) at weed, pest, and disease control.

Plastic Mulch

Application of plastic mulches in agriculture is a common practice by specialty crop producers worldwide. It conserves water, and helps in weed, pest, and disease control, subsequently improving crop yield and quality. Because PE is durable and does not degrade in the soil, you cannot leave it in the field, which ultimately leads to the question of disposal.  When PE is buried in the field, it becomes contaminated with soil and can’t be recycled but instead requires transport to a landfill, increasing production costs. Another problem arises when landfill facilities are not available. When this is the case, growers stockpile PE on their farm, where the rain can wash the mulch down to streams and water bodies. Henry Sintim and his team are investigating whether or not biodegradable plastic mulches (BDMs) could be a viable alternative.

Researchers digging a site up for installation

The team installs a lysimeter beneath the mulches.

Biodegradable Alternatives

Substituting PE with BDM could alleviate the need for disposal. However, Sintim says the potential impact on agricultural soil ecosystems needs to be assessed before adopting biodegradable mulch for field use. For instance, do biodegradable mulches really degrade?  Sintim explains, “By BDM, we mean it is plastic mulch, but it has been made from pure or partial biobased materials. Though there are plastic mulches advertised as biodegradable, none have actually been proven to biodegrade, so the team is examining degradation of different commercial BDM types over time. They have also included an experimental BDM, in which the constituents were specified by the team.”

Sintim is monitoring the degradation of BDM by assessing the material properties and measuring the particle size and surface area via photography: digitizing and analyzing them using Image J software.

Researchers standing at an installation getting data

There are indications that some of the BDMs are performing well.

How Well Do the Mulches Compare?

Sintim also wants to find out how well BDMs maintain microclimate in comparison to PE. Since soil temperature and moisture content are important parameters that govern chemical reaction rates and microbial activity, and are likely to vary among the different BDM treatments, he is monitoring soil moisture dynamics using soil moisture and temperature sensors installed at 10 cm and 20 cm depths. In addition, the team has installed sensors directly underneath the mulches to measure surface temperature and light penetration. Reduction of light penetration is the attribute that helps plastic mulches to control weeds. The team is also assessing soil quality using the USDA Soil Quality Test Kit.  

Sintim says so far one of the commercial BDMs and the experimental BDM had the same yield performance as PE.  He adds, “We don’t have final results yet, and there are a lot of variables that could come into the picture. But I will say there is an indication that some of the BDMs are performing well.”

Next week:  Find out how Sintim will determine what’s leaching into the soil and another alternative for polyethylene plastic mulch.

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May 14

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Is Average Relative Humidity A Meaningless Measurement? (Part II)

Scientists often misunderstand average relative humidity (see part I).  In fact, it’s not uncommon to encounter average relative humidity being misused in scientific literature.  This week, learn which measurement should be used instead.

Fog in trees

Humid conditions in a pine forest.

What is Wrong with Average Relative Humidity?

We often use average values to illustrate the behavior of parameters over time.  One of the most common is air temperature, where we effectively graph average half-hourly temperature across a day or daily temperature across a year to show important details about the environment. But, consider what average relative humidity would look like.  

As noted above, a general rule, though not consistent everywhere, is that the temperature at night cools down to the point where the air is saturated and the relative humidity is 100% (1).  During the day, depending on the climate and weather, the saturated vapor pressure may increase roughly two to five times ea and relative humidity would be between 0.2 to 0.5. If we calculated an average for the day, it would most likely be between 0.6 and 0.75, no matter what environment was being measured.  Of course, if it were raining or in the winter with low incoming radiation, this would be higher.  Still, it is easy to see that an average relative humidity does not do much to define meteorological conditions.  

Image: Britannica.com/

The title of this chart is misleading because they were not averaging across the day, but only daily at noon. Image: Britannica.com/

What Should We Use Instead?

The measurement that should be reported is vapor pressure. Not only is it independent of temperature, but it can also be effectively averaged over time to show ecosystem behavior.  However, this value will not be helpful to scientists who are identifying the pull generated by the atmosphere for water vapor in the plant or soil. This quantity is called vapor deficit and is calculated by taking the difference between the saturation vapor pressure and ea.

boy-drinking-from-bottle-738210_640 (1)

We sense water deficit in the atmosphere through our skin.

