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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 25

The Tensiometer: Micro-sized

A strand of a spider’s web is 5 micrometers in width. Microelectromechanical systems (MEMS) devices range in size from 20 micrometers to one millimeter. That’s the incredibly small size of the components used in the tensiometer being developed by PhD candidate, Michael Santiago, and his collaborators, professors Abraham Stroock and Alan Lakso  at Cornell University.

Spider wed with dew water on the strands

MEMS devices can be as small in width as 4 strands of a spider’s web.

The engineer/research team is using MEMS technology to develop a miniature tensiometer (microtensiometer) that has a 100 times larger range than existing tensiometers, is stable for months, communicates digitally, and can be embedded into plant stems to directly measure plant water potential.

Existing Tensiometer Limitations:

Water potential is the best measure of a plant’s hydration relative to growth and product yield. Unfortunately, directly measuring water potential in plant tissue is only possible through labor-intensive, destructive methods such as the leaf pressure bomb and stem psychrometer. A common alternative is to use ‘set-and-forget’ soil tensiometers to measure soil water potential as a proxy for plant water potential, but this method is unreliable for plants with high hydraulic resistance (vines and woody species), where plant water potential is often much less than the water potential in soil. Although soil tensiometers are very accurate and simple to use, they can be large and bulky, and cavitate as soils dry.

A 25 cent coin next to a prototype microtensiometer

Prototype microtensiometer made with MEMS components.

Solution:

The Cornell University research team wants to improve the design of the tensiometer so it can be used in the field for applications such as continuously monitoring and controlling plant water potential in vineyards to consistently produce high-quality wine grapes with an exact flavor/aroma profile.  Santiago says, “We’ve basically miniaturized a tensiometer using microchip technology to the point where it’s this tiny chip inside a wafer. Because of the way we fabricated it, we are hoping to make it an embeddable tensiometer that can go in anywhere and measure tension down to about -100 bars (-10 MPa).”

Developing and Calibrating

Santiago is using a chilled mirror hygrometer to produce solutions of specific water potential to test, calibrate, and characterize the microtensiometer.  He comments, “We’ve been testing it in osmotic solutions. We use the water potential meter for calibrating a solution of PEG (polyethylene glycol), and then we measure it with the tensiometer.”

One hurdle the team has to overcome is finding a membrane that keeps small molecules and ions out of the tensiometer pores: these pollute the water inside the tensiometer and cause measurement errors. Santiago explains, “Our solution right now is to test in solutions of large molecules, such as PEG of 1400 molecular weight. The tensiometer pores are about 3-4 nanometers, extremely small, but small molecules, such as sugars and salts, can still get through. It’s not a problem for the short term because we are directly submerging into solutions of just water and large molecules, but our goal is to go into the environment and insert the tensiometer into soils and plant stems where small molecules are ubiquitous, so we’ll have to find a membrane that works and can handle field testing.”

The team has been experimenting with materials such as Gore-Tex and reverse osmosis membranes [M5]  [M6] hoping to find a membrane that allows water through and keeps ions out, but does not slow the measurement.

Close up of a plant

Researchers want be able to insert the device directly into plant xylem.

What’s Next?

Santiago says the calibrations have worked well. Now the challenge will be putting the tensiometer into different environments such as soil, concrete, and plants. For example, they want be able to insert the device directly into plant xylem, which will require a seal so water is not exiting the system.  And that’s not the only complication. Santiago explains, “We are getting ready to do some testing in soils. The challenge will be getting good data because soil can be really heterogeneous, and we have this sensor with a much larger range than the usual tensiometer. So what do we compare it with? That’s going to be a bit of a challenge.” Santiago says the next few months will be spent getting into some different materials and obtaining some initial publishable data.

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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 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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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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