As humans, we intuitively sense the deficit when we feel that the atmosphere is dry through drying of our lips or our skin.  The same is true for plants. The dry atmosphere will exert a higher pull on the water, pulling it out through the leaves.  The higher the difference between the vapor pressure and the saturation vapor pressure, the more pull for water. Although sometimes reported in literature, the most common use for vapor pressure is as a standard input to evapotranspiration models like FAO56 or Penman-Monteith.

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May 5

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Is Average Relative Humidity A Meaningless Measurement?

Relative humidity is one of the most widely reported weather parameters and is familiar to most people.

Grass with dew on it

Scientists sometimes misunderstand relative humidity.

Still, it is not uncommon to encounter it being misused.  Here are two examples:  

  1. My sister recently stated that her son was experiencing 45℃ and 100% humidity while walking around during the day in the Philippines.
  2. In scientific literature, I often find figures displaying daily average relative humidity over a period of weeks or months.  

Both of these examples show a misunderstanding of what relative humidity is and how it can be used.

What is relative humidity?

Relative humidity (hr) is the ratio of the vapor pressure (ea) in the air over how much vapor pressure there could be if the air were saturated at that air temperature (saturated vapor pressure, es(Ta)).

Relative Humidity equation

While vapor pressure is a reasonably conservative quantity, meaning it doesn’t change drastically with time (i.e.hours), es(Ta) is solely tied to temperature, shown by the empirical Tetens equation:

Relative Humidity equation

where Ta is air temperature, and b =17.502 and c = 240.97℃ (constants).  As the equation shows, saturated vapor pressure is only a function of temperature, so relative humidity in natural conditions will simply show a sinusoidal pattern that is inverse to air temperature.  

Army soldier wiping his eyes from dirt

When humidity is higher, the vapor concentration difference is smaller so we lose less water, reducing our ability to cool.

Why do we estimate it poorly?

When temperatures are elevated above our comfort zone, we begin to feel hot. Our bodies, which are adept at keeping us cool, evaporate water from our skin to return us to a comfortable skin temperature.  When humidity is higher, the vapor concentration difference is smaller so we lose less water, thus reducing our ability to cool.  In an attempt to balance the humidity, our body moistens the skin surface with sweat, leaving us feeling damp and sticky. This makes us feel like the air is nearly saturated, but in reality, the higher humidity has simply limited our ability to cool ourselves.

It is a relatively simple thing to convince ourselves that daytime humidities are never 100% unless it’s raining. We know that daytime temperatures are almost always higher than nighttime, due to solar radiation. And, we are familiar with dew that forms on surfaces as nighttime temperatures cool to the point that they begin to condense water out of the air (dew point temperature). If we assume that the vapor pressure of the air (ea) is the same as the saturation vapor pressure when the dew began to form (nighttime low temperature), then any air temperature throughout the day (Ta, which we assume would be higher) generates a saturation vapor pressure (es(Ta)) that is higher than ea and thus, relative humidity would be less than 1.

So, what about my nephew in the Philippines? Right now, a typical low temperature is 24℃ with a high of 34℃ (when it’s not raining).  Under that scenario, the relative humidity, although it would feel quite high, would only be around 56% at midday.

Next Week: Learn what’s wrong with using average relative humidity in scientific papers and what measurement should be used instead.

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Apr 29

Does Early Planting Increase Risk to Winter Canola?

Many dryland winter canola growers assume that if they plant earlier, they will establish a stronger plant, but Washington State University researcher Megan Reese recently found that this was not the case.  She and her team discovered that planting earlier increases risk to the plant, as more water is used, and the reduced amount of water then left after the winter season limits spring regrowth. Megan’s findings could be valuable as water is the most yield-limiting factor in eastern Washington state’s wheat-dominated dryland systems, where winter canola has newly emerged as a rotational crop.

Bright yellow canola field in full bloom

Winter canola is cold hardy, but it’s not as resilient as wheat.

Early Planting:

Winter canola is cold hardy, but it’s not as resilient as wheat.  It’s planted in August, much earlier than winter wheat, which is planted in the late fall.  In order to survive, winter canola has to establish a hardy taproot system so that plants have reserves to survive the winter. Megan says, “Opinions vary, but anecdotally, a dinner plate sized plant can survive winter fairly well, so that’s why winter canola is planted in August . However, because establishment and germination can be an issue, we decided to try planting in June at Ritzville, Washington, thinking the soil would be more moist and have a cooler seedbed.  However, the early planting date had a negative effect on winter survival. Not one of the early plants survived.  We found the plants that started earlier used a lot more water, and consequently, the winter rains weren’t enough to refill the soil profile.  Excessive growth and bolting also contributed to low survivorship.”

Methods and Moisture Release Curves:

Megan monitored soil water in the profile several different ways.  At one location she used a neutron probe and hand-sampled gravimetric soil moisture in the top 30 cm of the profile, and in other locations, she was limited to  hand samples.  Then she combined those measurements with local weather stations to provide the crop water balance for the canola.  Using these data, she was able to determine soil water use as indicated by the water content change through the growing season and calculate the depletion of soil water.  

Image of blooming winter Canola

Anecdotally, a dinner plate sized plant can survive winter fairly well.

Megan also took soil samples into the lab from each depth increment at every site and used a chilled mirror hygrometer to construct a moisture release curve.  This helped her to define the apparent permanent wilting point at -1.5 MPa.  She says, “I was able to then see how efficient canola was at extracting available water, and I could look at available water instead of total water contents, which was more useful in terms of plant accessible moisture in the soil profile. It allowed me a consistent platform to compare actual water amounts across sites with differing soil types.  At one site, 12.5% of the water was unavailable, but in the sandier soils at another site, it was 4%.  So there were significant differences in permanent wilting point.”

Water and Physiological Challenges Affect Winter Survival:

Megan found that the June planted canola used every milliliter of available water in the soil profile by late October/early November, but August-planted canola still had some water above wilting left in the profile over the winter, which helped the plants in the spring.  She says, “It was a milder winter, so we didn’t get the usual amount of snow and rain, which probably played a role, but we did not see the profile refilled in the June-planted canola.  In addition, those June plants were purple and wilted by November, so water stress could have hurt the plants in terms of its defenses. However, I think a larger issue was that they grew so large (the crowns actually elongated and bolted so they weren’t close to the soil) they were more susceptible to the harsh temperatures, whereas the August planted canola were much smaller and their crowns stayed right on the soil surface.”  These findings are based on only one year of data, and Megan notes that early plantings have worked well in the milder climate of Pendleton, OR.

What Does it Mean for Farmers?

Megan says, “We were able to surprise a lot of farmers by showing that canola roots access water down to 1.5 to 1.7 m in the fall; it was hard to believe that a winter crop would do that. Also, in my second year’s data, we followed water use all the way through harvest, so we were able to show how much yield we gained for every millimeter of water used, and farmers liked hearing that number as well.  I think it’s useful information that incorporates biophysics principles and answers some questions that these new canola producers are interested in.  I have three locations this season that we are currently following to give farmers a further idea of what the water use looks like, when canola uses that water, and from where in the soil profile.  Hopefully, this research will help them manage their rotations and look at the possibility of adopting canola.”

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Apr 7

Green Roofs—Do They Work? (Part II)

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

Green and whited plant on a garden rooftop with orange rocks

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

New Challenges for Green Roofs

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

Overhead close up of garden roof plant

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

Measurements Required for Green Roof Validation

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

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

Plants poking out of the soil in front of a house

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

Other Projects and Future Plans

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

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Apr 1

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Green Roofs—Do They Work?

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

Garden on a rooftop with flowers and a city around it

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

Why Green Roofs?

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

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

Researcher kneeling testing soil with a soil sensor

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

Finding Answers

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

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

Green Roof Runoff Reduction graph

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

What Has Been Learned?

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

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

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Mar 23

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

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

Abu Dhabi from the ocean looking at the city

Abu Dhabi

Infiltrometers Predict Dripper Behavior:

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

researcher recording data while sitting on the floor

Wafa on site, using the twin head method.

Treated Effluent Resolves Salinity Issues

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

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

Large white irrigation tanks sitting in sand in the desert

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

What’s The Future?

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

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Mar 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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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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